SIGHT IN MAMMALIA. 473 



branch of the fifth cranial nerve ; the optic nerve is said to be absent, although 

 the optic tracts and commissures exist. Amongst the Pachydermata, the 

 eyes are small. In the Cetacea, the thickness of the coats of the eyeball, 

 especially at its hinder part, is enormous, in order to preserve its shape, under 

 the pressure of different and extreme depths of water. 



The eyeballs are of great relative size in the Ruminants and the Solipeds ; 

 they are generally far apart, and very prominent. In the camel and giraffe, 

 their position is so lateral and . projecting, that those quadrupeds can look 

 backwards, without turning their heads. The usual direction of the antero- 

 posterior axes of the eyeballs in Mammalia, is obliquely forwards and out- 

 wards ; but in the Quadrumana, the eyes are directed forwards, as in man ; 

 whilst in the Kodentia, the direction of the axes of the two eyes, is completely 

 lateral. 



In such cases, the relation between the parts of the retina, must be peculiar. 

 The images of objects, seen in front of the animal, must fall, not as in man, 

 on the outer half of one retina, and on the inner half of the other, but on the 

 outer halves of both retinae, and on identical points of the two retinae, occupy- 

 ing inverse positions as regards the axes of the eyes. Objects placed directly 

 to either side, must be seen independently in the corresponding eye, and the 

 points of the retinae on which they are received, can have no correspondence 

 or identity with each other ; otherwise, two similar objects, seen, one on the 

 right, and the other on the left hand, would be combined, and appear as one 

 object. It is not yet known whether the decussation of the optic nerves, pre- 

 sents peculiarities, in accordance with the special seats of the identical points 

 of the retinae in these animals, and with the absence of identity in other parts 

 of the retinae. 



In certain Mammalia, the orbital fkscia or membrane, which completes the 

 orbit, contains plain muscular fibres, and when the sympathetic nerve is irri- 

 tated, it contracts, and presses the eyeball forwards. In the Mammalia gen- 

 erally, the third eyelid, or membrana nictitans, is well developed ; in the ele- 

 phant, it is provided with two special muscles, and has a very strong fibro- 

 cartilage embedded in it. In the Kuminants, the third eyelid is provided with 

 a large gland, named the Harderian gland, the mucous secretion of which 

 facilitates the motion of that membrane over the eyeball. In the Sirenia, 

 amongst Cetacea, the eyelids are represented only by a border of loose skin, 

 the margins of which are provided with small mucous glands ; a membrana 

 nictitans is present ; in the Cetacea proper, this is the only structure which 

 supplies the place of the eyelids. No lachrymal gland exists in the Cetacea. 

 This gland, however, is very large and lobulated in all the Ruminantia. In 

 the deer, and in some antelopes, the so-called tear-pits, formed by recesses in 

 the lachrymal bones, between the orbit and nose, are met with ; they are 

 lined by an extension of the integuments, and open externally by a gutter-like 

 aperture. 



The muscles of the eyeball in almost all Mammalia, resemble those of man ; 

 but, except in the Quadrumana, there generally exists a seventh ocular mus- 

 cle called the retractor oculi, or choanoid muscle, interposed between the recti 

 muscles and the optic nerve. In seals, the crystalline lens is more nearly 

 spherical than in other Mammalia, in accordance with the requirements for 

 the production of distinct images under water, as will be mentioned in speak- 

 ing of the eyes of fishes. In the castor-beaver, the cornea is likewise flattened, 

 and the lens nearly globular, to suit its aquatic habits. The vitreous humor 

 in the Cetacea, is much flattened from before backwards ; the cornea is also 

 flat, but the crystalline lens is, as in seals, nearly spherical. In the Ornitho- 

 rhynchus, a cartilaginous plate, projecting from the orbit, protects the eye 

 above ; the sclerotic coat is also cartilaginous, the cornea is flat, and the lens 

 small. 



In many Mammalia, the bottom of the eyeball is partially lined by a mem- 

 brane, called the tapetum, which presents different brilliant hues ; it consists 

 of a layer of thin fibres, or, as in Carnivora, of nucleated cells, of metallic 

 brilliancy ; this reflects the rays of light from the bottom of the eye, like a 

 concave mirror, and causes a luminous appearance in the eyes of those ani- 

 mals in the dark. In Kuminants, Solipeds, and Pachyderms, the tapetum 



