HISTOLOGICAL ELEMENTS RETINA NECTURUS 407 
the ganglion cells, which are described by Ramén y Cajal (94) 
as sending free branching processes into the inner reticular layer, 
and axis-cylinders to the central organ through the optic stalk. 
My problem is concerned with enumerations of the rods, cones, 
and double-cones, and the nuclei associated with them, of the 
nuclei of the inner nuclear layer, and the ganglion layer, of the 
Miiller’s fibers, and the fibers of the optic nerve. 
All my investigations have been made in the Zoélogical Labor- 
atory of Harvard University under the personal supervision of 
Prof. G. H. Parker, to whom I am greatly indebted for advice 
and valuable criticism. 
Il. HISTORICAL REVIEW 
A. Retina 
Although the literature on the retina and optic nerve in ver- 
tebrates is extensive, it is surprising that so little has been done 
on the numerical relations of the retinal elements. For the most 
part, those who have investigated the retina in this respect have 
limited their statements to the fovea and to comparisons between 
the number of visual and ganglion cells in central and peripheral 
regions. An epitome of the morphology and physiology of the 
retina has been published by Greeff (00), but it is not satisfactory 
for numerical relations of the elements because of the lack of suffi- 
cient data. 
Much of what has been written concerning the number of 
retinal elements in mammals relates to the human retina. One 
of the earliest to consider the question was Krause (’76), who 
estimated the number of rods in the human retina to be 130,000,- 
000 and of cones to be 7,000,000. Salzer (80) also worked on 
the human retina and estimated the number of cones to lie be- 
tween 3,000,000 and 3,600,000, but gave no estimate of the num- 
ber of rods or of other retinal elements. Foster (’91) and many 
other physiologists have apparently accepted the estimates of 
Salzer. In regard to the distribution of the rods and cones Foster 
writes ‘“‘Over the retina (including the ora) the rods are much 
more numerous than the cones, there being two or three rods 
