VISION. 



Pio. 395.— Diagrammatic section of the eye of the horse (Chanvean). a, optic nerve: 

 ft, sclerotic coat; c, choroid; d, retina; e, cornea; /, ins; g, A, ciliary ligament and 



firocesses of choroid represented as separated from it, the better to denne its lim- 

 ts; i, insertion of ciliary processes on crystalline lens; J, crystalline lens; k, lens 

 capsule; I, vitreous body; «i, m, anterior and posterior chambers; o, theoretical 

 indication of agneous hamor; p,p, tarsi (eyelids); g, g, fibrous membrane of eye- 

 lids; r, elevator muscle of upper eyelid; b, a, orbicularis muscle of lids; t^ t, skin 

 of eyelids; u, conjunctiva; t), epidermic layer of the latter covering cornea; x. 

 posterior rectus muscle; y, superior rectus; u, inferior rectus; w, fibrous sheath or 

 orbit (orbital membrane). 



drawn inward and thus protected the more effectually against 

 blows and obstacles. It seems to be of special importance in 

 animals that feed with the head 

 down for long periods, as in the 

 ruminants, in which class it is 

 most highly developed. 



The macula lutea is believed 

 to exist only in man, the quadru- 

 mana, and certain of the lizard 

 tribe — ^i. e., in animals in which 

 the axes of the eyeballs are parallel 

 to each other. Nevertheless, there 

 is no reason to doubt that the cen- 

 tral part of the retina is more sensitive than the periphery or 

 that there is a central retinal zone for distinct vision in all 

 vertebrates, though not so limited in all cases as in man. 



Fia. 896.— Diagram to illustrate de- 

 cussation of fibers in the ojptic 

 conunissnre of man (after Flint). 



