GENERAL ZOOLOGY 



■ Lamina vitrea 

 Pigment epithelium 



Int. lim. memb 



Rg. 4.14. The human retina (cf. Fig. 4.3). Lejl, drawing of vertical section; the region at 

 the bottom is adjacent to the chamber of the vitreous humor; x500. Middle, diagram showing 

 relationships of principal types of cells. Light passing through the lens, vitreous humor, and 

 inner layers of the transparent retina reaches the receptor layer made up of the outer segments 

 (cf. Fig. 4.15), or photosensitive regions, of the cones (c) and rods (r). Nerve impulses 

 established in the region called the outer plexiform layer are conducted by way of neurons of 

 dififerent types, the cell bodies of which are shown at a, b, cb, db, g, h, and pg. These 

 impulses finally are conducted by way of the nerve fibers which form the innermost layer of the 

 retina and which extend outward through the retina in the region of the blind spot to make 

 up the optic nerve. Right, diagram of a Miiller fiber (nucleus at A/), which functions to 

 hold the other retinal elements together; opposite ends of the Miiller fibers form the external 

 limiting membrane (ext. lim. memb.) and the internal limiting membrane (mt. lim. memb.), 

 which lies adjacent to the vitreous humor. (From G. L. Walls, The Vertebrate Eye, copyright 

 1942 by Cranbrook Institute of Science, reprinted by permission.) 



the retina. This accommodation involves the contraction of the ciliary muscle 

 which releases the tension on the lens capsule and permits the lens to become 

 more nearly spherical. After 40 years of age, there is a gradual loss of the 

 capacity to accommodate in man; this can be corrected by wearing reading 

 glasses. 



The retina contains the light-sensitive receptors known as rods and cones 

 (Figs. 4.14 and 4.15). Rods are exceedingly sensitive to light and are 

 responsible for vision under conditions of dim illumination. The rods of 

 marine fishes and land vertebrates contain a light-sensitive red pigment, 



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