It is true that the majority of muscular movements are determined8 by a reflex from centres; and that any break228 in the triple process of the ingoing nerve, centre, and outgoing nerve, prevents such movements. It is true that the more conspicuous9 and harmoniously10 co-ordinated phenomena11 belong to this class. But it is also demonstrable that many nerve-actions may, and some do, take place by direct stimulation12 of the nerve, or direct stimulation of the muscle, without the intervention of a centre, without even the intervention of a ganglion. This must obviously be the case in animals which have no centres; and even in some which have well-developed nervous centres, there is every reason to believe that these centres often act rather in the way of co-ordinating than of directly stimulating13 actions.
90. I was first led to doubt the reigning14 doctrine15 by a surprising observation (frequently repeated) after I had removed the whole nervous centres from a garden snail16 (Helix pomatia). The muscular mass called “the foot” was thrown into slow but energetic contraction17 whenever the skin was pricked18 with the point of a scalpel, or touched with acid; nay19, even when a glass rod dipped in the acid was brought close to, without absolutely touching20, the skin, the foot curled up, and then slowly relaxed. The same effect was produced on the “mantle”—where there was of course no centre. But direct irritation21 of the muscles under the skin produced no such contraction; only through the skin could the stimulation take effect. In one case I observed this strange phenomenon five hours after removal of the centres. It was a great puzzle. At first I concluded that there must be minute ganglia in the skin, serving as reflex-centres. I searched for them in vain; and although a longer search on better methods might possibly have detected ganglionic cells, I soon relinquished22 the search, because I had other grounds for believing that even the presence of abundant ganglia would not suffice, until some better proof were afforded that such ganglia were reflex-centres.
229 91. That direct stimulation of the nerve suffices to move the muscles, is familiar to all experimenters. There is no centre, or ganglion, in the amputated leg of the frog, which nevertheless contracts whenever the sciatic nerve is stimulated23. And after the nerve has been exhausted24, and refuses to respond to any stimulus25, the muscle itself may be directly stimulated. Inasmuch as the movement depends on the contractility of the muscles, a stimulation through centre, through motor-nerve, or through muscle, will be followed by contraction. Let us take a clear case of reflex action. The pupil of the eye contracts when a beam of light falls on it, and dilates27 when the beam is shut off. The path of the neural28 process is normally this: the light stimulates29 the optic nerve, which in turn stimulates the corpora quadrigemina; (here the nerves which move the eye are experimentally proved to be stimulated;) and it is through these that the pupil is caused to contract. If the optic nerve be divided, no such reflex takes place—proving that the contraction does not, at least normally, come from the ciliary ganglion.
But now it is matter of observation that the pupil will contract and dilate26 under the stimuli30 of light and darkness, when there is no such reflex pathway open. Removal of the eye from the body obliterates31 this path, cuts the eye off from all connection with the centre. Brown Séquard removed both eyes from a frog, placed one in a dark box, and left the other exposed to the light: the pupil of the former was found dilated32, that of the latter contracted. On reversing the experiment, and placing the eye with contracted pupil in the dark box, he found it there dilate, while the dilated pupil exposed to the light contracted.117 In frogs with very irritable33 tissues, I have230 found not only the pupil contracting, after the whole cranial cavity has been emptied, but even the eyelid34 close, on irritating the conjunctiva118—yet this is one of the typical reflex actions! I am disposed to think that even the action of swallowing may be faintly excited by stimulation of the pharynx of a brainless frog; but I have not observations sufficiently35 precise to enable me to speak confidently. Goltz has, however, shown that after removal of brain and spinal36 cord and heart, there is spontaneous and active movement in ?sophagus and stomach.119 This will no doubt be referred to the agency of the ganglionic plexus; but similar movements have been observed by Engelmann in the ureter, and in isolated231 fragments of the ureter in which not a ganglionic cell was present.120
92. That nerves are stimulated by internal changes has long been recognized with reference to “subjective sensations.” The divided nerve, in that portion which remains38 connected with the centre, will at times cause great pain. Obscure organic conditions, changes of temperature, states of the blood, excite the nerves, and the patient feels as if the surface of the amputated limb were irritated. It is all very well to call these “subjective sensations”; that does not alter the fact of the nerve being called into activity by other than the normal stimuli from the surface; in like manner muscular movements (which are not to be explained as “subjective movements”) will be excited by organic stimuli when motor-nerves are separated from their centres. In each case it has sufficed that the nerve should be excited; and when excited, no matter by what means, the effect is always similar.
93. Here are a few facts. Stimulation of the nerves which send filaments39 to the chromatophores of the skin in reptiles40 causes the skin to become paler, and even colorless: the color-specks disappear under this contractile stimulus. This being known, Goltz deprived a frog of brain, spinal cord, and heart, thus eliminating all possible influence from them, slit41 up the skin of the back, and displayed the nerves which pass from each side of the spine42 to the skin; these nerves he then divided on the right side, and observed the skin on this side slowly become paler and paler, till finally it was as yellow as wax; the left side, having its nerves intact, retained its color. Two conclusions seemed to him warranted by this experiment: First, that even in the dead frog the nerves separated from their centre were still active; secondly,232 that the irritation of the nerves resulting from their section was the cause of the color-specks disappearing. This second conclusion was strengthened when he found that the irritation was increased when he cut the nerves bit by bit.
It is not at present, I believe, clearly made out that the color-specks of the Cephalopoda are in direct connection with nerves; but it is tolerably certain that they are in some way under the influence of nervous stimulation, directly or indirectly43. D’Orbigny, indeed, goes so far as to say they are dependent on the will of the animal.121 This seems very lax language; but restricting ourselves to the fact of nervous influence, the experiments of Goltz receive further illustration in an observation I have elsewhere recorded.122 I found that a strip of skin taken from the dead body of a calamary (Loligo) showed the color-specks expanding and contracting with vigor44.
94. The heart is well known to beat after death, if death be not the result of a gradual decay. Sometimes, indeed, its muscular irritability45 is so active that the heart will beat for hours. E. Rousseau observed it beating in a woman twenty-seven hours after she had been guillotined.123 Not only will it beat after death, but in many animals even after removal from the body: the heart of a young puppy, or kitten, will beat for three or four hours after its removal; that of a full-grown dog, or cat, not one hour; whereas the beating of that of a tortoise, or a frog, will, under proper precautions, be preserved for days—and even after it has stopped, it may be stimulated to fresh pulsations.
Physiologists47 explain this spontaneous movement of the heart as due to the ganglia in its substance. This233 explanation, which is founded on what I cannot but regard as a purely48 imaginary view of the functions of ganglionic cells, must stand or fall with that hypothesis. A long and arduous49 investigation50 has led me to doubt whether in any case the heart’s movements are primarily due to its ganglia; at all events, the same spontaneous movements are observed in the hearts of molluscs and crustaceans51, which are without even a trace of ganglia; and in the hearts of mammalian embryos52 long before ganglia or nerve-fibres make their appearance. Not less certain is it that movements of contraction and dilatation are produced in the blood-vessels53 independently of all central influence. This has been decisively proved by the Italian physiologist46, Mosso, when experimenting on an organ isolated37 from the organism; and although the vessels have their nerve cells and fibres, he justly doubts whether it is to these that the stimulation is due, because the phenomena are observed after the nervous vitality54 has disappeared. Goltz severed55 all the tissues in the leg of a rabbit, so that the only connection of the leg with the rest of the body was through the crural vein56 and artery57, which kept up the circulation; yet although the nerves of the skin were thus separated from their centre, so that no sensation could be produced by stimulating the skin of the leg, consequently no reflex from the centre on the vessels, Goltz found that a marked reddening of the skin from congestion58 of the capillaries59 followed the application of mustard to the skin. Physiologists who believe that the constriction60 and dilatation of blood-vessels are due to the action of the ganglionic cells distributed over the walls of the vessels will explain Goltz’s observation as a case of reflex action; but those who agree with me that such an hypothesis respecting the part played by the cells is untenable, will class the observation among other cases of direct stimulation.
234 95. But passing from these perhaps questionable61 cases, let us glance at other cases. The mobile iris62 of the bird displays movements after the nerves have been divided. Even the voluntary striped muscles are not altogether motionless. Schiff divided the hypoglossus on one side, and found, of course, the tongue paralyzed on that side; but he also found that on the third day after the operation some of the muscles of that side were quivering: the agitation63 spread to others, till by the end of the fourth day all the fibres were rhythmically65 contracting. From this time onwards, the contractions66 were incessant67; though they were never able to move the tongue, because the fibres did not contract simultaneously68.
Schiff also observed that the hairs over the eyes and the “whiskers” of cats, rabbits, and guinea pigs were for months after section of their nerves in incessant rhythmical64 vibration69. This was observed when the animals were asleep as when awake. Valentin records the spontaneous movements in the diaphragm of animals just killed; and this even after section of the phrenic nerve. The same movements may be seen in the operculum of fishes. Henle observed the spontaneous contractions of the intercostal muscles; which Schiff confirms, adding that the movements observed by him in cats and birds were not simply contractions of some fibres, but of all the muscles, so that three or four excised70 ribs71 rhythmically contracted and expanded.
I have performed a great many experiments with a view of determining this question, but the phenomena were so variable that I refrain from adducing any,124 and235 merely state the general result as one in harmony with the foregoing examples. The great variability of the phenomena depends upon the variable conditions of muscular irritability and anatomical relations. When the heart of one woman is found beating twenty-seven hours after death, while in most men and women it ceases after a few minutes, we must be prepared to find different, and even contradictory72 phenomena under varying unknown conditions. There is, however, a general agreement among experimenters that muscular irritability increases after separation from nerve-centres, and then quickly decreases again.
96. Although the stimulation of muscles usually comes through a nerve-centre, yet since the muscles do not derive their Contractility from nerve-centres any stimulation will suffice. Now since we have abundant proof that sensory73 nerves are stimulated by certain organic changes, by poisons in the blood, excess of carbonic acid, etc., we are justified74 in concluding that motor nerves will be stimulated in like manner, and thus muscular movement be produced occasionally without the intervention of a centre. Pressure on a motor nerve, or the irritation which results from inflammation, will determine contraction, or secretion75 directly. Recently, Erb and Westphal have disclosed the fact that the leg will be suddenly jerked out if the patella be gently tapped; and they prove this not to be a reflex action, because it follows with the same certainty after the skin has been made insensible.125
There are doubtless many other phenomena which, though commonly assigned to reflex stimulation, are236 really due to direct stimulation. Research might profitably be turned towards the elucidation76 of this point. Since there is demonstrable evidence that a nerve when no longer in connection with its centre, or with ganglionic cells, may be excited by electricity, pressure, thermal77 and chemical stimuli, we must conclude that even when it is in connection with its centre, any local irritation from pressure, changes in the circulation, etc., will also excite it. But as such local excitations will have only local and isolated effects, they will rarely be conspicuous.
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1
postulates
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| v.假定，假设( postulate的第三人称单数 ) | |
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postulate
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| n.假定，基本条件；vt.要求，假定 | |
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physiology
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| n.生理学，生理机能 | |
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intervention
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| n.介入，干涉，干预 | |
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derive
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| v.取得；导出；引申；来自；源自；出自 | |
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explicitly
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| ad.明确地，显然地 | |
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formulated
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| v.构想出( formulate的过去式和过去分词 )；规划；确切地阐述；用公式表示 | |
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determined
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| adj.坚定的；有决心的 | |
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conspicuous
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| adj.明眼的，惹人注目的；炫耀的，摆阔气的 | |
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harmoniously
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| 和谐地，调和地 | |
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phenomena
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| n.现象 | |
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stimulation
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| n.刺激，激励，鼓舞 | |
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stimulating
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| adj.有启发性的，能激发人思考的 | |
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reigning
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| adj.统治的，起支配作用的 | |
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doctrine
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| n.教义；主义；学说 | |
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snail
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| n.蜗牛 | |
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contraction
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| n.缩略词，缩写式，害病 | |
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pricked
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| 刺，扎，戳( prick的过去式和过去分词 )； 刺伤； 刺痛； 使剧痛 | |
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nay
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| adv.不；n.反对票，投反对票者 | |
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touching
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| adj.动人的，使人感伤的 | |
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irritation
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| n.激怒，恼怒，生气 | |
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relinquished
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| 交出，让给( relinquish的过去式和过去分词 )； 放弃 | |
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stimulated
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| a.刺激的 | |
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exhausted
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| adj.极其疲惫的，精疲力尽的 | |
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stimulus
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| n.刺激，刺激物，促进因素，引起兴奋的事物 | |
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dilate
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| vt.使膨胀，使扩大 | |
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dilates
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| v.（使某物）扩大，膨胀，张大( dilate的第三人称单数 ) | |
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neural
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| adj.神经的，神经系统的 | |
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stimulates
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| v.刺激( stimulate的第三人称单数 )；激励；使兴奋；起兴奋作用，起刺激作用，起促进作用 | |
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stimuli
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| n.刺激（物） | |
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obliterates
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| v.除去( obliterate的第三人称单数 )；涂去；擦掉；彻底破坏或毁灭 | |
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dilated
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| adj.加宽的，扩大的v.（使某物）扩大，膨胀，张大( dilate的过去式和过去分词 ) | |
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irritable
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| adj.急躁的；过敏的；易怒的 | |
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eyelid
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| n.眼睑，眼皮 | |
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sufficiently
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| adv.足够地，充分地 | |
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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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isolated
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| adj.与世隔绝的 | |
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remains
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| n.剩余物，残留物；遗体，遗迹 | |
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filaments
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| n.（电灯泡的）灯丝( filament的名词复数 )；丝极；细丝；丝状物 | |
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reptiles
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| n.爬行动物，爬虫( reptile的名词复数 ) | |
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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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spine
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| n.脊柱，脊椎；(动植物的)刺；书脊 | |
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indirectly
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| adv.间接地，不直接了当地 | |
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vigor
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| n.活力，精力，元气 | |
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irritability
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| n.易怒 | |
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physiologist
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| n.生理学家 | |
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physiologists
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| n.生理学者( physiologist的名词复数 )；生理学( physiology的名词复数 )；生理机能 | |
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purely
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| adv.纯粹地，完全地 | |
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arduous
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| adj.艰苦的，费力的，陡峭的 | |
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investigation
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| n.调查，调查研究 | |
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51
crustaceans
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| n.甲壳纲动物（如蟹、龙虾）( crustacean的名词复数 ) | |
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embryos
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| n.晶胚；胚，胚胎( embryo的名词复数 ) | |
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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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vitality
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| n.活力，生命力，效力 | |
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severed
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| v.切断，断绝( sever的过去式和过去分词 )；断，裂 | |
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vein
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| n.血管，静脉；叶脉，纹理；情绪；vt.使成脉络 | |
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artery
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| n.干线，要道；动脉 | |
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congestion
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| n.阻塞，消化不良 | |
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capillaries
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| 毛细管，毛细血管( capillary的名词复数 ) | |
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constriction
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| 压缩； 紧压的感觉； 束紧； 压缩物 | |
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questionable
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| adj.可疑的，有问题的 | |
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iris
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| n.虹膜，彩虹 | |
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agitation
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| n.搅动；搅拌；鼓动，煽动 | |
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rhythmical
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| adj.有节奏的，有韵律的 | |
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rhythmically
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| adv.有节奏地 | |
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contractions
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| n.收缩( contraction的名词复数 )；缩减；缩略词；（分娩时）子宫收缩 | |
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incessant
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| adj.不停的，连续的 | |
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simultaneously
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| adv.同时发生地，同时进行地 | |
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vibration
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| n.颤动，振动；摆动 | |
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excised
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| v.切除，删去( excise的过去式和过去分词 ) | |
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ribs
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| n.肋骨( rib的名词复数 )；（船或屋顶等的）肋拱；肋骨状的东西；（织物的）凸条花纹 | |
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contradictory
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| adj.反驳的，反对的，抗辩的；n.正反对，矛盾对立 | |
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sensory
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| adj.知觉的，感觉的，知觉器官的 | |
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justified
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| a.正当的，有理的 | |
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secretion
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| n.分泌 | |
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elucidation
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| n.说明，阐明 | |
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thermal
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| adj.热的，由热造成的；保暖的 | |
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