406 £ SAMUEL C. PALMER 
of Golgi’s bichromate method and of Ehrlich’s methylene-blue 
method for staining nervous tissues, this difficulty has been, 
at least partially, overcome, and as a result great advance has 
been made in our knowledge of the visual apparatus. We owe 
much, also, to the use of osmic acid, which has been so success- 
fully employed, not only in the study of the finer structure of 
nerve cells and their processes, but also in the enumeration of 
nervous elements. In the optic nerve this has not always been 
satisfactorily carried out, because the fibers of this nerve are 
very small and difficult to stain. The usual treatment has been 
to blacken the medullary sheaths with osmie acid, which, however, 
leaves the axis-cylinders unstained. 
Many theories have been elaborated to explain the structure 
and functions of the retinal elements, but where the numerical 
relations of these units have been concerned investigators have, 
for the most part, confined their enumerations to regions of 
special interest, leaving untouched the broader aspect of the 
entire retina and optic nerve. It has seemed to me, therefore, 
that a complete enumeration of the histological elements of the 
retina and of the optic nerve, which have to do directly with the 
transmission of visual impulses, would be a desirable addition 
to our knowledge of the eye; and I have, therefore, undertaken 
to complete such an investigation on one of the lower vertebrates. 
There is great similarity in the structure of the retina in all 
classes of vertebrates. From the standpoint of the neurone 
theory, the visual apparatus in these animals consists of three 
distinct layers of neurones. These neurones bear so constant 
a relation to one another that a definite terminology has become 
associated with them. The first neurones to receive the visual 
impulses are the visual cells, which include the rod-and-cone 
layer, a majority of the nuclei in the outer nuclear layer, and 
processes terminating in the outer reticular layer. The second 
set of neurones is represented by the nuclei of the inner nuclear 
layer exclusive of Miiller’s fibers and are of two distinct forms, 
viz., amacrine cells and bipolar cells. Processes from the latter 
form synapses with similar processes arising from the visual and 
from the ganglion cells. The third set of neurones consists of 
