396 



Comparative Animal Physiology 



fibers of the retina pass across the inner surface of the retina and enter the 

 optic nerve. The region of exit of the nerve contains no rods and cones, is 

 thereby insensitive to hght, and is known as the "bhnd spot." 



In one region close to the optical axis of the eye the rods are fewer in 

 number and the cones are much more numerous. This area is called the area 

 centralis. In birds, in a few mammals, and in reptiles and fishes the area 

 centralis is thinner than the surrounding retina, because of a decreased thick- 

 ness of the layer of secondary and tertiary neurones, and is referred to as 

 the fovea centralis. In the fovea of certain birds (e.g., hawks, swallows, 

 terns) these neuronal layers are not merely thinner, but their thickness va- 



«(»M<ii«^«ij/»^«i:',v^/'«^*^M»>.' ' 



i lamina vitrea 

 ^pigmeni epithelium 



receptor layer 



'^' (<"j<^~^ ■'■rr^ - -.-ex-trlimrmemk 

 . %*.^'v,"'{' outer nuclear 







outer plexi- 

 form layer 



% inner nuclear 

 layer 



inner plexi- 

 form layer 



ganglion layer ^ 



nerve fibers 



Fig. 1 10. Histology of the human retina. Left, vertical section through the retina in 



the nasal fundus; right, neuronal connections in the retina as revealed by silver impreg- 

 nation methods (after Polyak). a, Amacrine cell; h, bipolar cells; c, cones; ch, centrifugal 

 bipolar, believed to conduct outward through the retina; dh, diffuse bipolar; g, ganglion 

 cells; h, horizontal cells; m, Miiller fibers, forming limiting membranes; pg, parasol gan- 

 glion cell; r, rods. From Walls.^"' 



ries in such a way as to spread the image over a larger surface, thereby per- 

 mitting a higher degree of visual acuity. This spreading lens effect is caused 

 by the fact that the refractive index of the retinal tissue is greater than that of 

 the vitreous body. The fovea of man is a broad area with very little if any 

 spreading power. 



The two types of sense cells, rods and cones, are adapted to function most 

 effectively over different intensity ranges. The rods are much more sensi- 

 tive to light than the cones and thereby are adapted to function at very low 

 intensities of light, ranging from moderately bright moonlight downward to 



