Book II Chapter 13

Men, and Birds, and Quadrupeds, viviparous and oviparous alike, have their eyes protected by lids. In the Vivipara there are two of these; and both are used by these animals not only in closing the eyes, but also in the act of blinking; whereas the oviparous quadrupeds, and the heavy-bodied birds as well as some others, use only the lower lid to close the eye; while birds blink by means of a membrane¹ that issues from the canthus. The reason for the eyes being thus protected is that nature has made them of fluid consistency², in order to ensure keenness of vision. For had they been covered with hard skin, they would, it is true, have been less liable to get injured by anything falling into them from without, but they would not have been sharp-sighted. It is then to ensure keenness of vision that the skin over the pupil is fine and delicate; while the lids are superadded as a protection from injury. It is as a still further safeguard that all these animals blink, and man most of all; this action (which is not performed from deliberate intention but from a natural instinct) serving to keep objects from falling into the eyes; and being more frequent in man than in the rest of these animals, because of the greater delicacy³ of his skin. These lids are made of a roll of skin; and it is because they are made of skin and contain no flesh that neither they, nor the similarly constructed prepuce, unite again when once cut.

As to the oviparous quadrupeds, and such birds as resemble them in closing the eye with the lower lid, it is the hardness of the skin of their heads which makes them do so. For such birds as have heavy bodies are not made for flight; and so the materials which would otherwise have gone to increase the growth of the feathers are diverted thence, and used to augment⁴ the thickness of the skin. Birds therefore of this kind close the eye with the lower lid; whereas pigeons and the like use both upper and lower lids for the purpose. As birds are covered with feathers, so oviparous quadrupeds are covered with scaly⁵ plates; and these in all their forms are harder than hairs, so that the skin also to which they belong is harder than the skin of hairy animals. In these animals, then, the skin on the head is hard, and so does not allow of the formation of an upper eyelid⁶, whereas lower down the integument⁷ is of a flesh-like character, so that the lower lid can be thin and extensible.

The act of blinking is performed by the heavy-bodied birds by means of the membrane already mentioned, and not by this lower lid. For in blinking rapid motion is required, and such is the motion of this membrane, whereas that of the lower lid is slow. It is from the canthus that is nearest to the nostrils⁸ that the membrane comes. For it is better to have one starting-point for nictitation than two; and in these birds this starting-point is the junction⁹ of eye and nostrils, an anterior¹⁰ starting-point being preferable to a lateral¹¹ one. Oviparous quadrupeds do not blink in like manner as the birds; for, living as they do on the ground, they are free from the necessity of having eyes of fluid consistency and of keen sight, whereas these are essential requisites¹² for birds, inasmuch as they have to use their eyes at long distances. This too explains why birds with talons¹³, that have to search for prey¹⁴ by eye from aloft, and therefore soar to greater heights than other birds, are sharpsighted; while common fowls¹⁵ and the like, that live on the ground and are not made for flight, have no such keenness of vision. For there is nothing in their mode of life which imperatively¹⁶ requires it.

Fishes and Insects and the hard-skinned Crustacea present certain differences in their eyes, but so far resemble each other as that none of them have eyelids¹⁷. As for the hard-skinned Crustacea it is utterly¹⁸ out of the question that they should have any; for an eyelid, to be of use, requires the action of the skin to be rapid. These animals then have no eyelids and, in default of this protection, their eyes are hard, just as though the lid were attached to the surface of the eye, and the animal saw through it. Inasmuch, however, as such hardness must necessarily blunt the sharpness of vision, nature has endowed the eyes of Insects, and still more those of Crustacea, with mobility¹⁹ (just as she has given some quadrupeds movable ears), in order that they may be able to turn to the light and catch its rays, and so see more plainly. Fishes, however, have eyes of a fluid consistency. For animals that move much about have to use their vision at considerable distances. If now they live on land, the air in which they move is transparent²⁰ enough. But the water in which fishes live is a hindrance²¹ to sharp sight, though it has this advantage over the air, that it does not contain so many objects to knock against the eyes. The risk of collision being thus small, nature, who makes nothing in vain, has given no eyelids to fishes, while to counterbalance the opacity²² of the water she has made their eyes of fluid consistency.

1 membrane   

n.薄膜，膜皮，羊皮纸

参考例句：

• A vibrating membrane in the ear helps to convey sounds to the brain.耳膜的振动帮助声音传送到大脑。
• A plastic membrane serves as selective diffusion barrier.一层塑料薄膜起着选择性渗透屏障的作用。

2 consistency   

n.一贯性，前后一致，稳定性；(液体的)浓度

参考例句：

• Your behaviour lacks consistency.你的行为缺乏一贯性。
• We appreciate the consistency and stability in China and in Chinese politics.我们赞赏中国及其政策的连续性和稳定性。

3 delicacy   

n.精致，细微，微妙，精良；美味，佳肴

参考例句：

• We admired the delicacy of the craftsmanship.我们佩服工艺师精巧的手艺。
• He sensed the delicacy of the situation.他感觉到了形势的微妙。

4 augment   

vt.(使)增大，增加，增长，扩张

参考例句：

• They hit upon another idea to augment their income.他们又想出一个增加收入的办法。
• The government's first concern was to augment the army and auxiliary forces.政府首先关心的是增强军队和辅助的力量。

5 scaly   

adj.鱼鳞状的；干燥粗糙的

参考例句：

• Reptiles possess a scaly,dry skin.爬行类具有覆盖着鳞片的干燥皮肤。
• The iron pipe is scaly with rust.铁管子因为生锈一片片剥落了。

6 eyelid   

n.眼睑，眼皮

参考例句：

• She lifted one eyelid to see what he was doing.她抬起一只眼皮看看他在做什么。
• My eyelid has been tumid since yesterday.从昨天起，我的眼皮就肿了。

7 integument   

n.皮肤

参考例句：

• The first protector against the entry of microorganisms is the integument.抗御微生物进入体内的第一道防线是皮肤。
• The cells of the integument and nucellus of some plants form perfectly normal embryos.某些植物的珠被和珠心细胞形成完全正常的胚。

8 nostrils   

鼻孔( nostril的名词复数 )

参考例句：

• Her nostrils flared with anger. 她气得两个鼻孔都鼓了起来。
• The horse dilated its nostrils. 马张大鼻孔。

9 junction   

n.连接，接合；交叉点，接合处，枢纽站

参考例句：

• There's a bridge at the junction of the two rivers.两河的汇合处有座桥。
• You must give way when you come to this junction.你到了这个路口必须让路。

10 anterior   

adj.较早的；在前的

参考例句：

• We've already finished the work anterior to the schedule.我们已经提前完成了工作。
• The anterior part of a fish contains the head and gills.鱼的前部包括头和鳃。

11 lateral   

adj.侧面的，旁边的

参考例句：

• An airfoil that controls lateral motion.能够控制横向飞行的机翼。
• Mr.Dawson walked into the court from a lateral door.道森先生从一个侧面的门走进法庭。

12 requisites   

n.必要的事物( requisite的名词复数 )

参考例句：

• It is obvious that there are two requisites. 显然有两个必要部分。 来自辞典例句
• Capacity of donor is one of the essential requisites of \"gift\". 赠与人的行为能力是\"赠与\"的一个重要前提。 来自口语例句

13 talons   

n.（尤指猛禽的）爪( talon的名词复数 )；（如爪般的）手指；爪状物；锁簧尖状突出部

参考例句：

• The fingers were curved like talons, but they closed on empty air. 他的指头弯得像鹰爪一样，可是抓了个空。 来自英汉文学 - 热爱生命
• The tiger has a pair of talons. 老虎有一对利爪。 来自辞典例句

14 prey   

n.被掠食者，牺牲者，掠食；v.捕食，掠夺，折磨

参考例句：

• Stronger animals prey on weaker ones.弱肉强食。
• The lion was hunting for its prey.狮子在寻找猎物。

15 fowls   

鸟( fowl的名词复数 )； 禽肉； 既不是这； 非驴非马

参考例句：

• A great number of water fowls dwell on the island. 许多水鸟在岛上栖息。
• We keep a few fowls and some goats. 我们养了几只鸡和一些山羊。

16 imperatively   

adv.命令式地

参考例句：

• Drying wet rice rapidly and soaking or rewetting dry rice kernels imperatively results in severe fissuring. 潮湿米粒快速干燥或干燥籽粒浸水、回潮均会产生严重的裂纹。 来自互联网
• Drying wet rice kernels rapidly, Soaking or Rewetting dry rice Kernels imperatively results in severe fissuring. 潮湿米粒的快速干燥，干燥籽粒的浸水或回潮均会带来严重的裂纹。 来自互联网

17 eyelids   

n.眼睑( eyelid的名词复数 )；眼睛也不眨一下；不露声色；面不改色

参考例句：

• She was so tired, her eyelids were beginning to droop. 她太疲倦了，眼睑开始往下垂。
• Her eyelids drooped as if she were on the verge of sleep. 她眼睑低垂好像快要睡着的样子。 来自《简明英汉词典》

18 utterly   

adv.完全地，绝对地

参考例句：

• Utterly devoted to the people,he gave his life in saving his patients.他忠于人民，把毕生精力用于挽救患者的生命。
• I was utterly ravished by the way she smiled.她的微笑使我完全陶醉了。

19 mobility   

n.可动性，变动性，情感不定

参考例句：

• The difference in regional house prices acts as an obstacle to mobility of labour.不同地区房价的差异阻碍了劳动力的流动。
• Mobility is very important in guerrilla warfare.机动性在游击战中至关重要。

20 transparent   

adj.明显的，无疑的；透明的

参考例句：

• The water is so transparent that we can see the fishes swimming.水清澈透明，可以看到鱼儿游来游去。
• The window glass is transparent.窗玻璃是透明的。

21 hindrance   

n.妨碍，障碍

参考例句：

• Now they can construct tunnel systems without hindrance.现在他们可以顺利地建造隧道系统了。
• The heavy baggage was a great hindrance to me.那件行李成了我的大累赘。

22 opacity   

n.不透明；难懂

参考例句：

• He insisted that the mineral content of the water determined the opacity.他坚持认为水的清澈程度取决于其中矿物质的含量。
• Opacity of the eye lens can be induced by deficiency of certain vitamins.眼球晶状体的混浊可由缺乏某些维生素造成。