ORGANS OF VISION. 25 



such as the Mole, have very delicate optic nerves. The fifth pair 

 is frequently very much developed ; the infra-orbital branch in par- 

 ticular is often of remarkable size, particularly in those animals that 

 are provided with a snout, or with large vibrissse upon the upper* lip, 

 to the follicles of which, it gives considerable branches. 



ORGANS OF THE SENSES. 



Organs of Vision. 



IT is only in the Apes that the eye is situated, as in Man, in a 

 complete bony cavity closed both from without and within ; in all 

 the rest of the Mammalia, and also in the Makis, the orbital and 

 zygomatic spaces blend upon the sides of the skull. Both spaces 

 are nevertheless separated by a peculiar membrane, which is called 

 the orbital membrane, and was formerly regarded incorrectly as 

 muscular ; it arises from the periosteum, lines the orbits funnel-wise, 

 and consists partly of elastic tissue. It seems to act as an antag- 

 onist to the retractor muscle of the globe of the eye, protruding or 

 pushing the eyeball forward so soon as the action of that muscle 

 ceases. 



In other respects the Mammalia in general exhibit the greatest 

 conformity in the structure of the eye with that of Man. Yet there 

 are some species among the Insectivora and Rodentia, for example 

 the Talpa caeca, Spalax typhlus, Chrysocloris, living under ground, 

 in which the eye is very rudimentary, and nearly closed by the skin, 

 so that they possess an extremely imperfect sense of vision. The 

 globe of the eye is here very small, but appears to contain all 

 its principal parts. In the Mammalia the transverse diameter of 

 the eyeball is generally the largest, as in the Whale, the Walrus, 

 and the Seal ; the sclerotic is of enormous thickness in the Whales, 

 and in the Ornithorynchus it is enclosed by a bony plate. In the 

 Apes and in Man the longitudinal axis of the eye exceeds the trans- 

 verse in diameter. The cornea is flat in the aquatic Mammalia. 

 Between the sclerotic and chqroid lies as usual the pigmentary layer. 

 In addition to this there occurs in many Mammalia a membrane 

 composed of an interlacement of delicate fibres, and having a metallic 

 brilliancy, called the tapetum. In the Ruminantia, Solidungula 

 and Pachydermata, it has a variegated lustre of blended green and 



