Monthly MlcroscoplcaH 
Journal, I^fov. 1, 1869. J 
Histology of the Eye. 
241 
cetaceans, and upon the glistening silvery portion of the choroid, 
called the tapetum lucidum in ruminants, solipedes, and carnivores, 
the epithelium is also present, hut it is devoid of pigment. In birds, 
reptiles, fish, and amphibia, brushes of pigmented tissue pass in- 
wards from the epithelial cells between the retinal bacilli. In man, 
the diameter of the cells ranges between xrooth and ^yVoth of an 
inch ; their average is about ttV o th. 
The epithelium rests on a very distinct structureless mem- 
brane — the elastic lamina. This is often the seat of circumscribed 
thickenings, which begin as little elevations of the inner surface, 
and grow into knobs, and globes, and glandiform masses, large 
enough to be seen, in a strong hght, with the unaided eye. The 
affection is one of those degenerations common in old age, but 
which also occurs in young persons as a sequel of long-continued 
local inflammation. 
The choroid is supplied with arterial blood by the short posterior 
and the anterior ciliary arteries. The former, about twenty in 
number, pierce the posterior segment of the sclerotic, some near the 
posterior pole, others farther forwards. The hindermost are distri- 
buted to the sclera and choroid around the optic nerve : and, here 
inosculating with the capillaries of the nerve, they establish a col- 
lateral channel, through which a little blood can enter the retina 
when the trunk of the arteria centralis is plugged by an embolus. 
The remaining short posterior ciliary arteries run forwards with a 
straight course, sending off short branches through the stroma to 
the capillary net, where they break up quickly in an arborescent 
manner. The foremost of these arteries inosculate in front of the 
equator with the anterior ciliary arteries (branches of the muscular), 
which supply this region of the choroid. The capillaries form a 
net immediately at the outer surface of the elastic lamina, the 
meshes of which are smaller and less regular in the posterior 
segment of the choriod than in the anterior, where they are wider 
and longer. The vessels are large ; and in all situations the inter- 
stices of the net are relatively narrow, less broad than the diameter 
of one of the overlying epithelial cells. "We can recognize the col- 
lective effect of the capillary net, but not the individual vessels com- 
posing it in the living eye. (Fig. 11 ; p. 242.) 
The blood of all the choroidal capillaries is collected by the well- 
known venus whorls, vasa vorticosa, which empty their contents by 
four short wide trunks which pierce the sclerotic very obliquely a 
little behind the equator. The valvular form of these sclerotic 
canals has been noticed by Leber, who adds the remark that it 
v/ould tend to hinder the exit of the venous blood whenever there 
is an increased pressure on the inner surface of the eye-ball. 
The stroma in which all the larger arteries and veins are bedded 
is a modified connective substance. It contains, like that of the 
VOL. II. S 
