Integrative Systems 



203 



FlG. 176. Diagrammatic section of half a mammalian eye. (ac) \nterior cham- 

 ber; (c) eyelash (cilium); (ca) ciliary arteries; (c/) conjunctiva; (co) cornea; (cp) 

 ciliary process; (cs) conjunctival sac; (c/) choroid tunic; (c») central retinal artery 

 and vein; (d) dura of optic nerve; (i) iris; (on) optic nerve; (os) ora serrata; (p) 

 posterior chamber; (pe) pigmented epithelium; (r) retina; (sc) sclera; (tg) tarsal 

 gland; (vv) vorticose vein; (zz) zonula zinii. (Courtesy, Kingsley, "Comparative 

 Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



lens to the ciliary body, forming a suspensory ligament (or zonula) 

 which holds the lens in place back of the iris. 



The cavities within the eyeball (Fig. 175) are filled by liquid or 

 semifluid substances, the "humors" of the eye. Between cornea and 

 lens is the aqueous humor. The large central space behind the lens 

 is occupied by the vitreous humor, a transparent substance of jelly- 

 like consistency. 



The retina lines the cavity of the eyeball, but thins out and is 

 non-nervous in the zone adjacent to the ciliary body, that being a region 

 where little or no light strikes it. This non-nervous zone continues 

 outward, covering the ciliary body and the entire inner surface of the 

 iris even to the edge of the pupil (Fig. 176). 



In its functional region, the retina contains three layers of neurons 

 (Figs. 177, 178), those of one layer being joined to those in adjacent 

 layers, presumably by synaptic connection. The neurons of the most 

 peripheral layer (i.e., the layer most remote from the center of the 

 eyeball) are the receptors. They are much-elongated cells whose 

 peripheral regions are cylindric in some and conic in others. The re- 

 ceptors are accordingly known as "rods" and "cones." Beyond the 

 rod-shaped or cone-shaped region, the nucleus occasions an expansion 

 of the body of the cell. Beyond the nucleus, a slender process continues 

 toward the middle one of the three retinal layers. In a rod, this process 

 ends in a knoblike enlargement which is in close (synaptic) relation to 



