660 



BIRDS 



In the inner plexiform layer a variable number of faint bands can 

 always be seen, running parallel to the retinal surfaces. These mark the 

 distinct levels at which the various types 

 of amacrines expand their terminal ar- 

 borizations. At these same levels, for the 

 obvious purpose of binding together the 

 synapses at each level, there are vari- 

 cosities on the filaments of the Miiller 

 fibers. These cells are very different from 

 those of other retinae, for in their course 

 through the inner half of the retina they 

 depart from the usual sponge-like struc- 

 ture, and each cell breaks up into a great 



Fig. 193 — The avian retina and its visual cells. 



a, portion of fundal retina of week-old chick. x500. 

 p- pigment epithelium; v- visual cells; o- outer nu- 

 clear layer; b- bipolar elements of inner nuclear 

 layer; m- row of Miiller-fiber nuclei; a- amacrine 

 elements of inner nuclear layer; /'- inner plexiform 

 layer (note stratification); g- ganglion-cell layer; 

 n- nerve-fiber layer. 



b, peripheral single cone and double cone, periph- 

 eral and fundal rods, of Passer Jomesticus. x 1000. 

 p- paraboloid. 



