305 



As one approaches the fovea the rods and cones have less diameter, and are 

 more numerous per given area. This necessitates an increase in the number of 

 cells which form the connection with the nerve fibres (ganglion or nerve cells and 

 cells of the inner and outer nuclear layers). But this is not the only cause for 

 an increase in number of these cells. Raymon y Cajal has carefully worked out 

 the manner in which these cells form the connection between the rods and the 

 cones and the nerve fibres. In general, processes pass outward (detidrites) from 

 the ganglion cells and branch profusely among the ingoing processes (neurites) 

 from the cells of the inner nuclear layer. A similar relation obtains in the outer 

 molecular layer between the dendrites and the neurites of the cells of the inner 

 and outer nuclear layers. Each ganglion cell, and consequently each nerve fibre, 

 comes in contact with from ten to thirty rods or cones. But in the region of the 

 area the dendrites and neurites of these cells branch less and finally reach that 

 condition in the center of the fovea where each ganglion cell is in contact with 

 but a single cone. 



In the peripheral part of the retina the rods generally exceed the cones in 

 number, but as one approaches the area or fovea the cones become more numer- 

 ous, and finally in the center of the fovea the rods are entirely wanting. 



The fovea varies greatly in form, number and position in different animals. 

 It varies from a very questionable depression found in the domestic guinea hen 

 to a very sharp and deep funnel-like pit found in most birds, especially birds of 

 prey, and in many lizards. The depression may be very broad, as seen in man 

 and some fishes; or, according to Chievitz, we may have a trough-like fovea of 

 various depths, extending horizontally across the retina. In my researches I 

 have not been able to find such a fovea. It is true that in many birds I have 

 seen what appears, to the unaided eye, to be a trough-like depression, but when 

 sections were made across such a region and examined microscopically such a 

 fovea was not discerned. I have been able to examine but one of the species 

 which he has mentioned as having this peculiar fovea, so have included them in 

 my tabulation as he has described them. 



As a rule but one fovea is present, but twelve birds have been examined in 

 which two distinct fovese have been found. Chievitz has described some as having 

 also a trough-like fovea. A double fovea has been discovered only in birds. 

 Among those which I have found to possess double fovea^ are three species of 

 hawks, the white-bellied swallow, the common tern and the kingfisher. 



The position of the fovea may be either on the nasal side of the entrance of 

 the optic nerve (fovea nasalis), as in most birds, or it may be situated on the 

 temporal side (fovea temporalis) as in man and the owls. The fovea nasalis 



