THE SIMPLE AND COMPOUND EYES OF INSECTS. 
585 
retina is very difficult to determine, but I have no doubt, from the examination oi 
many hundred preparations, that the ganglion cells of the outer layer are continuous 
with the protoplasm of the rhabdia by their outer processes, and that the stellate cells 
of the two outer layers form a complex network with each other by their lateral pro- 
cesses. I have been unable to determine whether the inner processes of these cells 
pass into the small round cells of the third layer, but I suspect they do ; they certainly 
communicate with the fusiform cells of the fourth layer. 
Figs. 10 , Jc, and 13, f', represent the bundles of cells in this layer. The first is a 
transverse section through a bundle from a stained specimen. I shall call this the 
facelloid layer of the retina. The bundles of cells consist of five or six cells. ( I am at 
a loss to explain this deviation from the number of structures in the rhabdion, but it 
will be remembered that the number of cells in the facellus of Tipala is not the same 
as the number of rod-like elements.) 
The innermost or fifth layer of the ganglionic retina ({/') is formed of stellate nerve 
cells like those of the outer layer. These rest on a membrane of extreme tenuity : the 
inner limiting membrane. This is connected with the outer or basal membrane on 
which the rhabdia rest by a fine connective network, or neuroglia, in the spaces of 
which the elements already described are situated. The number of layers of elements 
is very much reduced in those portions of the retina which correspond to the peripheral 
portions of the eye. The outer ganglion cells are reduced to a single layer, and the 
facelloid layer exhibits fewer sets of cells. 
The optic nerve (w') consists of clear, often varicose fibres. These unite the inner 
and outer ganglia, and form a complete decussation from above downwards, as well as 
from behind forwards. The inner half of each of these fibres is surrounded by a vast 
number of minute nuclei, which refract light highly. I have been unable to satisfy 
myself of their connection with the fibres, but I am inclined to the belief that they 
are united with them, as they move with them when the glass cover is shifted, and 
are only separated from them with great diffi culty. The inner ganglion (c) consists of 
five or six layers of fusiform cells of granular protoplasm. 
Musca vomitona (figs. 14 to 17).- — In this insect the chamber of the eye (fig. 14) 
is shorter than in Eristalis, and the tetrasome (a') is placed in a small ovoid cavity at 
its inner extremity, surrounded by a dense layer of pigment, so that only its apex is 
exposed to the light. The segments of the tetrasome are finely striated in a longi- 
tudinal direction. 
The cornea has the curvature of an epicycloid in section (fig. 51, page 596). The 
facets are Towoth of an inch in diameter ; the radius of curvature of the outer surface 
is roVoth of an inch, and that of the inner surface is - 7 -l p-th of an inch. The focal 
length of the lens is 400 th of an inch, measured in air. The distance of the outer 
extremity of the tetrasome to the inner surface of the cornea is as nearly as possible 
TWoth of an inch. The focal length of the lens is given from the same surface of the 
cornea, so that the tetrasome lies considerably within the focus of the lens. 
4 f 2 
