The fourth nerve supplies the superior oblique muscle with motor power, while the sixth nerve similarly regulates the movements of the external straight muscle—the only two muscles in the orbit which are not supplied by the third pair.
Although not entitled to be termed a nerve of the orbit, the facial nerve deserves mention as sending a motor branch to the orldeularis muscle, by which the eye is closed.
2. Comparative Anatomy of the mammals, the structure of the eye is usually almost identical with that of man. The organ is, however, occasionally modified, so as to meet the peculiar wants of the animal. Thus, in the cetacea, and in the amphibious carnivora that catch their prey in the water, the shape of the lens is nearly spherical, as in fishes, and there is a similar thickening of the posterior par of the sclerotic, so as to thrust the retina sufficiently forward to receive the image formed by such a lens. (See the subsequent remarks on the eyes of fishes.) Again, instead of the dark-brown or black pigment which lines the human choroid, a pigment of a brilliant metallic lus ter is secreted in many of the carnivora, forming the so-called tapetum lucidum at the bottom of the eyeball, which seems (according to Bowman) to act as a concave reflec tor, causing the rays of light to traverse the retina a second time, and thus probably increasing the visual power, particularly where only a feeble light is admitted to the eye. The pupil, moreover, varies in form, being transversely oblong in the ruminants and many other herbivora, and vertically oblong in the smaller genera of cats. These shapes are apparently connected with the positions in which the different animals look for their food. Lastly, in some mammals (for example, the horse), there is a rudiment ary third eyelid, corresponding to the membrana nictitans of birds.
In birds, the eye, though presenting the same general composition as in man, differs i from the mammalian eye in several important. points. From our knowledge of the habits of birds (especially birds of prey), we should naturally expect that from their rapid movements they should be able readily to alter the focus between the extremes of long and short sighted vision, and the modifications we shall now proceed to notice clearly have this object in view.
By examining a longitudinal section of the eye of the owl, we see (1) that the shape of the organ is not spherical, as in mammals, nor flattened anteriorly, as in fishes and aquatic reptiles, but that the cornea is very prominent, and the antero-posterior dianieter lengthened; the consequence of this arrangement being to allow room for a large quantity of aqueous humor, and to increase the distance betwCen the lens and the pos terior part of the retina, and thus to produce a greater convergence of the rays of light, by which the animal is enabled to discern near objects, and to see with a faint light. In order to retain this elongated form, we find a series of bony plates, forming a broad zone, extending backwards from the margin of the cornea, and lying embedded in the sclerotic. The edges of the pieces forming this bony zone overlap each other, and are slightly movable, and hence, when they are compressed by the action of the muscles of the ball, there is protrusion of the aqueous humor and of the cornea, adapting the eye for near vision; while relaxation of the muscles induces a corresponding recession of the humor and flattening of the cornea, and fits the eye for distant vision. The focal distance is fUrther regulated by a highly vascular called the marsupium, or pecten, which is lodged in the posterior part of the vitreous humor. It is attached to the optic nerve at the point where it expands into the retina, and seems to be endowed with a power of dilatation and contraction; as it enlarges, from distension of its blood-vessels, it causes the vitreous humor to push the lens forwards, while, as it collapses, the lens falls backwards again towards the retina.
In addition to an upper and lower eyelid, birds have an elastic fold of conjunctiva, which, in a state of repose, lies in the inner angle of the eye, but is movable by two distinct muscles, which draw it over the cornea. It is termed the membrana nictitans; it is to a certain degree transparent, for (according to Cu vier) birds sometimes look through it, as, for example, the eagle when looking at. the sun. The lachrymal gland is situated as in mammals, but there is here a second gland, the glandula Harderi, which yields a lubricating secretion.