The orbital portion of the superior maxillary nerve may now be exposed by cutting through the external rectus muscle and drawing the eye with its muscles toward's the inner part of the orbit. The nerve having crossed the spheno maxillary fossa enteri the orbit through the spheno-maxillary fissure; in company with a branch of the internal maxillary artery it passes along the infra-orbital groove, covered by a layer of periosteumiiit then passes through the canal and emerges from the infra-orbital fora men. The trunk of the nerve is but little vi sible on the floor of the orbit. While the supe rior maxillary nerve is in the foramen rotun dum, or during its passage across the fossa, it sends off a temporo-malar branch which passes through the spheno-maxillary fissure superior and external to the trunk of the nerve; it passes along the floor of the orbit, beneath the inferior rectus muscle, and about the middle divides into two branches, a temporal and a molar.
The temporal branch goes towards the miter wall of the orbit, passes up between the bone and the external rectus muscle, and joins with a temporal branch from the lachrymal ; it then pierces the orbital process of the molar bone and enters the temporal fossa, .where it com municates with the anterior deep temporal nerve, sends branches to the temporal muscle, and piercing the fascia is distributed to the skin over the temporal region.
The molar br' aneh passes on to the inferior external angle of the orbit, where it sometimes communicates with the lachrymal nerve ; it enters one or more canals in the malar bone, and appears on the facial surface, supplying the orbicularis and the integuments, and com municating with the portio dura.
In addition to the structures ordinarily described as existing within the orbit, Mr. O'Ferrall has described* a fibrous structure, tri which he gives the name of " tunica vaginalis,", and which invests the globe of the eye, sepa rating it frorn the muscles and fat of the orbit. In order to expose the outer surface of this structure, a vertical incision must be made through the integument of the upper lid ; after removing carefully the orbicularis and a fascia, between the two layers of which the tendon of the levator palpebree is inserted, the part next in order is the tarsal mrtilage. Tracing this upwards and backwards its thin'margin is found to be continuous with a fibrous lamina, which passes back into the orbit and separates the globe of the eye from the superior rectos muscle, but presenting a svell-defined opening, through which the tendon of the muscle passes as over a pulley, to be inserted into the scle rotic coat. In order to examine the ocular sur face of this membrane, 111r. O'Ferrall advises a vertical division of both palpebrre, then an incision through the conjunctiva at the angle of reflection from the eyelid to the ball of the eye.
The incision being made and the edges of the divided membrane separated, we expose the ocular surface of " a distinct tunic of a yellowish white colour and fibrous consistence, continuous in front with the posterior margin of -the tarsal cartilage, and extending back wards to the bottom or apex of the orbit, where its consistence becomes less strongly marked." This ocular surface is smooth where the eye glides over it in its movements, and is con nected to the surface of the globe by fine cel lular tissue. The muscular substance of the recti muscles is on the outside of this tunic and invisible through it; but about half an inch posterior to its anterior margin are six well-defined openings through which the ten dons of the muscles emerge in passing to their insertion in the sclerotic coat. Mr. O'Fer rall was induced to look for this structure in consequence of meeting with cases in which the globe of the eye and the conjunctiva were protruded in a manner not satisfactorily ex plained by reference to any previously described structure. Ile believes these to have been aises of inflammation of this tunic, with effusion between its deep surface and the globe of the eye.
Mr. O'Ferrall believes that " the uses of this tunic are to present a smooth surface, faci litating the movements of the eye; and by its density and tension, to protect it from the pres sure incidental to the swelling of its muscles during their action. That the openings in this tunic perform the office of pulleys, giving a proper direction to the force exerted by the inuseles,—securing the motions of rotation, a lid opposing those of retraction, which would other wise predominate." Action of the muscles. —The action of the levator palpebrte muscle is to raise the upper lid, and thus to expose the anterior part of the eye-ball. In this action it is an associate of the frontal portion of the occipito-frontalis, and an antagonist of the orbicularis palpebrarum. Sir C. Belli affirmed that the action of the levator palpebrx is not simply that of raising the upper lid, but that the swelling and tension of the muscle during i LS action have the effect of pushing forwards the eye-ball, thus causing the lower lid to slide off the convex surface of the eye, and to be depressed whilst the upper lid is elevated. There is no proof of any such action of the levator palpebrx, and it seems improbable that it should exert any such in fluerice, separated as it is from the eye-ball by the superior rectus, by a considerable quantity of fat and by the " tunica vaginalis." The re sult of paralysis of this muscle is a dropping of the upper lid, to which the term ptosis is applied.