" From this curious disposition of the muscles," says Dr. Porterfield,* "it is easy to conceive, how this internal eyelid is extended over the cornea far enough to cover all the pupil, though the muscles themselves are con tained in a small space. Every body knows, that the contraction of all muscles is only in a certain given proportion to their length ; and therefore that the eyelid might be drawn far enough over the cornea, nature was obliged to make use of a long muscle, which could not be contained in so small a place as the orbit, without being bent or inflected ; and therefore the one muscle is bent upwards near the optic nerve, making an acute angle, where it passes through the perforated end of the other muscle, by which means its action is greatly increased. But its action is yet more increased by the contraction of the square muscle itself, which must draw the cord or tendon of the py ramidal muscle which passes through it, through a space double of what it moves itself; and thus the membrane nictitans is extended far enough to cover the whole cornea though its muscles are contained in a small space." In Owls Nitzsch discovered a small hone on the lower surface of the bony ring of the sclerotica, ossiculum tuberculare, for the support of the long tendon of the pyramidalis. In par rots the third eyelid is small.
Among reptiles there is in chelonia and lizards a third eyelid much the same as in birds, but smaller and less moveable. It is moved only by a single muscle analogous to the pyramidalis of birds. " An allied struc ture," says Muller, " is a spectacle-like trans parent part in the lower eyelid of some lizards, as several acinci, which may be drawn over the eye without interrupting vision, the cornea cor responding to it." This is the structure in the frog; for what Cuvier admitted as a third eye lid is horizontal instead of vertical, and, as Carus has shown, nothing more than the lower In the frog the upper eyelid follows the motions of the eyeball. The lower eyelid has independent motion; admitting of being drawn over the eye and falling into a fold when this is open.
The anterior and posterior semilunar folds in certain fishes have been already alluded to.
The glandule if Harden—The glandule of Harder belongs peculiarly to the membrana nietitans or third eyelid ; it therefore does not exist in man and the quadrumana. The lacry mal caruncle is not the representative of it, as is asserted in Dr. Grant's Treatise on Compara tive Anatomy, for both may exist together. I do not even know if it is correct to say that the two structures are developed in an inverse ratio, for in the sheep, in which the glandule of Harder is of considerable size, the caruncula is absolutely as great, if not greater, than that of man. Even among the hare kind, which have the rnembrana nictitans and glandule of Harder much developed, I find, particularly in the rabbit, a trace of lacrymal caruncle as scattered follicular grains along with small hairs at the inner canthus. It is only in birds that we lose all trace of the lacrymal caruncle.
The glandule of Harder is situated in the orbit between its inner wall and the globe of the eye. In the sheep, for instance, the glan dular substance is collected around the cartila ginous pedicle of the membrana nictitans, on the inner surface of which it opens by two or three small ducts. In the elephant, in which the lacrymal gland is said to be wanting, or very small, the glandule of Harder is said to be very large. It opens between the membrana nictitans and the eyeball by an opening the size of a quill. In the hare kind, as has been said, the glandule of Harder is immense. It presents two lobes, and its duct opens in a wide lacuna within the inembrana nictitans. According to Muller,* the elementary particles or ends of the ducts are minute vesicles every where equal and joining in the mariner of branches into irregular oblong lobes. The excretory duct at the external surface of the gland opposite the eye is divided in the bi lobated glandular mass into a great number of smaller ducts, which divaricating are joined each to a branch of the lobules.
In birds the glandule of (larder is commonly much more considerable than the lacrymal gland. It lies as usual at the nasal canthus and opens within the third eyelid towards the eyeball. There is never any caruncle. Mailer tells us the glandule of Harder in birds is easily injected with mercury after its secretion has been all pressed out. The surface of the organ is divided into many smaller lobes. The internal ramification of the ducts does not ap pear to be complicated.
In reptiles the glandule of Harder is smaller than the lacrymal gland.
The secretion of the glandule of Harder is a thick transparent viscid matter.
Ill. Secreting and derivative lacrimal apparatus.—The development of the lacrymal gland and of that of Harder is generally in an inverse ratio. it would appear that the deriva tive lacrymal apparatus is more in relation wit the lacrymal gland than with the glandule Ilarder, as it is much developed in man, i whom there is no glandule of Harder, and, as reported, it is wanting in the elephant, in %%hi the glandule of Harder is said to be ve large. The lacrymal gland exists in man, ape sapajous, and snakes, but no glandule Harder. In all other mammifera there is both a lacrymal gland and a glandule of Harder. I proportion as the latter enlarges the former h comes smaller. Among the cetacea, the do phin possesses a lacrymal gland which su rounds the eyeball like a ring. Its excreto ducts, which are numerous, open on the inn surface of both upper and lower eyelids. Tl derivative lacrymal apparatus is wanting; i this respect, seals and walrusses agree with t cetacea.* Other mammals, as moles and shrei mice, are said to present no trace of lacryin apparatus. In the elephant, small glandul grains the size of a pea are said to represent II lacrymal gland. Camper says the hippo tames has no puneta, from which may be ferred the absence of any lacrymal passage in the nose. The elephant is also said to wa the derivative lacrymal apparatus.