In birds the lacrymal gland is small,and Ii at the posterior angle of the eye, either towar the roof or the floor of the orbit The derivati lacrymal organs in the common fowl, for i stance, consist of two large lacrymal poin upper the larger, and a membraneous leading into the nose.
In reptiles, the lacrymal gland lies behind t eyeball, and is of considerable size, especial in harmless serpents. According to Duverney, the lacrymal gland in one species of typhl is six times larger than the extremely drininu tive eyeball ; but even in poisonous serpents, the viper for instance, it is large.
The sauria and chelonia have for the most part both a lacrymal gland and a glandulc el Harder, the former the larger. Batrachia want the lacrymal apparatus.
In serpents the secretion of the lacrymal gland is poured into the oculo-palpebral space, from which a lacrymal duct leads. In the colubri there is, in the fore and lower part of the oculo-palpebral space, a hole or round pore, in some individuals seen with difficulty, very distinct on the contrary in others, and which may admit a bristle. This is the lacry• mal point ; it is single and is continuous walk a very slender membranous duct, semi. transparent, which forms the lacrymal canal This, in harmless serpents, opens into a large pouch communicating with the mouth in floe of the palatine branch of the upper jaw. Cloquet* calls this pouch intermaxillary sinus or sac. In venomous serpents, the lacrymal canal opens, as in the mammifera, in the ex ternal wall of the nasal foss.
There is no lacrymal apparatus in fishes.
In the description of the lacrymal gland in man, the intimate structure of it in the lower animals has been already alluded to. The re mark of Muller may be repeated here, that similar glands have often a perfectly different structure in different animals ; of which the lacrymal gland examined in the chelonia, birds, and mammifera affords an example.
The lacrymal bone contributes to separate the orbit from the cavity of the nose. It is wanting in certain mammifera, as the phocm and most cetacea. It is enormously developed, on the contrary, in certain others, as the giraffe, stag, &c. it exists also in birds, and forms in them often the greatest part of the inferior margin of the orbit. In reptiles, its existence is variable.
It is found in crocodiles. It is absent in the chelonia, ophidia, and batrachia. It is also wanting in fishes, unless the first infra-orbital be assumed as analogous to it.
In ruminating animals, remarkably so in deers and antelopes, the infra-orbital fossa of the su perior maxillary bone is very large, and is lined by a reflection of the skin, more or less in the form of a sac. The skin, which has assumed the characters of a mucous membrane, contains in its substance numerous follicles,which secrete a thick blackish unctuous humour—a secretion which appears to have some relation with the sexual function. This matter has been impro perly called tears, hence the French name larmiers of the infra-orbital glandular sacs of ruminants. In the sheep these organs are re presented by a mere fissure extending on the side of the nose from the nasal canthus. Heckel compares to this structure the foveae in the face, behind the nostrils, of several poi sonous serpents, such as the rattle-snake ; but the membrane lining these parts scarcely ap pears to secrete anything. The temporal gland of the elephant seems to be of the same nature as the infra-orbital glandular sacs of rumi nants.
Development of the accessory parts the eyert—The accessory parts of the eye appear subsequently to the eyeball, and, as is the case with the accessory parts of the organ of hearing in reference to the labyrinth or ear-bulb, have quite a separate and distinct origin from it. That the development of the accessory parts of the eye is independent of that of the eyeball is confirmed by the anomalous conformation which the organ has been sometimes found to present; thus Malacarne relates a case in which the eye balls, their nerves and muscles were wanting, whilst the lacrymal apparatus and eyelids were regularly developed.
Up to the eighth week the external integu ment passes quite smoothly over the eyeball. The conjunctiva is then partitioned off by the formation of a linear fold, which, in the ninth week, surrounds the anterior surface of the eyeball like a small ring. The upper and lower parts of the fold progressively enlarge until they meet each other over the eyeball, which takes place about the twelfth week.