RETINAL STRUCTURE AND VISUAL ACUITY 107 



It cannot be sheer accident that diurnal birds with their 

 high visual acuity should have such a strikingly broad 

 internal nuclear layer, and that in the nocturnal mammals 

 it should be so narrow as compared with the external nuclear 

 layer. Conduction must be influenced also by the number 

 of ganglion cells in relation to the other two nuclear layers. 

 In the nocturnal mammals with very broad external nuclear 

 layers, much narrower internal nuclear layers, and still 

 thinner ganglion cell layers, impulses from a great many 

 photoreceptors must be concentrated into a few final paths. 

 It reminds one of a series of reducing valves. In the diurnal 

 lacertilians and birds, relatively fewer receptors must make 

 connections with many bipolar and association cells in this 

 internal nuclear layer, which in turn also must discharge 

 into a reduced number of ganglion cells. In all retinas, 

 whether diurnal or nocturnal, whether the photoreceptors 

 are very numerous per unit area or less so, the number of 

 ganglion cells is never as great as the visual cell nuclei or 

 the nuclei of the internal nuclear layer. Visual acuity, there- 

 fore, from a structural standpoint, appears to be influenced 

 not only by the fineness and distance apart of the receiving 

 elements, but it is apparent that the great differences exist- 

 ing in the numbers of the conducting and association elements 

 may also play an important role. 



