848 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



must be lost by absorption by the pigment, and, as a consequence, 

 the clearness of the image must be sacrificed. The field of vision in 

 such an e3 T e will, of course, depend upon the segment of the sphere 

 represented by the termination of the cones, since, although the eye is 

 convex, no movement in an orbit is possible. So, again, accommodation 

 is not required in such an eye, since all the rays of light appreciated by 

 each cone must be coincident with the axis of each cone. Therefore, 

 such a compound eye may be regarded simply as a combination of an 

 immense number of ocelli compressed together and taking an angular 

 form, in insects six-sided and in crustaceans four-sided. The color of 

 the pigment in such eyes varies, being white, yellow, red, green, purple, 



FIG. 367. DIAGRAM OP A HORIZONTAL SECTION THROUGH THE HUMAN 



EYE. (Yeo.) 



1, cornea; 2, sclerotic; 3, choroid ; 4, ciliary processes ; 5, suspensory ligament of lens; 6, so-called 

 posterior chamber between the iris and the lens : 7, iris ; 8, optic nerve ; 8', entrance of central artery of 

 retina; 8", central depression of retina, or yellow spot; 9, anterior limit of retina; 10, hyaline mem- 

 brane; 11, aqueous chamber; 12, crystalline lens; 13, vitreous humor; 14, circular venous sinus which 

 lies around the cornea ; a a, antero-posterior, and b b, transverse axes of bulb. 



or black. Each cornea, or the termination of each ocellus or tube, is 

 convex on one side and convex or flat on the other, so that, to a certain 

 extent, it takes the part of a lens. 



Among the invertebrates the eyes of the cuttle-fish are the largest 

 and most perfect, resembling the eyes of higher animals in possessing a 

 crystalline lens and a chamber behind filled with vitreous humor. 



In the vertebrates the eye is formed by a folding in of the external 

 integument to form a lens and an outgrowth from the optic vesicles of 

 the brain to form a sentient surface. The eyeball in vertebrates consists 

 of an external white, spherical case, or sclerotic coat, which serves to 



