514 
PHYSIOLOGY OF THE EYE OF THE HORSE. 
demonstrated ; but we can imagine that the function of the mem¬ 
brane would be more perfectly performed by its possessing some 
muscular power of its own, instead of being the mere passive 
thing which it is supposed to be : and we are compelled to admit 
the power of many an agent whose actual existence has never been 
demonstrated to our senses. I would ask, how is the protrusion 
of the tongue out of the mouth accomplished? there is no mus¬ 
cle at its termination to effect this; or how do we account for 
the projection of the eye out of the socket when under the in¬ 
fluence of fear ? The lachrymal gland, placed in the outer corner 
of the upper lid, is most admirably adapted for its particular 
function. It secretes the tears, which are composed of water 
and saline matter; and, from the obliquity of the canthi, and 
the close apposition of the lids, this fluid, after having discharged 
its duty, is conveyed into the puncta lachrymalia. When an 
offending body rests on the conjunctiva, an increased quantity 
of tears is secreted, in consequence of the irritation produced; 
and these, by the action of the lids, flow over, and are pressed 
upon every part of the eye ; the membrana nictitans then ren¬ 
ders its assistance, by protruding over the globe, and thus the 
offending body is removed. The puncta lachrymalia are the 
receptacles for the tears, which are guided into them by the ca- 
runcula lachrymalia, placed in the inner canthus : thence the 
tears are conveyed into the ductus ad nasum, which is a canal 
first bony and then membranous, and terminating on the cuti- 
cular portion of the nostril, and thus avoiding irritation of the 
Schneiderian membrane. 
The four straight muscles of the eye perform the functions 
which their names imply; the two oblique ones will give the 
eye a kind of rotatory motion, but in contrary directions to each 
other. Mr. Hunter directs a person to look at the sails of a 
windmill when in motion as a proof of this; but 1 cannot here 
agree with him, for I am certain that I could do so and keep 
my eye perfectly fixed and steady. The retractor oculi is a 
muscle peculiar to quadrupeds, and accounts for the difficulty, 
or almost impossibility, of operating for cataracts on such ani¬ 
mals. Our Professor told us, not long since, that he had to use 
a counteracting force of twenty pounds, in order to fix the eye 
ball. 
The sclerotic coat, from its density and elastic property, pre¬ 
serves the form of the globe, gives attachment to the muscles, 
aflbrds protection to the internal parts, and allows a passage to 
the optic nerve. The choroid coat, from its vascularity, is evi¬ 
dently for the purpose of the growth and nourishment of parts; 
it has a black secretion—the pigmentum nigrum, which is to 
absorb the too divergent rays of light passing through the cornea. 
