THE EYE AND OPTIC TRACT OF INSECTS. 
219 
stained specimens it is easy to distinguish the cell-protoplasm 
and the anastomosing branches (PI. XV, fig. 6). 
I have spoken of these cells as nerve-cells because it is 
necessary to distinguish them from ganglion-cells, which are 
more isolated in position, and possess a considerable quantity 
of cell-protoplasm. I shall refer to this point again in 
Section 3. 
The opticon itself consists of a very fine granular matrix, 
traversed throughout by a fine meshwork of minute fibrillse, 
similar to the minute network of primitive fibrillse described 
by Gerlach in the mammalian brain and spinal cord. This 
description of the minute structure of the opticon applies 
equally to the epi-opticon and principal ganglia of the body. 
As this tissue is very commonly met with in the animal 
kingdom, and has not, as far as I am aware, yet received any 
separate name, I propose to call it a neurospongium. In 
many insects the neurospongium of the opticon is traversed by 
fibrils, and in some cases it contains a few scattered nerve- or 
even ganglion-cells. 
The epi-opticon is connected with the peri-opticon by a 
bundle of decussating nerve-fibrils, and as these fibrils approach 
the peri-opticon they are connected with a number of scattered 
nerve-cells (fig. 2, n. c.), and at the decussation two or three 
larger nerve-cells may be seen in each section (fig. 2, ys.) 
The peri-opticon of Musca is composed of a number of 
cylindrical masses of neurospongium arranged side by side, which 
I shall refer to as the elements of the peri-opticon (fig. 2, p. op.), 
and between them a single nerve-cell is very frequently seen. 
As this ganglion has been recently subjected to considerable 
investigation I have taken considerable pains to study it 
thoroughly. In addition to numerous sections of hardened 
specimens, made both transversely and horizontally, I have 
succeeded in isolating the peri-opticons of fresh flies, and 
teasing them carefully with needles, both before and after 
staining in gold and other reagents, and the following account 
is based on these investigations. 
The nerve-fibrils coming from the epi-opticon divide into two 
