552 STUDIES IN ADVANCED PHYSIOLOGY. 



matic representation of the relative arrangement of these 

 layers. It will be seen that the retina consists of a number 

 of neurons, the cell body of the first neuron being in the 

 third layer. From these large cells nerve fibers extend in 

 one direction along the optic nerve to the brain, but toward 

 the fourth layer are given off short dendrons which meet 

 with similar dendrons from the second neurons, the cells of 

 which lie in the inner nuclear layer. These cells send their 

 dendrons into the sixth layer to meet similar dendrons be- 

 longing to neurons whose cells lie in the outer nuclear 

 layer. 



These cells extend a nerve fiber into the rod and cone 

 layer. In fact, the rods are merely special terminations of 

 these nerves. The cones seem to be partly of a cellular na- 

 ture themselves, which connect with the lower neurons in 

 the outer molecular layer and end in the rod and cone layer 

 in the characteristic points found on these cones. 



To resume, then, the retina consists of three systems of 

 neurons super-imposed one above the other, the individual 

 neurons being exceedingly small, but, on the other hand, 

 exceedingly numerous. This decrease in size and cor- 

 responding increase in numbers is necessitated by the deli- 

 cacy which the retina must possess to be affected by light 

 and so make possible the perception of two points of light 

 very close to each other. The touch areas for light in the 

 retina must be very much smaller indeed than the touch 

 areas for the portions of the skin since we are able to see 

 two points of light which are separated by a distance meas- 

 urable only with difficulty. 



A somewhat surprising arrangement of the retina at first 

 sight consists in the relative position of the first and ninth 

 layer. Naturally one would expect to see the rods and 

 cones turned towards the light. In reality they are in the 

 human eye, as in the eye of all vertebrates, turned away 

 from the light and the light has to permeate the eight 

 super-imposed layers before affecting the sensitive rods and 

 cones. This inversion of the retina is easily understood 



