Organ of Sight. 147 



tlian the transverse*: but the relative size of the eyeball 

 seems to increase directly with the increasing power of loco- 

 motion of the different species, even in those of the same 

 Order. The eyes are generally small in those species that 

 burrow. 



In no Mammal is bone developed in the Sclerotic ; but this 

 investing membrane is itself occasionally ^fibro-cartilaginous', 

 and may be present in great thickness especially round the 

 entrance of the Optic Nerve. The Cornea, which generally 

 presents a more or less convex surface externally, is flat in 

 the Aquatic Mammalia. In all Mammalia, Monodelphous, 

 and Didelphous, as far as at present observed, the Lem and 

 Ciliary muscle do not differ in any essential point from those 

 of Man. In all other Mammalia the Lens is more spherical 

 than in Man, especially in Aquatic species : in most, the 

 central planes are three, as in the human foetus : in a fewf 

 there is but one. The Capsule, its epithelium, and the lens- 

 fibres, are essentially like those of Man. The Ciliary pro- 

 cesses are simple ; the suspensory ligament and its connexions, 

 the arrangement of the Ciliary muscle (which derives its 

 nerves from the lenticular ganglion), and the kind of muscular 

 tissue (imstriped), are such as we find in the Human eye:|:. 

 The Pupil is almost invariably circular, but is sometimes 

 capable of being contracted to form a vertical slit : occasion- 

 ally also the pupil is found transversely oblong, with the 

 upper border somewhat festooned by the presence of a Liga- 

 mentum pectinatum. 



Tuft-shaped flakes of pigment, called the Racemiform, or 

 sponge-like bodies, are occasionally found projecting from the 

 pigment layer (Uvea) at the posterior surface of the Iris, and 

 tending to interrupt the regular margin of the Pupil. 



* Except in Man and Apes, 

 t Cete and some Rodentia. 



