ANIMAL PHYSIOLOGY. 



M 



ANI.MAI, I'llYSluLoCY. 11. 



TIII: 

 i HO classes of animals wlii< 



of \\lli.-ll . .. i.i.IH- of picci-i joint. '1 t'l on.' 



! in a I'.n;' f the body 1 



tho otlirr, t! 



cases additional p:irt ., ii.ro found, w)ii!<! 

 in others tho i-\.- i-. .-o redo ;radod an to be of littlo 



VERTICAL SECTION OF THE E7E OF A SOARING BIRD. 



1, Sclerotic ; 2, Choroid ; 3, Betiim ; b, Pecten ; 4, Vitreous humour ; 

 5, Bony support of sclerotic or hard coat ; 6, Iris ; 7. Cornea ; 

 8, Lens; 9, Aqueous humour; 10, Lens ligament; 11, Ciliary pro- 

 oesses; 12, Optic nerve. 



r no use ; but in tho majority of cases in brutes, reptiles, and 

 fishes, and in all birds, the eye is well developed, and even where 

 it can be of no use, still indications of it are found. 



Our English molo is an instance of an animal with a degraded 

 condition of eye. It is in this animal smaller than a pin's head, 

 and has to be looked for carefully in the midst of the velvet fur. 

 Of course, to an animal which lives underground, burrowing 

 continually in soft earth, an eye would be useless, and even in- 

 convenient ; yet the rudiment of an eye is found. 



Besides man, only apes (and some lizards, such as the chame- 1 

 leon, and perhaps some fish) have tho yellow spot of distinct! 

 vision. Vision in some apes must be very powerful, for it is) 

 said a gentleman who owned a baboon used to ride away across 

 the plain until he could only just sue his dog-ape with tho naked 

 eye ; then using his telescope, he made a number of gestures, 

 which were immediately mimicked with precision by tho animal. 



Jn looking into tho open eye tho white is part of the opaque 

 sclerotic. The coloured part is tho iris seen through the trans- 

 parent cornea and vitreous humour, while tho pupil is the hole 

 through the middle of this, which seems black because of tho 

 dark non-reflecting choroid at the back of the eye. 



The iris gives the colour to the eye. When there is Only a 

 layer of pigment on the back part of this, the eye is blue ; bat 

 when, in addition, specks or sheets of pigment are distributed 

 through the substance of tho iris, eyes of various colours are 

 produced. Thus, fair people have usually blue eyes, and black 

 eyes accompany an olive complexion and dark hair. In other 

 words, people that have a surplus of internal paint elsewhere 

 have it in tho iris too. 



Again, tho lack of pigment is sometimes so groat that oven 

 tho choroid has none, and then tho pupil looks red because the 

 blood-vessels of the choroid can be seen through its front layer. 

 Albinos, as individuals with tho last peculiarity are called, are 

 found among rubluts, mice, cats, and many other species, and 

 are especially prone to occur under domestication. These 

 '. an appearance which is very ethereal and 

 fairy-like, so that artists havo often introduced them into their 

 fanciful pictures, as in Landse<Vs " Bottom and Titania." But 

 however they iruy grace the ideal creation of the painter, they 

 are less suited to this working-day world than their coarser 

 brothers. 



On the other hand, in some species a further dcoosit takes 



VOL. i. 



place in the choroid of pigment of metallic brilliancy. This 

 .veil aeen at the bottom of the eye of the ox inside ; in 

 other*, the sclerotic w coloured, M any visitor at the Zoological 

 . -i may MO to be the ca*e in the eye of the chimpanzee. 



return, with many others, snob ae the contraction 



M of the cat, HO an to leave a -lit instead of a circular 



opening, are interesting, bat by no mean* no functionally iin- 



<> be mentioned hereafter, when we describe 



oyea BUI itions altogether different, snob, for instance, 



as the fi*h'u eye, which is constructed to see in water. 



some of which are almost exclusively denizens of the 

 uir, uml mot of which have the power of betaking tbenselree 

 to flight occasionally to escape pursuit, to hunt octire prey, to 

 search for new feeding-ground*, or to select a more genial 

 climate at tho change of the neasons, must have eyes suited to 

 distant vision. Hfiicc the lens is of a very flattened form, and 

 docs not increase in density from the outside to the inside as it 

 mammalia, and more strikingly in fish. The distance 

 from tho lens to the back part of the eye u small, and to the 

 cornea large relatively; in other vordu, they have a larger 

 amount of aqueous and a smaller amount of vitreous humour 

 than brutes have. The back part of the eye too is natter, and 

 is a portion of a larger sphere in relation to the rest of the eye 

 than in animals. The shape will be boot seen by the aid of the 

 diagram of the vortical section of the eye of a soaring bird. 



When the eye is spherical and distended with fluid, M in 

 man, there is no tendency of the procure within to alter the 

 shape of the ball ; but when, as in :" bird*, it has any 



other form, tho internal pressure would r. train the elastic capsule 

 of the eye in some parts more than in others. This strain can 

 only be prevented by rendering those parts of the capsule which 

 are exposed to tho extra pressure more solid. In the case of 

 the bird, this is effected by means of a series of bony platen 

 which encircle tho sclerotic, bedded in its substance, and 

 stretching from tho rim of tho cornea to tho circumference of 

 the large segment of the eye, on the inside of which the- retina 

 is spread out. 



The structures described above, conducive to long sight in a 

 thin medium, are more especially to bo remarked in soaring, 

 raptorial birds, like the eagles, vultures, and hawks. These, as 

 they wheel round at a great height, survey a large extent, ct 



VERTICAL SECTION OF THK BTE OF A FISH. 



1, Solerotic ; 2, Choroid , 2", Inuer layer of Choroid ; 3. Retin* ; , 

 Choroid gland ; 4, Vitreous humour ; 5, Bouy support of sclerotic 

 or hard coat ; G, Iris ; 7. Cornea ; 8, Lens ; 9, Aqueous unmoor; 

 10, Lens ligamant ; 11, Ciliary processes ; 12, Optic nrre. 



country ; yet their sight is so keen at that elevation that no 

 young unprotected animal, or maimed and disabled prey, escapes 

 their sight. So keen is the sight of the condor of .the Andes, 

 that if a carcase be exposed where the naked eye can detect 

 none of those creatures in tho horizon, yet in a few minute* 

 they ore seen streaming in from all directions straight towards 

 their hoped-for meal. 



But though birds be long-sighted, it is also highly necessary 

 that they should see minute objects at a short dw tan- 

 entomologist will deny that on insectivorous bird must hare 

 keen eyes for short distances, if it is to get its living with 

 ease. A microscopic sight is scarcely less requisite for a graiar 



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