Chapter 9

The fact that all animals have an even number of feet, and the reasons for the fact have been set forth¹. What follows will explain that if there were no point at rest flexion and straightening would be impossible. Flexion is a change from a right line to an arc or an angle, straightening a change from either of these to a right line. Now in all such changes the flexion or the straightening must be relative to one point. Moreover, without flexion there could not be walking or swimming or flying. For since limbed creatures stand and take their weight alternately on one or other of the opposite legs, if one be thrust forward the other of necessity must be bent². For the opposite limbs are naturally of equal length, and the one which is under the weight must be a kind of perpendicular³ at right angles to the ground.

When then one leg is advanced it becomes the hypotenuse of a right-angled triangle. Its square then is equal to the square on the other side together with the square on the base. As the legs then are equal, the one at rest must bend either at the knee or, if there were any kneeless animal which walked, at some other articulation⁴. The following experiment exhibits the fact. If a man were to walk parallel to a wall in sunshine, the line described (by the shadow of his head> would be not straight but zigzag⁵, becoming lower as he bends, and higher when he stands and lifts himself up.

It is, indeed, possible to move oneself even if the leg be not bent, in the way in which children crawl. This was the old though erroneous account of the movement of elephants. But these kinds of movements involve a flexion in the shoulders or in the hips⁶. Nothing at any rate could walk upright continuously and securely without flexions at the knee, but would have to move like men in the wrestling schools who crawl forward through the sand on their knees. For the upper part of the upright creature is long so that its leg has to be correspondingly long; in consequence there must be flexion. For since a stationary⁷ position is perpendicular, if that which moves cannot bend it will either fall forward as the right angle becomes acute or will not be able to progress. For if one leg is at right angles to the ground and the other is advanced, the latter will be at once equal and greater. For it will be equal to the stationary leg and also equivalent to the hypotenuse of a right-angled triangle. That which goes forward therefore must bend, and while bending one, extend the other leg simultaneously⁸, so as to incline forward and make a stride and still remain above the perpendicular; for the legs form an isosceles triangle, and the head sinks lower when it is perpendicularly⁹ above the base on which it stands.

Of limbless animals, some progress by undulations (and this happens in two ways, either they undulate on the ground, like snakes, or up and down, like caterpillars), and undulation is a flexion; others by a telescopic action, like what are called earthworms and leeches¹⁰. These go forward, first one part leading and then drawing the whole of the rest of the body up to this, and so they change from place to place. It is plain too that if the two curves were not greater than the one line which subtends them undulating animals could not move themselves; when the flexure is extended they would not have moved forward at all if the flexure or arc were equal to the chord subtended; as it is, it reaches further when it is straightened out, and then this part stays still and it draws up what is left behind.

In all the changes described that which moves now extends itself in a straight line to progress, and now is hooped¹¹; it straightens itself in its leading part, and is hooped in what follows behind. Even jumping animals all make a flexion in the part of the body which is underneath¹², and after this fashion make their leaps. So too flying and swimming things progress, the one straightening and bending their wings to fly, the other their fins¹³ to swim. Of the latter some have four fins, others which are rather long, for example eels¹⁴, have only two. These swim by substituting a flexion of the rest of their body for the (missing) pair of fins to complete the movement, as we have said before. Flat fish use two fins, and the flat of their body as a substitute for the absent pair of fins. Quite flat fish, like the Ray, produce their swimming movement with the actual fins and with the two extremes or semicircles of their body, bending and straightening themselves alternately.

1 forth   

adv.向前；向外，往外

参考例句：

• The wind moved the trees gently back and forth.风吹得树轻轻地来回摇晃。
• He gave forth a series of works in rapid succession.他很快连续发表了一系列的作品。

2 bent   

n.爱好，癖好；adj.弯的；决心的，一心的

参考例句：

• He was fully bent upon the project.他一心扑在这项计划上。
• We bent over backward to help them.我们尽了最大努力帮助他们。

3 perpendicular   

adj.垂直的，直立的；n.垂直线，垂直的位置

参考例句：

• The two lines of bones are set perpendicular to one another.这两排骨头相互垂直。
• The wall is out of the perpendicular.这墙有些倾斜。

4 articulation   

n.（清楚的）发音；清晰度，咬合

参考例句：

• His articulation is poor.他发音不清楚。
• She spoke with a lazy articulation.她说话慢吞吞的。

5 zigzag   

n.曲折，之字形；adj.曲折的，锯齿形的；adv.曲折地，成锯齿形地；vt.使曲折；vi.曲折前行

参考例句：

• The lightning made a zigzag in the sky.闪电在天空划出一道Z字形。
• The path runs zigzag up the hill.小径向山顶蜿蜒盘旋。

6 hips   

abbr.high impact polystyrene 高冲击强度聚苯乙烯，耐冲性聚苯乙烯n.臀部( hip的名词复数 )；[建筑学]屋脊；臀围（尺寸）；臀部…的

参考例句：

• She stood with her hands on her hips. 她双手叉腰站着。
• They wiggled their hips to the sound of pop music. 他们随着流行音乐的声音摇晃着臀部。 来自《简明英汉词典》

7 stationary   

adj.固定的，静止不动的

参考例句：

• A stationary object is easy to be aimed at.一个静止不动的物体是容易瞄准的。
• Wait until the bus is stationary before you get off.你要等公共汽车停稳了再下车。

8 simultaneously   

adv.同时发生地，同时进行地

参考例句：

• The radar beam can track a number of targets almost simultaneously.雷达波几乎可以同时追着多个目标。
• The Windows allow a computer user to execute multiple programs simultaneously.Windows允许计算机用户同时运行多个程序。

9 perpendicularly   

adv. 垂直地, 笔直地, 纵向地

参考例句：

• Fray's forehead was wrinkled both perpendicularly and crosswise. 弗雷的前额上纹路纵横。
• Automatic resquaring feature insures nozzle is perpendicularly to the part being cut. 自动垂直功能，可以确保刀头回到与工件完全垂直的位置去切割。

10 leeches   

n.水蛭( leech的名词复数 )；蚂蟥；榨取他人脂膏者；医生

参考例句：

• The usurers are leeches;they have drained us dry. 高利贷者是吸血鬼，他们吸干了我们的血汗。 来自《简明英汉词典》
• Does it run in the genes to live as leeches? 你们家是不是遗传的，都以欺压别人为生？ 来自电影对白

11 hooped   

adj.以环作装饰的；带横纹的；带有环的

参考例句：

• Will the joint area with dense hooped reinforcement enhance the bearing capacity of the frame column? 节点区箍筋加密是否有利于框架柱承载能力的提高？ 来自互联网

12 underneath   

adj.在...下面，在...底下；adv.在下面

参考例句：

• Working underneath the car is always a messy job.在汽车底下工作是件脏活。
• She wore a coat with a dress underneath.她穿着一件大衣，里面套着一条连衣裙。

13 fins   

[医]散热片；鱼鳍；飞边；鸭掌

参考例句：

• The level of TNF-α positively correlated with BMI,FPG,HbA1C,TG,FINS and IRI,but not with SBP and DBP. TNF-α水平与BMI、FPG、HbA1C、TG、FINS和IRI呈显著正相关，与SBP、DBP无相关。 来自互联网
• Fins are a feature specific to fish. 鱼鳍是鱼类特有的特征。 来自辞典例句

14 eels   

abbr. 电子发射器定位系统（=electronic emitter location system）

参考例句：

• Eels have been on the feed in the Lower Thames. 鳗鱼在泰晤士河下游寻食。
• She bought some eels for dinner. 她买回一些鳗鱼做晚餐。