Many objects are almost worthless to the microscopist until the extraneous3 matter is removed from them; and this is frequently difficult in the extreme to perform satisfactorily. As an instance, certain Foraminifera may be mentioned in which the cells are placed one upon another, consequently the object must be reduced to a certain degree of thinness before a single uniform layer of these cells can be obtained to show something of the internal arrangements.
Most animal and vegetable forms require an examination of the separate parts before much can be known about them. The mass must be divided into separate portions, each part intended to be preserved being cleaned from the useless matter with which it is surrounded. It will frequently be found necessary to make thin sections, which from a very tender substance is no easy matter; and much patience will be necessary to attain4 anything like proficiency5.
This making of sections was not until very recently entered into by many except those belonging to the medical profession, but I do not see why this should be so, as much may be accomplished6 by a persevering7 and interested mind where there is time for entering into the subject. I will therefore make an attempt to give some instructions on this subject also. We will first consider the cutting of sections from hard substances, in which the ordinary knife, chisel8, &c., are of no avail. Most of these require no particular care in mounting, but are placed in balsam like the other objects noticed in Chapter III.: where, however, any special97 treatment is necessary it will be commented upon as we proceed.
Shells, &c.—It is seldom, if ever, necessary to possess apparatus9 for this process except a small thin saw made with a steel blade, for which a piece of watch-spring serves very well; a fine stone such as is used for sharpening pen-knives; and two smooth leather strops, one of which is to be used with putty-powder to polish the section after grinding, and the other dry, to give the final surface. It is, however, very convenient to have three or four files of different degrees of fineness. The shell, if very thick, may be divided by using the watch-spring saw; and this section may then with ordinary care be rubbed down with water on the stone until one side of it is perfectly10 flat. When this is accomplished it must be again rubbed with putty-powder upon the strop, and finally upon the other strop without the powder. This surface will then be finished and must be firmly united to the slide in the position it is intended to occupy. To do this a small quantity of Canada balsam may be dropped upon the middle of the slide and heated over the lamp until on cooling it becomes hard; but this must be stopped before it is rendered brittle11. Upon this the polished surface must be laid, and sufficient heat applied12 to allow the object to fall closely upon the slide, when slight pressure may be used to force aside all bubbles, &c. On cooling, the adherence13 will be complete enough to allow the same grinding and polishing upon the upper surface which the lower received. Whilst undergoing this, the section must be examined from time to time to ascertain14 whether the necessary degree of thinness has been reached. When this is the case the section should be washed thoroughly15 and dried. It must then be covered, which is best done by using the ordinary Canada balsam, as recommended in Chapter III.
Sections of some exquisitely16 beautiful objects are cut with much less trouble than the above. The Orbitolite, for instance, may be prepared in this manner. Take the object and by pressure with the finger rub the side upon98 a flat and smooth sharpening stone with water until the portion is reached which it is wished to show. The strength of the object will easily allow this to be accomplished with ordinary care. This side may then be attached to the glass slide with heated balsam, as above described, and the object may then be gently rubbed down to the degree of thinness required to show it to the best advantage. After removing all disengaged matter from the object by washing and thoroughly drying, it may be mounted in balsam in the usual manner, when it is equally beautiful as a transparent17 or opaque18 object. From this it will be seen that in many instances where a smooth stone is found sufficient for the work (which is often the case when the section is mounted in balsam) the final process of polishing advised above may be dispensed19 with, as in the Orbitolite, Nummulite, &c., &c. It is quite necessary that the stones on which the objects are rubbed be perfectly flat, otherwise one side must be acted upon before the other, and it will be found impossible to attain anything like uniformity. Where it is not practicable to cut a section and the object is very thick, a coarse stone may be first used to reduce it and the smoother afterwards.
The consideration of the cutting of sections from shells would scarcely be deemed complete without some mention of what Dr. Carpenter terms the decalcifying process. Muriatic acid is diluted20 with twenty times its volume of water, and in this the shell is immersed. After a period, differing according to the thickness of the shell, the carbonate of lime will be dissolved away, and a peculiar22 membrane23 left, showing the structure of the shell very perfectly. This may be mounted dry, in balsam, or sometimes in liquid, according to the appearance of the object; but no rule can be given. The discretion24 of the student, however, will enable him to choose the most suitable method.
From some shells it is easy to divide thin plates, or “lamin?,” which require nothing but mounting in Canada balsam to show the texture25 very well. In working, however,99 with those which are “pearly,” it will be found that experience and patience are needed, as they are very brittle and peculiarly hard; but a little practice will overcome these difficulties.
Amongst the Echinodermata, which include the star-fishes, sea-hedgehogs, &c., there are many whose outer surface is covered with “spines,” or thin projections26. Some of these are sharp and thorn-like, others blunt, longer or shorter, and, indeed, of endless variety. In many of these, when a section is made, rings are seen which have a common centre, with radiating supports, resembling sections of some of the woods. These are very beautiful objects, and methods of procuring27 them may now be considered. It is the best to cut as thin a section as can safely be got with the watch-spring saw first, when the smooth “sharpening stone” may be used to polish one side, which is easily accomplished with water only. When this is effected, it must be washed clean, and thoroughly dried, and then may be united to the slide in the same manner as before recommended for the Orbitolite, &c. If it is ever necessary to displace it on account of inequalities, bubbles, or other remediable fault, this may be done by warming the slide; though too much heat must be avoided, otherwise fresh bubbles will certainly be produced. The covering with thin glass, balsam, &c., will present no difficulty to the student; but he must remember that the transparency is somewhat increased by this last operation.
Corals are often treated in this way, in order to reveal their structure. Except, however, the student has had much practice, he will often find this a most difficult task, as many of them are exceedingly brittle. He will find the method before described equally applicable here, and should take both horizontal and vertical28 sections.
Coal.—This substance is one of the most interesting objects to the microscopist. It is, of course, of a vegetable origin; and though this is in many cases of such minute separate portions as to have lost all appearance of vegetation,100 yet it is very frequently met with in masses, bearing the form, even to the minute markings, of wood, in various directions. To see this and prepare it for microscopic29 research, a suitable piece of coal must be obtained; but in every case the cutting and preparation of these sections require great care and skill. Sometimes the coal is first made smooth on one side, fastened to the glass, reduced to the requisite30 degree of thinness, and finished in the method before described. This mode of treating it is sometimes, however, very tantalizing31, as, at the last moment, when the section is about thin enough, it often breaks up, and so renders the trouble bestowed32 upon it fruitless. The dark colour and opacity33 of coal render an extraordinary thinness necessary, and so increase the liability to this accident.
Perhaps the best method which can be pursued is that recommended in the “Micrographic Dictionary,” which is as follows:—“The coal is macerated for about a week in a solution of carbonate of potash; at the end of that time it is possible to cut tolerably thin slices with a razor. These slices are then placed in a watch-glass with strong nitric acid, covered and gently heated; they soon turn brownish, then yellow, when the process must be arrested by dropping the whole into a saucer of cold water, else the coal would be dissolved. The slices thus treated appear of a darkish amber34 colour, very transparent, and exhibit the structure, when existing, most clearly. We have obtained longitudinal and transverse sections of coniferous wood from various coals in this way. The specimens36 are best preserved in glycerine in cells; we find that spirit renders them opaque, and even Canada balsam has the same defect. Schulz states that he has brought out the cellulose reaction with iodine37, in coal treated with nitric acid and chlorate of potash.”
Cannel-coal is so close and firm in its structure as to be much used instead of jet in the manufacture of ornaments38: it takes a beautiful polish, and consequently presents the student with none but ordinary difficulties in getting101 sections of it. Its formation is somewhat different from that of coal, sometimes showing the transition very clearly.
In flint there are often found remains39 of sponges, shells, Diatomace?, &c.; but to show these well, sections must be cut and polished by the lathe40 and wheel of the lapidary41, which the microscopic student seldom possesses. Thin chippings may, however, be made, which when steeped in turpentine and mounted in balsam, will frequently show these remains very well.
Teeth are very interesting objects to all microscopists, more especially to those who give much study to them; as the class of animal may very frequently be known from one solitary42 remaining tooth. To examine them thoroughly, it is necessary to cut sections of them; but this is rather difficult to perform well, and needs some experience. Some instructions, however, will at least lessen43 these difficulties, and we will now endeavour to give them.
It is generally thought that Canada balsam injures the finer markings of these sections, consequently, they are almost invariably mounted dry. A thin piece is first cut from the tooth with the saw of watch-spring before mentioned, if possible; but should the substance be too hard for this, the wheel and lathe must be used with diamond dust. If this cannot be procured44, there is no alternative but to rub down the whole substance as thin as practicable on some coarse stone or file. The surface will then be rough; but this may be much reduced by rubbing upon a flat sharpening stone with the finger, or a small piece of gutta-percha, upon the object to keep it in contact. The scratches may be much lessened46 by this, but not so thoroughly removed as microscopic examination requires in dry sections. It must, therefore, be polished with the putty-powder and dry strop, as recommended in the working of the shell-sections. The other side of the section of the tooth may then be rubbed down to the requisite thinness, and polished in the same manner, when the dust and other impurities47 must be removed by washing, after which the section must be carefully dried102 and mounted. Some of these sections are equally interesting as opaque or transparent objects.
The dentine of the teeth may be decalcified by submersion of the section in dilute21 muriatic acid; after drying and mounting in Canada balsam it presents a new and interesting appearance, showing the enamel48 fibres very beautifully when magnified about three hundred diameters. A friend tells me that after submersion of the whole tooth in the acid he has been able to cut sections with a razor.
Sections of Bone.—With the aid of the microscope few fragmentary remains have proved so useful to the geologist49 and students of the fossil kingdom as these. From a single specimen35 many of our naturalists50 can tell with certainty to what class of animal it has once belonged. To arrive at this point of knowledge much study is necessary, and sections of various kinds should be cut in such a manner as will best exhibit the peculiarities51 of formation. The methods of accomplishing this will now be considered. It may, however, be first mentioned that the chippings of some bones will be found useful now and then, as before stated with flint, though this is by no means a satisfactory way of proceeding52. Sometimes the bones may be procured naturally so thin that they may be examined without any cutting; and only require mounting dry, or in fluid, as may be found the best.
When commencing operations we must provide the same apparatus as is needed in cutting sections of teeth, before described. A fine saw, like those used for cutting brass53, &c.; two or three flat files of different degrees of coarseness; two flat “sharpening” stones; and a leather strop with putty-powder for polishing. As thin a section as possible should first be cut from the part required by the aid of the fine saw; and it is better when in this state to soak it for some short time in camphine, ether, or some other spirit to free it from all grease. With the aid of a file we may now reduce it almost to the necessary degree of thinness, and proceed as before recommended with teeth. The103 “sharpening” stone will remove all scratches and marks sufficiently54 to allow it to be examined with the microscope to see if it is ground thin enough; and if it is to be mounted dry we must polish it with putty-powder and water upon the strop to as high a degree as possible, and having washed all remains of polishing powder, &c., from the section we must place it upon the slide and finish it as described in Chapter II.
If the bone is not sufficiently hard in its nature to bear the above method of handling whilst grinding and polishing—as some are far more brittle than others—as thin a section as possible must first be cut with the saw, and one surface ground and polished. The piece must then be dried and united to the glass by heated balsam in the same manner as shells, &c. After which the superabundance of balsam must be removed from the glass; then rub down upon the stone and strop as before. When the polishing is completed the whole slide must be immersed in chloroform, ether, or some other spirit, to release and cleanse55 the section, when it may be mounted as the one above mentioned.
Some have recommended a strong solution of isinglass to affix56 the half-ground teeth or bones to the glass as causing them to adhere very firmly and requiring no heat, and also being readily detached when finished.
The reason why the sections of bone are usually mounted dry is that the “lacun?,” bone cells, and canaliculi (resembling minute canals) show their forms, &c., very perfectly in this state, as they are hollow and contain air, whereas if they become filled with liquid or balsam—which does sometimes occur—they become almost indistinguishable. There are some dark specimens, however, where the cells are already filled with other matter, and it is well to mount these with balsam and so gain a greater degree of transparency.
To gain a true knowledge of the structure of bone, sections must be cut as in wood, both transversely and longitudinally; but with fossil bones, without the lapidary’s104 wheel, &c., it is a laborious57 task, and indeed can seldom be properly accomplished. In this place, also, it may be mentioned that by submitting bone to the action of muriatic acid diluted ten or fifteen times with water, the lime, &c., is dissolved away and the cartilage is left, which may be cut into sections: in caustic58 potash the animal matter is got rid of. Both of these preparations may be mounted in fluid.
The method of cutting thin sections of bone may be also employed with the stones of fruit, vegetable ivory and such like substances; many of which show a most interesting arrangement of cells, especially when the sections are transverse. Most of these objects present a different appearance when mounted dry to that which they bear when in balsam, owing to the cells becoming filled; and to arrive at a true knowledge of them we must have a specimen mounted in both ways.
To those who study polarized light, few objects are more beautiful than the sections of the different kinds of horn. We will briefly59 inquire into the best method of cutting these. There are three kinds of horn, the first of which is hard, as the stag’s, and must be cut in the same manner as bone. The second is somewhat softer, as the cow’s. The third is another and still softer formation, as the “horn” (as it is termed) of the rhinoceros60. In cutting sections of the two last we should succeed best by using the machine invented for these purposes which I shall shortly describe when the method of cutting wood is considered. To aid us in this when the horn is hard it must be boiled for a short time in water, when the cutting will be more easily accomplished. The sections should be both transverse and longitudinal, those of the former often showing cells with beautiful crosses, the colours with the selenite plate being truly splendid. Of this class the rhinoceros horn is one of the best; but the buffalo61 also affords a very handsome object. The cow’s, and indeed almost every different kind of horn, well deserves the trouble of mounting. Whalebone, when cut transversely,105 strongly resembles those of the third and softer formation. All these are best seen when mounted in Canada balsam, but care must be taken that they have been thoroughly dried after cutting, and then steeped in turpentine.
An interesting object may also be procured from whalebone by cutting long sections of the hairs of which it is composed. Down the centre of each hair we shall find a line of cells divided from one another very distinctly. And (as recommended in the “Micrographic Dictionary”) if whalebone be macerated twenty-four hours in a solution of caustic potash it will be softened63, and by afterwards digesting in water, the outer part will be resolved into numerous transparent cells, which will show more plainly the structure of this curious substance.
In a former chapter, hairs were mentioned, their many and interesting forms, and their beauty when used with polarized light. The sections of them, however, are no less a matter of study, as this mode of treatment opens to sight the outer “casing,” and the inner substance somewhat resembling the pith of plants.
It would be out of place to enter into the description of the different forms met with; but the ways in which sections are to be procured may be glanced at. If transverse sections are required, some place a quantity of hairs betwixt two flat pieces of cork64, which by pressure hold them firmly enough together to allow the required portions to be cut with a razor. Others take a bundle of the hairs and dip it into gum or glue, which gives it when dry a solidity equal to wood. Sections of this are then cut with the machine mentioned a little further on, and these may be mounted in balsam. The human hair is easily procured in the desired sections by shaving as closely as possible a second time and cleansing65 from the lather66, &c., by carefully washing. Most hairs, however, should be examined both transversely and longitudinally. It is not difficult to procure45 the latter, as we may generally split them with the aid of a sharp razor. In a great number of hairs there is a quantity of greasy67 matter106 which must be got rid of by soaking in ether or some other solvent68 before mounting.
We may next consider the best method of procuring sections of wood, which must be cut of such a degree of thinness as to form transparent objects, and so display all the secrets of their structure. There is no monotony in this study, as the forms are so various, and the arrangement of the cells and woody fibre so different, that the microscopist may find endless amusement or study in it. From a single section the class of trees to which it has belonged may be known, often even when the wood is fossil. The apparatus best adapted for cutting these sections is made as follows:—A flat piece of hard wood, about six inches long, four wide, and one thick, is chosen, to which another of the same size is firmly fixed69, so as to form, in a side view, the letter T. On one end of the upper surface is fastened a brass plate, perfectly flat, in the centre of which a circular opening is cut about half an inch in diameter. Coinciding with this opening is a brass tube, fixed in the under side of the table (if it may be termed so). This tube is so cut at the bottom as to take a fine screw. Another screw is also placed at the same end of the “table,” which works at right angles to this, so that any substance in the tube may be wedged firmly by working this last screw. To use this instrument, the piece of wood or other object of which a section is required must be placed in the tube, when, by turning the screw underneath70, the wood is raised above the brass plate more or less as wished, and by using the screw at the end, it is held firmly in the same position. With a flat chisel the portion of the object which projects above the surface of the brass plate may now be cut off, and by means of the bottom screw another portion may be raised and treated in the same manner. As to the thickness of which objects should be cut, no proper directions can be given, as this differs so greatly that nothing but experience can be any guide. The same thickness can be obtained by working the screw underneath in uniform107 degrees, the head being marked for this purpose; and when the substance to be cut is very much smaller than the hole in the brass plate, it may be wedged with cork.
As this instrument is peculiarly adapted for cutting wood (though used for other substances, as before mentioned), I shall notice a few particulars concerning this branch of sections. It may here be remarked, that to obtain anything like a true knowledge of the nature of wood, it should be cut and examined in at least two directions, across and along. The piece of wood is often placed in spirits for a day or two, so that all resinous71 matter may be dissolved out of it; it must then be soaked in water for the same length of time, so as to soften62 and render it easy to cut. Sections are obtained in the manner described above, but often curl to such a degree as to make it necessary to immerse them in water, from which they may be taken and dried under slight pressure. They are often mounted dry, and require no care beyond other objects, as in Chapter II. Some, however, are best mounted in balsam, particularly the long sections when used for the polariscope; these must be soaked in turpentine, and the greatest care taken that all air bubbles are got rid of. Others are thought to be most useful when mounted in shallow cells with some of the preservative72 liquids mentioned in Chapter IV.—weak spirit and water, chloride of calcium73 solution of the strength of one part of the salt to three parts of distilled74 water, &c.
The above “section-cutter” may not be within the reach of every student, nor is it absolutely necessary; though where any great number of specimens is required it is very useful, and insures greater uniformity in the thickness. Many employ a razor for the purpose, which must always be kept sharp by frequent stropping. Sections of leaves also may be procured by the same means, though, as before mentioned, they are sometimes easily divided by stripping the coatings off with the fingers. The cells which come to sight by cutting some of the orchideous108 plants are most interesting. To cut these leaves they may be laid upon a flat piece of cork, thus exposing the razor to no danger of injury by coming in contact with the support. It may be mentioned here that the razor may also be used in cutting sections of the rush, than which a more beautiful object can scarcely be found when viewed transversely, as it shows the stellate arrangements of the ducts to convey the liquids to the different parts of the plant very clearly. This should be mounted dry. In the same way sections of the leaf-stalks of ferns may also be cut, some of which, as Dr. Carpenter states, show the curious ducts very beautifully, especially when cut rather obliquely75.
When sections of the softer substances are required, no instrument can be compared with “Valentin’s knife,” which consists of two steel blades lying parallel with one another and attached at the lower end. The distance of separation may be regulated at will by a small screw near the handle. When, therefore, a section is wanted, the substance must be cut through, and betwixt the blades a thin strip will be found, which may be made of any thickness, according to the distance of their separation. By loosening the screw the blades may be extended, and the section may be floated out in water if the damp will not injure it. The knife cuts much better if dipped in water immediately before use and also when the substance to be operated upon is wet, or even under water altogether; but care must be taken, after use, to clean the blades thoroughly and oil them before laying by, if the place is at all damp. This instrument is most useful in such subjects as anatomical preparations where the sections are required to show the position of the different vessels76, &c.; but, as before stated, is very valuable for all soft substances. As an instance of this, it may be mentioned, that it is frequently used in cutting sections of sponges; but as these are often very full of spicula, &c., it is much better to press the sponge flat until dry, and then cut off thin shavings with a very sharp knife; these shavings will109 expand when placed in water. After this they may be laid betwixt two flat surfaces and dried, when they may be mounted as other dry objects, or, when desirable, in balsam.
Valentin’s knife is very much used in taking sections of skin, which are afterwards treated with potash solution, acids, &c., to bring out in the best way the different portions. Dr. Lister’s mode, however, of getting these is thus given in the Microscopic Journal:—“But I afterwards found that much better sections could be obtained from dried specimens. A portion of shaved scalp being placed between two thin slips of deal, a piece of string is tied round them so as to exercise a slight degree of compression; the preparation is now laid aside for twenty-four hours, when it is found to be dried to an almost horny condition. It then adheres firmly by its lower surface to one of the slips, and thus it can be held securely, while extremely thin and equable sections are cut with great facility in any plane that may be desired. These sections, when moistened with a drop of water and treated with acetic78 acid, are as well suited for the investigation79 of the muscular tissue as if they had not been dried.”
There are many who almost confine their attention to polarized light and its beautiful effects. Such would not deem these efforts to aid the student in cutting sections complete without some notice of those which are taken from various crystals, in order to display that curious and beautiful phenomenon, the rings with a cross. The arrangement of these is somewhat changed by the crystal which affords the section; but nitrate of potash gives two sets of rings with a cross, the long line of which passes through both, the short line dividing it in the middle.
The process of cutting these sections is rather difficult, but a little care and perseverance80 will conquer all this. The following is extracted from the Encyclop?dia Metropolitana:—“Nitre crystallizes in long six-sided prisms whose section, perpendicular81 to their sides, is the regular hexagon. They are generally very much interrupted in110 their structure; but by turning over a considerable quantity of the ordinary saltpetreF of the shops specimens are readily found which have perfectly transparent portions of some extent. Selecting one of these, cut it with a knife into a plate above a quarter of an inch thick, directly across the axis82 of the prism, and then grind it down on a broad wet file till it is reduced to about one quarter or a sixth of an inch thick, smooth the surface on a wet piece of emeried glass, and polish on a piece of silk strained very tight over a strip of plate-glass, and rubbed with a mixture of tallow and colcothar of vitriol. This operation requires practice. It cannot be effected unless the nitre be applied wet and rubbed till quite dry, increasing the rapidity of the friction83 as the moisture evaporates. It must be performed in gloves, as the vapour from the fingers, as well as the slightest breath, dims the polished surface effectually. With these precautions a perfect vitreous polish is easily obtained. We may here remark, that hardly any two salts can be polished by the same process. Thus, Rochelle-salt must be finished wet on the silk, and instantly transferred to soft bibulous84 linen85 and rapidly rubbed dry. Experience alone can teach these peculiarities, and it is necessary to resort to contrivances (sometimes very strange ones) for the purpose of obtaining good polished sections of soft crystals, especially of those easily soluble86 in water.
“The nitre is thus polished on both its surfaces, which should be brought as near as possible to parallelism.”
F Sometimes the saltpetre of the shops is nitrate of soda87, and as this is slightly deliquescent, it is well to be certain that we have the nitrate of potash, which is free from this defect.
Some sections of the naturally formed crystals also show the “rings” very well,—as Iceland Spar, which gives a single ring and cross; but the difficulty of cutting and polishing them is almost too great for the amateur, and must be left to the lapidary. This curious phenomenon, however, may be seen by using a plate of ice uninterruptedly formed of about one inch in thickness.
111 Before concluding these remarks on sections, I must mention a few difficulties which may be met with, and their remedies. The foremost of these is the softness of some objects, which have not resistance enough in themselves to bear cutting even with the sharpest instruments. This may often be removed by soaking in a solution of gum, and then drying, which will render the substance firm enough to be cut, when the sections must be steeped in water, and the gum thus got rid of. Small seeds, &c., may be placed in wax when warmed, and will be held firmly enough when it is again cold to allow of them being cut into sections, &c. And, lastly, where a substitute for a microscopist’s hand-vice is required, a cork which fits any tube large enough may be taken and split, the object being then placed between the two parts, and the cork thrust into the tube, a sufficient degree of firmness will be obtained to resist any necessary cutting, &c.
Dissection.—As I stated at the commencement of this chapter, no written instructions can enable any student to become an adept88 in this branch without much experience and no little study. I will, however, describe the necessary apparatus, and afterwards mention the mode of treatment which certain objects require.
A different microscope is manufactured for the purpose of dissection, most first-rate makers89 having their own model. The object-glasses of many of these are simple, and consequently not expensive; but one of the great requisites90 is a stage large enough to hold the trough, in which the operation is often performed. Where this is the case it would scarcely be worth the expense of getting a dissecting91 microscope if the student were pursuing no particular study, but merely made use of the instrument when an object to be operated upon turned up accidentally. The ordinary form is much improved for this purpose, by having two wooden rests placed at the sides of the microscope, upon which the hands may be supported when working upon the stage. These should be weighty enough to be free from danger of moving.112 These supports will also be found to be a remedy against much of the weariness which inevitably92 arises from having to sustain the hands as well as work with them. The erector, as I before observed, is necessary to a young student; but with a little practice he may work very well without it.
We will now notice some of the instruments which are most useful in dissection. Two or three different sizes of ordinary scissors should be possessed93, but the shapes must be modified in others for many purposes, as those used by surgeons; a pair with the cutting parts bent94 in a horizontal direction, and another pair slightly curved in a perpendicular; so that parts of the substance operated upon may be reached, which it would be impossible to touch with straight scissors. One point of these is sometimes blunt, and the other acute, being thus made very useful in opening tubular formations. Another form of these is made, where the blades of the scissors are kept open by a spring, the handles being pressed together by the fingers. Where it is desirable, one or both of these handles may be lengthened95 to any degree by the addition of small pieces of wood.
The Knives which are most useful are those of the smallest kind which surgeons employ in very delicate operations. These are made about the length of an ordinary pen-knife, and are fixed in rather long flattish handles; some are curved inwards, like the blade of a scythe96, others backwards97; some taper98 to a point, whilst others again are broad and very much rounded. Complete boxes are now fitted up by the cutlers, of excellent quality and surprisingly cheap.
Needles.—These are very useful and should be firmly fixed in handles as recommended in Chapter I. It is convenient to have them of various lengths and thicknesses. If curved by heating and bending to any required shape they may be re-hardened by putting them whilst hot into cold water. Dr. Carpenter also makes edged instruments by rubbing down needles upon a hone. They are more pleasant to work with when short, as the spring they have whilst long robs them of much of their firmness.
113 A glass syringe is also useful in many operations, serving not only to cleanse the objects but to add or withdraw liquids from the dissecting-trough. This trough will now be described, as many substances are so changed by becoming dry that it is impossible to dissect2 them unless they are immersed in water during the operation. If the object is opaque and must be worked by reflected light, a small square trough may be made to the required size of gutta-percha, which substance will not injure the edge of the knives, &c.; but where transparency is necessary, a piece of thin plate-glass must be taken, and by the aid of marine99 glue (as explained in Chapter IV.) the sides affixed100 of the required depth. As pins, &c., cannot be used with the glass troughs and the substance must be kept extended, a thin sheet of cork loaded with lead in order to keep it under water may be used; but this, of course, renders the bottom opaque. When working with many thin substances, a plate of glass three or four inches long and two wide will serve every purpose, and be more pleasant to use than the trough. A drop or two of water will be as much liquid as is needed, and this will lie very well upon the flat surface. As these are the principal apparatus and arrangements which are requisite in dissection, the method of proceeding in a few cases may now be noticed.
Vegetables.—The dissection of vegetable matter is much less complicated than that of animal; maceration101 in water being a great assistant, and in many cases removing all necessity for the use of the knife. This maceration may be assisted by needles, and portions of the matter which are not required may be removed by them. When, for instance, the spiral vessels which are found in rhubarb are wanted, some parts containing these are chosen and left in a small quantity of water until the mass becomes soft, and this is more quickly effected when the water is not changed. The mass must be then placed upon a glass plate when practicable, or in the trough when large, and with the aid of two needles the matter may be removed from the spiral vessels,114 which are plainly seen with a comparatively low power; and by conveying these to a clean slip of glass, repeating the process, and at last washing well, good specimens may be procured. Most of these should be mounted in some of the preservative liquids in the manner described in Chapter IV. Many, however, may be dried on the slide, immersed in turpentine, and then mounted in balsam; but liquid is preferable, as it best preserves the natural appearance. Certain kinds of vegetables require a different treatment to separate these spiral vessels. Asparagus is composed of very hard vegetable matter, and some have recommended the stems to be first boiled, which will soften them to such a degree that they may easily be separated. Dilute acids are also occasionally used to effect this; and in some instances to obtain the raphides caustic potash may be employed; but after any of these agents have been made use of, the objects must be thoroughly cleansed102 with water, else the dissecting instruments (and perhaps the cell) will be injured by the action of the remaining portion of the softening103 agent.
For the dissection of animal tissues it is necessary that the instruments be in the best order as to sharpness, &c.; and as the rules to be observed must necessarily be somewhat alike in many instances, the treatment required by some of the objects most frequently mounted will now be described. We may here remark that cartilage can be best examined by taking sections, which will show the arrangement of the cells very perfectly. This, however, is plainly seen in the mouse’s ear without any section being necessary. Glycerine, the preservative liquids before mentioned, and Canada balsam are all used to mount it; but perhaps the first named may be preferred in many cases.
Muscle.—This is what is commonly called the “flesh” of animals. If a piece be laid upon the slide under the microscope, bundles of “fibres” will be perceived, which with needles and a little patience may be separated into portions, some of these being “striated,” or marked with alternate spaces of dark and light. Some of the non-striated115 or smooth class of muscle, such as is found in intestines104, may be prepared for the microscope by immersing for a day or two in nitric acid diluted with three or four parts of water, and then separating with needles and mounting as soon as possible. Sometimes boiling is resorted to to facilitate the separation, and occasions little or no alteration105 in the material. Specimens are often taken from the frog and the pig, as being amongst the best, Goadby’s solution being generally used in mounting them. The muscle of insects also shows the stri? very perfectly.
Nerve-tissue.—This is seldom mounted; as Dr. Carpenter observes, “no method of preserving the nerve-tissue has been devised which makes it worth while to mount preparations for the sake of displaying its minute characters,” but we will mention a few particulars to be observed in its treatment. The nerve should be taken from the animal as soon as possible after death, and laid upon a glass slide, with a drop or two of serum106 if possible. The needles may be used to clean it, but extreme delicacy107 is necessary. It will be found that the nerve is tubular and filled with a substance which is readily ejected by very slight pressure. When the nerve is submitted to the action of acetic acid, the outer covering, which is very thin, is considerably108 contracted, whilst the inner tube is left projecting; and thus is most distinctly shown the nature of the arrangement. Mr. Lockhart Clarke, who has made great researches into the structure of the spinal109 cord, gives a part of his experience as follows:—He takes a perfectly fresh spinal cord and submits it to the action of strong spirits of wine. This gives the substance such a degree of hardness that thin sections may be readily cut from it, which should be placed upon a glass in a liquid consisting of three parts of spirit and one of acetic acid, which renders them very distinct. To mount these sections, they must now be steeped in pure spirit for two hours and afterwards in oil of turpentine, and lastly must be mounted in Canada balsam.
Trache? of Insects, &c.—The nature of these116 was described in Chapter III., but the method of procuring them was not explained, as this clearly belongs to “dissection.” The larger tubes are readily separated by placing the insect in water, and fixing as firmly as possible, when the body must be opened and the viscera removed. The trache? may then be cleaned by the aid of a camel-hair pencil, and floated upon a glass, where they must first be allowed to dry, and then be mounted in balsam. Mr. Quekett gives the following method of removing the trache? from the larva of an insect:—“Make a small opening in its body, and then place it in strong acetic acid. This will soften or decompose110 all the viscera, and the trache? may then be well washed with the syringe, and removed from the body with the greatest facility, by cutting away the connections of the main tubes with the spiracles by means of fine-pointed scissors. In order to get them upon the slide, it must be put into the fluid, and the trache? floated upon it; after which they may be laid out in their proper position, then dried and mounted in balsam.” If we wish them to bear their natural appearance, they must be mounted in a cell with Goadby’s fluid; but the structure is sometimes well shown in specimens mounted dry. As before mentioned, these trache? terminate on the outside in openings termed spiracles, which are round, oblong, and of various shapes. Over these are generally a quantity of minute hairs, forming a guard against the entrance of dust, &c. The forms of these are seldom alike in two different kinds of insects, so that there is here a wide field for the student. The dissection, moreover, is very easy, as they may be cut from the body with a sharp knife or scissors, and mounted in balsam or fluid. Many of the larv? afford good specimens, as do also some of the common Coleopterous insects.
Tongues, or Palates, of Molluscs.—Of the nature of these, Dr. Carpenter gives the following description:—“The organ which is commonly known under this designation is one of a very singular nature; and we should be altogether wrong in conceiving of it as having any likeness111 to that on117 which our ordinary ideas of such an organ are founded. For, instead of being a projecting body, lying in the cavity of the mouth, it is a tube that passes backwards and downwards112 beneath the mouth, its higher end being closed, whilst in front it opens obliquely upon the floor of the mouth, being, as it were, slit113 up and spread out so as to form a nearly flat surface. On the interior of the tube, as well as on the flat expansion of it, we find numerous transverse rows of minute teeth, which are set upon flattened114 plates; each principal tooth sometimes having a basal plate of its own, whilst in other instances one plate carries several teeth.” These palates, or tongues, differ much amongst the Gasteropods in form and size, some of them being comparatively of an immense length. Many are amongst the most beautiful objects when examined with polarized light. They must, however, be procured by dissection, which is usually performed as follows:—The animal is placed on the cork in the dissecting-trough before mentioned, and the head and forepart cut open, spread out, and firmly pinned down. With the aid of fine scissors or knife, the tongue must be then detached from its fastenings, and placed in water for a day or two, when all foreign matter may with a little care be removed. In what way it should be mounted will depend on the purpose for which it is intended. If for examination as an ordinary object, it may be laid upon the slide and allowed to dry, which arrangement will show the teeth very well. If we wish to see it as it is naturally, it must be mounted in a cell with Goadby’s fluid; but if it is wanted as a polarizing object, it must be floated upon a slide, allowed to dry thoroughly, and then Canada balsam added in the usual manner.
In the stomach, also, of some of these molluscs teeth are found, which are very interesting objects to examine, and must be dissected115 out in the same manner as the “tongues.”
Since writing the above. Dr. Alcock (whose very beautiful specimens prove him to be a great authority in this118 branch) has published some of his experience in the second volume of the third series of “Memoirs of the Literary and Philosophical116 Society of Manchester.” By his permission I make the following extract:—
“This closes my present communication on the tongues of mollusca; but as some members may possibly feel inclined to enter upon the inquiry117 themselves, I think it will not be amiss to add a few remarks on the manner in which they are to be obtained.
“First, as to the kinds best worth the trouble of preparation. Whelks, Limpets, and Trochuses should be taken first. Land and fresh-water snails118 can scarcely be recommended, except as a special study,—their tongues being rather more difficult to find, and the teeth so small that they require a high power to show them properly. It would appear, from Spallanzani’s description of the anatomy120 of the head of the snail119, that even he did not make out this part, although, in his curious observations on the reproduction of lost parts, he must have carefully dissected more snails than any other man.
“As to preserving the animals till wanted, they should simply be dropped alive into glycerine or alcohol. Glycerine is perhaps best where only the tongues are wanted; but it leaves the animals very soft; and as it does not harden their mucus at all, they are very slippery and difficult to work upon when so preserved.
“Then as to the apparatus required for dissection. In the first place, all the work is to be done under water, and a common saucer is generally the most convenient vessel77 to use. No kind of fastening down or pinning out of the animal is needed; and, in fact, it is much better to have it quite free, that you may turn it about any way you wish. The necessary instruments are a needle-point, a pair of fine-pointed scissors, and small forceps; the forceps should have their points slightly turned in towards each other.
“A word or two on the lingual121 apparatus generally,119 and on its special characters in a few different animals, will conclude what I have to say.
“The mode of using the tongue can be easily seen in any of the common water-snails, when they are crawling on the glass sides of an aquarium122; it may then be observed that from between the fleshy lips a thick mass is protruded123, with a motion forwards and upwards124, and afterwards withdrawn125, these movements being almost continually repeated. The action has the appearance of licking; but when the light falls suitably on the protruded structure, it is seen to be armed with a number of bright points, which are the lingual teeth, so arranged as to give the organ the character and action of a rasp.
“If you proceed to dissection, and open the head of one of these mollusca (say, for instance, a common limpet), you will find the cavity of the mouth almost filled with the thick fleshy mass, the front of which is protruded in the act of feeding; and on its upper surface, extending along the middle line from back to front, is seen the strong membranous126 band upon which the teeth are set. The mass itself consists of a cartilaginous frame, surrounded by strong muscles; and these structures constitute the whole of the active part of the lingual apparatus....
“But the peculiarity127 of the toothed membrane, which makes its name of ‘ribbon’ so appropriate, is, that there is always a considerable length of it behind the mouth, perfectly formed, and ready to come forward and supply the place of that at the front, which is continually wearing away by use.
“In the limpet this reserve-ribbon is of great length, being nearly twice as long as the body, and the whole of it is exposed to view on simply removing the foot of the animal; nothing, then, can be easier than to extract the tongue of the common limpet. But, unfortunately, what you find in one kind of mollusc is not at all what you find in another. In the Acm?as, for instance, which are very closely related to the limpets, and have shells120 which cannot be distinguished128, the reserve portion of the ribbon has to be dug out from the substance of the liver, in which it is imbedded, that organ being, as it were, stitched completely through by a long loop of it.... It might be thought a comfortable reflection that, at all events, one end of the ribbon can always be found in the mouth; but in many cases this is about the worst place to look for it. Perhaps it may appear strange that in some of the smaller species, with a retractile trunk, a beginner may very likely fail altogether in his attempt to find the mouth; if, however, the skin of the back is removed, commencing just behind the tentacles129, there will be very little difficulty in making out the trunk, which either contains the whole of the ribbon, as in the whelk, or the front part of it, as in Purpura and Murex, where a free coil is also seen to hang from its hinder extremity130.... In the periwinkles the same plan of proceeding, by at once opening the back of the animal, is best; and on doing so, the long ribbon, coiled up like a watch-spring, cannot fail to be found.
“In the Trochuses, and indeed in all the Scutibranchiata, one point of the scissors should be introduced into the mouth of the animal, and an incision131 made directly backwards in the middle line above to some distance behind the tentacles; the tongue is then immediately brought into view, lying along the floor of the mouth.”
Dr. Alcock’s method of dissection will be found to differ in some degree from the general rules before given; and when the tongue is dissected out he washes it for one hour (shaking it now and then) in a weak solution of potash. After cleaning thoroughly in water, it must be mounted by one of the methods before mentioned.
Amongst insects, especially the grasshopper132 tribe, are found many which possess a gizzard, armed with strong teeth, somewhat similar to those of the molluscs. It requires great nicety of manipulation to obtain these for the microscope; but it would be useless to attempt any121 description of the process here, as the strident can only be successful by experience in dissecting objects less difficult to manage, and by using the most delicate instruments.
We have now considered most of those objects which require any peculiar treatment in section-cutting, &c.; but in no branch of microscopic manipulation is experience more necessary than in this.
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1
dissection
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| n.分析；解剖 | |
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dissect
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| v.分割；解剖 | |
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extraneous
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| adj.体外的；外来的；外部的 | |
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attain
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| vt.达到，获得，完成 | |
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proficiency
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| n.精通，熟练，精练 | |
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accomplished
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| adj.有才艺的；有造诣的；达到了的 | |
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persevering
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| a.坚忍不拔的 | |
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chisel
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| n.凿子；v.用凿子刻，雕，凿 | |
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apparatus
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| n.装置，器械；器具，设备 | |
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perfectly
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| adv.完美地，无可非议地，彻底地 | |
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brittle
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| adj.易碎的；脆弱的；冷淡的；(声音)尖利的 | |
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applied
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| adj.应用的；v.应用，适用 | |
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adherence
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| n.信奉，依附，坚持，固着 | |
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ascertain
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| vt.发现，确定，查明，弄清 | |
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thoroughly
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| adv.完全地，彻底地，十足地 | |
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exquisitely
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| adv.精致地；强烈地；剧烈地；异常地 | |
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transparent
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| adj.明显的，无疑的；透明的 | |
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opaque
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| adj.不透光的；不反光的，不传导的；晦涩的 | |
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dispensed
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| v.分配( dispense的过去式和过去分词 )；施与；配（药） | |
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diluted
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| 无力的，冲淡的 | |
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dilute
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| vt.稀释，冲淡；adj.稀释的，冲淡的 | |
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peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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membrane
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| n.薄膜，膜皮，羊皮纸 | |
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discretion
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| n.谨慎；随意处理 | |
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texture
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| n.(织物)质地；(材料)构造；结构；肌理 | |
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projections
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| 预测( projection的名词复数 )； 投影； 投掷； 突起物 | |
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procuring
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| v.（努力）取得， （设法）获得( procure的现在分词 )；拉皮条 | |
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vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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microscopic
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| adj.微小的，细微的，极小的，显微的 | |
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requisite
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| adj.需要的，必不可少的；n.必需品 | |
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tantalizing
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| adj.逗人的；惹弄人的；撩人的；煽情的v.逗弄，引诱，折磨( tantalize的现在分词 ) | |
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bestowed
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| 赠给，授予( bestow的过去式和过去分词 ) | |
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opacity
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| n.不透明；难懂 | |
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amber
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| n.琥珀；琥珀色；adj.琥珀制的 | |
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specimen
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| n.样本，标本 | |
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specimens
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| n.样品( specimen的名词复数 )；范例；（化验的）抽样；某种类型的人 | |
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iodine
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| n.碘，碘酒 | |
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ornaments
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| n.装饰( ornament的名词复数 )；点缀；装饰品；首饰v.装饰，点缀，美化( ornament的第三人称单数 ) | |
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remains
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| n.剩余物，残留物；遗体，遗迹 | |
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lathe
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| n.车床，陶器，镟床 | |
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lapidary
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| n.宝石匠；adj.宝石的；简洁优雅的 | |
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solitary
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| adj.孤独的，独立的，荒凉的；n.隐士 | |
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lessen
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| vt.减少，减轻；缩小 | |
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procured
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| v.（努力）取得， （设法）获得( procure的过去式和过去分词 )；拉皮条 | |
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procure
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| vt.获得，取得，促成；vi.拉皮条 | |
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lessened
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| 减少的，减弱的 | |
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impurities
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| 不纯( impurity的名词复数 )； 不洁； 淫秽； 杂质 | |
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enamel
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| n.珐琅，搪瓷，瓷釉；(牙齿的)珐琅质 | |
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geologist
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| n.地质学家 | |
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naturalists
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| n.博物学家( naturalist的名词复数 )；（文学艺术的）自然主义者 | |
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peculiarities
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| n. 特质, 特性, 怪癖, 古怪 | |
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proceeding
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| n.行动，进行，(pl.)会议录，学报 | |
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brass
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| n.黄铜；黄铜器，铜管乐器 | |
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sufficiently
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| adv.足够地，充分地 | |
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cleanse
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| vt.使清洁，使纯洁，清洗 | |
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affix
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| n.附件，附录 vt.附贴，盖(章)，签署 | |
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laborious
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| adj.吃力的，努力的，不流畅 | |
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caustic
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| adj.刻薄的，腐蚀性的 | |
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briefly
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| adv.简单地，简短地 | |
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rhinoceros
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| n.犀牛 | |
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buffalo
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| n.(北美)野牛；(亚洲)水牛 | |
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soften
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| v.(使)变柔软；(使)变柔和 | |
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softened
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| （使）变软( soften的过去式和过去分词 )； 缓解打击； 缓和； 安慰 | |
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cork
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| n.软木，软木塞 | |
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cleansing
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| n. 净化(垃圾) adj. 清洁用的 动词cleanse的现在分词 | |
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lather
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| n.(肥皂水的)泡沫，激动 | |
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greasy
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| adj. 多脂的，油脂的 | |
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solvent
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| n.溶剂；adj.有偿付能力的 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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underneath
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| adj.在...下面，在...底下；adv.在下面 | |
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resinous
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| adj.树脂的，树脂质的，树脂制的 | |
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preservative
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| n.防腐剂；防腐料；保护料；预防药 | |
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calcium
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| n.钙(化学符号Ca) | |
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distilled
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| adj.由蒸馏得来的v.蒸馏( distil的过去式和过去分词 )；从…提取精华 | |
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obliquely
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| adv.斜； 倾斜； 间接； 不光明正大 | |
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vessels
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| n.血管( vessel的名词复数 )；船；容器；（具有特殊品质或接受特殊品质的）人 | |
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vessel
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| n.船舶；容器，器皿；管，导管，血管 | |
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acetic
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| adj.酸的 | |
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investigation
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| n.调查，调查研究 | |
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perseverance
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| n.坚持不懈，不屈不挠 | |
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perpendicular
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| adj.垂直的，直立的；n.垂直线，垂直的位置 | |
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axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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friction
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| n.摩擦，摩擦力 | |
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bibulous
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| adj.高度吸收的，酗酒的 | |
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linen
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| n.亚麻布，亚麻线，亚麻制品；adj.亚麻布制的，亚麻的 | |
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soluble
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| adj.可溶的；可以解决的 | |
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soda
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| n.苏打水；汽水 | |
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adept
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| adj.老练的，精通的 | |
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makers
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| n.制造者，制造商（maker的复数形式） | |
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requisites
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| n.必要的事物( requisite的名词复数 ) | |
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dissecting
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| v.解剖（动物等）( dissect的现在分词 )；仔细分析或研究 | |
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inevitably
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| adv.不可避免地；必然发生地 | |
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possessed
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| adj.疯狂的；拥有的，占有的 | |
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bent
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| n.爱好，癖好；adj.弯的；决心的，一心的 | |
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lengthened
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| （时间或空间）延长，伸长( lengthen的过去式和过去分词 ) | |
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scythe
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| n. 长柄的大镰刀，战车镰； v. 以大镰刀割 | |
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backwards
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| adv.往回地，向原处，倒，相反，前后倒置地 | |
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taper
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| n.小蜡烛，尖细，渐弱；adj.尖细的；v.逐渐变小 | |
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marine
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| adj.海的；海生的；航海的；海事的；n.水兵 | |
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affixed
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| adj.[医]附着的，附着的v.附加( affix的过去式和过去分词 )；粘贴；加以；盖（印章） | |
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maceration
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| n.泡软，因绝食而衰弱 | |
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cleansed
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| 弄干净，清洗( cleanse的过去式和过去分词 ) | |
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softening
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| 变软，软化 | |
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intestines
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| n.肠( intestine的名词复数 ) | |
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alteration
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| n.变更，改变；蚀变 | |
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serum
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| n.浆液，血清，乳浆 | |
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delicacy
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| n.精致，细微，微妙，精良；美味，佳肴 | |
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considerably
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| adv.极大地；相当大地；在很大程度上 | |
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spinal
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| adj.针的，尖刺的，尖刺状突起的；adj.脊骨的，脊髓的 | |
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decompose
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| vi.分解；vt.（使）腐败，（使）腐烂 | |
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likeness
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| n.相像，相似（之处） | |
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downwards
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| adj./adv.向下的（地），下行的（地） | |
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slit
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| n.狭长的切口；裂缝；vt.切开，撕裂 | |
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flattened
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| [医]（水）平扁的，弄平的 | |
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dissected
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| adj.切开的，分割的，（叶子）多裂的v.解剖（动物等）( dissect的过去式和过去分词 )；仔细分析或研究 | |
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philosophical
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| adj.哲学家的，哲学上的，达观的 | |
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inquiry
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| n.打听，询问，调查，查问 | |
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snails
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| n.蜗牛；迟钝的人；蜗牛( snail的名词复数 ) | |
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snail
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| n.蜗牛 | |
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anatomy
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| n.解剖学，解剖；功能，结构，组织 | |
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lingual
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| adj.语言的；舌的 | |
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aquarium
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| n.水族馆，养鱼池，玻璃缸 | |
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protruded
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| v.（使某物）伸出，（使某物）突出( protrude的过去式和过去分词 ) | |
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upwards
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| adv.向上，在更高处...以上 | |
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withdrawn
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| vt.收回；使退出；vi.撤退，退出 | |
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membranous
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| adj.膜的，膜状的 | |
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peculiarity
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| n.独特性，特色；特殊的东西；怪癖 | |
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distinguished
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| adj.卓越的，杰出的，著名的 | |
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129
tentacles
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| n.触手( tentacle的名词复数 )；触角；触须；触毛 | |
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extremity
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| n.末端，尽头；尽力；终极；极度 | |
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incision
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| n．切口，切开 | |
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grasshopper
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| n.蚱蜢，蝗虫，蚂蚱 | |
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