NUNNELEY, ON THE RETINA. 237 
The retina is much more easily separated from the vitreous 
humour after a time than it is immediately after the death of 
the animal, which may arise from the change these cells 
undergo. 
7. Vascular layer.—The retina is a very vascular structure ; 
it is supplied by the central artery of the retina, which almost 
as soon as it enters the eye divides into two or three branches, 
which immediately subdivide and form a series of inoscula~ 
tions; the branches pass forwards and form a complete vascular 
network, until the vessels become capillary, which they do sud- 
denly from vessels of comparatively large size. At first the 
larger branches are on the inner side of the fibrous layer, but 
as they pass forwards they gradually penetrate this, some of 
the branches running parallel with the nerve fibres for a con- 
siderable distance, (Pl. X, fig. 4) ; but the smaller branches 
and capillaries, which form beautiful loops with each other, 
appear to be exclusively distributed in the vesicular and 
granular layers, on the outer surface of the fibrous, (Pl. XJ, 
fig. 7) ; none, so far as can be detected, passing into the 
bacillar layer. Many of them form terminal loops at some 
distance from the anterior termination of the retina, (Pl. XI, 
fig. 8). Near the ora serrata there is described to be a cir- 
cular vessel, into which many others pass; this is regarded 
by some anatomists as a vein. 
It is necessary in injecting these vessels to do so from the 
ophthalmic or carotid arteries ; but the larger branches are so 
commonly found congested after death, that they may be 
readily seen, and not unfrequently the smaller ones are so 
filled with blood, that.their minute ramifications may be 
examined with great facility, particularly where cerebral 
congestion has existed during life, or immediately after death 
the animal has been placed with the head in a depending 
position. (Pl. XI, fig. 7), is taken from the retina of a 
woman who died of cerebral congestion. In the other eye 
a small apoplectic clot was found, and several patches of con- 
voluted highly congested vessels. 
This communication has already occupied so much space, 
that I must reserve until another opportunity a description 
of the punctum centrale of Soémmerring. I append the size 
of the rods and granular cells in some mammalia, and of the 
rods, cones, and bulbs in some birds, reptiles, and fish, in 
parts of an English inch. I would, however, wish the mea- 
surement to be regarded as no more than an approximation 
to the absolute size of these bodies, which certainly differs 
in different parts of the same retina. 
