874 



COM PAR ATI VE A NATO M Y 



set 



pet 





Fig. 490. 



A and B, The eye of Columba livia. 



A, in sagittal section ; B, entire organ, external aspect, en., cornea ; ch., choroid ; cl.pr., 

 ciliary processes ; ir., iris ; 1., lens ; opt.nv., optic nerve ; pet., pecten ; rt., retina ; scl.ps., sclerotic 

 plates. (From Parker and Haswell after Vogt and Yung.) 



C. Schematic representation of the sensory apparatus in the retina of the human eye. 



1. layer of pigment cells next to the choroid ; 2, processes of the pigment cells ; 3, rods ; 

 4, bodies of rod-cells ; 5, cones ; 6, axones of cone cells ; 7, cone-bipolar cells ; 8, 9, ganglion cells ; 

 10, optic nerve fibers (axones of ganglion cells) ; 11, 12, horizontal cells ; 13, 14, 15, 16 y cells of 

 different type; functions unknown; 17, fibres (probably axones) of cells having bodies in the 

 brain; 18, neuroglia cells; 19, radial fiber (Miiller's fiber: part of the sustentacular syncytial 

 framework of modified neuroglia). (From Dunlap after Merkle-Henle). 



choroid contains a great many blood vessels and covers the greater por- 

 tion of the eyeball. It meets the front of the eyeball with the circular 

 ciliary process, in which there are various ciliary muscles by which the 

 lens is moved toward or away from the retina so as to alter its shape. 

 This changes its focal point. The change of focal point is known as 

 accommodation of the eye. The center of the ciliary process is the iris, 

 a sort of circular curtain, with a central opening. The opening is known 

 as the pupil. There are circular muscles called sphincter pupillae and 

 dilator pupillae, which contract and enlarge the pupil respectively. 



