60 EVERS'S COMPARATIVE ANATOMY. 



is rapidly and forcibly drawn downwards and outwards over the 

 front of the eye, and is restored to its former situation by its own 

 elasticity. The lachrymal gland is situated at the external angle 

 of the eye, and at the internal angle there is another gland named 

 Harderian, from its discoverer, but is nothing more than a con- 

 geries of mucous follicles to compensate for the absence of meibo- 

 mian glands. There is a third gland in or near the orbit, called 

 nasal, its secretion being wholly destined for the nose. 



In the mammalia the organs of vision are Constructed in perfect 

 accordance with the media the animals move in ; they are usually 

 placed laterally, but in the nocturnal quadrupeds they are directed 

 forwards as much as in man, and in the quadrumana even more 

 so. The eyes are of a large size in ruminantia, rodentia and many 

 pachydermata; and they are small in moles, shrews, whales, and 

 in most cheiroptera. In the mus typhlus the eye is covered by 

 hairy conjunctiva ; and in the mole of Libanus, it is so small as to 

 be almost invisible. The form of the eye is generally spherical, 

 but in the cetacea flattened anteriorly as in fish, and in bats it 

 approaches to that of birds. In the cetacea the aqueous and vitre- 

 ous humours are less abundant than in terrestrial quadrupeds, the 

 cornea is flat, the lens is large, dense, and round, and the sclerotic is 

 an inch thick posteriorly in a whale, with an eye the size of an 

 orange; the lids are imperfectly developed, the lachrymal apparatus 

 is absent, and the superior oblique muscle of the eye is destitute of 

 a pully ; but the smallness of the eye in this order of mammalia, 

 compared wi th that of fish, bespeaks a higher development of internal 

 perceptive organs. 



In the carnivora the cornea is prominent, the pupil vertically 

 oval, the thick choroid without pigmentum nigum posteriorly, and 

 a blue or green tapetum glitters in the bottom of the eye. The 

 cornea covers half the eye in the rat and porcupine, and is elongated 

 transversely in the marmot, and generally in ruminants. The 

 tapetum also exists in the ruminantia, solipeda, pachydermata, and 

 cetacea. In the dog, wolf, and badger it is white, bordered by blue. 

 The iris is subject to numerous varieties as regards its colour, 

 structure, and the shape of the pupil: the latter is of a circular 

 form in the rodentia, bats, and simise ; transversely oval in the ru- 

 minantia, solipeda and cetacea ; in these instances the oval form 

 does not seem to pervade the entire thickness of the iris, but only 

 its external layer. The retina in carnivora, and many rodentia, as 

 in some birds, is confined to the posterior half of the eye: in the 

 former on account of the breadth of the corpus ciliare ; in the 

 latter on account of the width of the iris. 



In the mammalia the eyelids are formed generally as they are in 

 man, the superior b«ing the larger and more movable, and the 

 membrana nictitans exist in the entire class, with the exception of 

 man, apes, and the cetacea. The ornithorhynchus histrix has but 

 a single circular lid. In addition to the lachrymal gland, which is 



