PHYSIOLOGY OF THE EYE OF THE HORSE. 
513 
from the irregular shootino- herbaore, or the noisome insect. The 
orbicularis muscle is in action while the animal is awake, and 
the lids close in sheep, because they have lost the elastic power 
which supported them. The tarsal cartilages preserve the shape 
of the lids, and adapt them to the convexity of the globe. The 
levator palpebrae and epicranius assist in raising the lids; the 
tarsal ligaments are the connexion of the lids with the rim of the 
orbit. The mebomian glands secrete a fluid for the purpose of 
lubricating the edges of the lids, and preventing adhesion from 
taking place between them w'hile asleep, or the tears from escap- 
The tunica conjunctiva acts as a covering to the eye, and 
helps to retain it in its situation in the socket, w’hile it also 
secretes a transparent mucus which prevents irritation from the 
saline particles of the tears; it also seems to be a connecting 
medium with or a prolongation of the skin which covers the 
whole body. We have a proof of this in most of the serpent 
tribe, which, when shedding their skins, cast off a delicate fine 
substance from the eye-ball—a reflection of the conjunctiva. 
We cannot but admire the beautiful construction of this mem¬ 
brane, transparent over the transparent cornea, and intercepting 
none of the rays of light, nor producing defective vision. The 
membrnna nicitans is also a wise provision of nature acting as a 
third eye-lid. In some quadrupeds, however, it is, in a manner, 
wanting (as in the monkey) ; but its absence can be accounted 
for : nature gives nothing in vain, and as this animal uses his 
paws with great dexterity, he can sufficiently protect his eyes. 
For the same reason it appears to be denied to man, to whom in 
form and use of the superior extremities he is so nearly allied. 
It is very well developed in the eagle and elephant, for the for¬ 
mer, when he soars aloft the better to observe his prey, requires 
a protection from the ardent rays of light; and the latter, in a 
state of nature, carrying his head upward in search of his food 
amidst the boughs of the trees, needs a similar protection. You 
are all well aware by what simple mechanism it has been sup¬ 
posed to act. I have described the mass of fat with which the 
back part of the eye is surrounded, and the connexion of the 
haw with it. The retractor muscle draws the eye within the 
orbit; this fatty matter cannot be compressed, but it is forced 
out of its place ; it glides over the os planum, pushing the mem- 
brana nictitans before it and over the eye, and it afterwards re¬ 
turns by its own elasticity. 
Our worthy Assistant Professor considers that there are muscu¬ 
lar fibres connected with the membrana nictitans ; and I am in¬ 
clined to agree with this opinion. They have not, it is true, been 
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