Of the two sets of vessels distributed to the conjunctiva, one set supplies the conjunctiva forming the palpebral sinuses, the other that part of the conjunctiva which corresponds to the tarsal cartilages. Both sets give off nu merous branches, which subdivide very mi nutely in the papillary body. The second set, after having given off their ramifications to the papilla, proceed towards the margins of the eyelids ; following, in the upper, a straighter and more parallel direction than in the lower.
2. Veins.—The veins from the eyelids dis charge their blood into the anterior and pos terior facial veins.
The blood from the other accessory parts of the eye is returned to the cavernous sinus by the ophthalmic veins, of which there are two to each eye :—one called the cerebral oph thalmic, and the other the facial ophthalmic vein.
Cerebral ophthalmic vein.—Larger than the facial ophthalmic vein, this begins at the inner angle of the eye, from the upper end of the an terior facial vein. From this it passes back wards through the orbit to the inner part of the superior orbital fissure, by which it enters the cranium, where it empties itself into the caver nous sinus—seldom into the circular sinus. In this course it has several communications with the facial ophthalmic. The cerebral oph thalmic vein receives directly or indirectly, besides the veins from the different parts of the eyeball and its muscles, a vein from the lacry mal sac, and from the parts lying at the inner canthus ; the anterior nasal vein, the lacrymal vein, and the posterior nasal vein.
Facial ophthalmic vein.—This receives the infra-orbital and some other deep veins of the face, besides some veins from the eyeball. The deep branch of the anterior facial vein takes one of its origins from it. The facial ophthalmic vein leaves the orbit by the supe rior orbital fissure, and opens into the cavernous sinus below the cerebral ophthalmic.
Comparative anatomy and development.— In the description just given of the accessory parts of the human eye, allusion has been occa sionally made to their structure in the lower animals; here such further observations will be offered as may tend to illustrate. their physiolo gical importance in the animal series.
And first, it may perhaps be well to keep in mind that, although generally speaking, organs, traced from the higher to the lower animals, are observed to become depreciated in develop ment ; still that this is by no means al the case in a ratio corresponding to the po lion of the animal in our classifications. F. the circumstances connected with the mode life of an animal, be it mammal, bird, reptil or fish, may be such as to call for a greater I less development of some particular orgy Thus, though in the mammifera we find tt eyelids very perfectly developed, and in fish in an extremely imperfect state, or entirel wanting, and though we find gradations b tween these two extremes in the animals hol ing an intermediate place, still there exi• mammiferous animals in which the inter ume.
passes right over the eyeball without formi. any palpebral fold ; and there are fishes i which there are not only palpebral folds, b also an orbicular muscle. Again, the sem lunar fold of man and the higher quadrum is enlarged in quadrupeds into the membra nictitans, and in birds forms the very artificial] constructed third eyelid, which subsists, thou_ in a less perfect state, in reptiles, but in fishe where the structure does exist, it is found reduced to a semilunar fold. In man the lac mal gland is large. In the lower mammife generally, in birds, and in the higher reptiles th lacrymal gland is also found. But it is sma in proportion to another gland situated at th nasal canthus of the eye, the glandule Harder, which is developed in a direct propo tion with the membrana nictitans, or third eye lid, and to which it therefore belongs. In in and the quadrumana there is no trace of th glandule of Harder. It is incorrect to view th lacrymal caruncle in that light, for both ma exist together.
Besides these differences in the development of the accessory parts of the organ of vision observed in the animal series and capable of being generalised, there exist specific and indi vidual differences which can only be noticed in detail.
1. Eyelids.
In subterranean mammifera, as the blind rat, the chrysocliloris of the Cape, the common mole ; among reptiles, in the pips, which lives in obscure places; and in perennibranchiate batrachia, which inhabit subterranean lakes or marshes, as the proteus and syren ; and in fishes which burrow in the mud or sand, as the anguilliform and certain cyclostomatous fishes, the eyes are very small, and the common integument passes right over them without forming any palpebral fold. In the Ophidian reptiles as first pointed out by Jules Cloquet; in Geckoes among the Saurian reptiles, as shewn by J. Muller; and even in the blind rat, according to the latter author, there is a com pounding of the simple continuation of the in teguments over the eyeball, as described above, with the existence of a conjunctiva underneath, enclosing an oculo-palpebral space. This struc ture will be described in speaking of the con junctiva. In birds there is no example of the eyeball being covered over in a similar manner by a continuation of the skin.