The lachrymal gland is contained in a de pression on the roof of the orbit, beneath the external angular process of the frontal bone. It is generally about as large as a filbert, of an irregular ovoid form, with its long diameter placed transversely. Its upper surface is con vex, and connected by means of fibrous pro cesses to the periosteum ; its under surface is concave, and is in relation with the external rectus muscle and the eye-ball. The excretory ducts of this gland, froni ten to twelve in num ber, run parallel to each other, and open by as many orifices beneath the upper lid, about a line from the tarsal cartilage.
The.fourth nerve enters the orbit by passing through the inner part of the sphenoidal fissure. At this point it is above the other nerves, which pass through the same ope»ing. It then passes forwards and inwards iinmediately under the periosteum, crossing over the origin of the leva tor palpebrm and superior rectus muscles, and it is distributed to the orbital surface of the superior oblique muscle.
Thefrontalnerve,a branch of the ophthalmic division of the fifth, enters the orbit with the fourth nerve, but a little below it and on its outer side. It passes forwards between the periosteum and the levator palpebrm, and soon divides into two branches, internal and external frontal, or supra-trochlcar, and supra-orbital. The supra-orbital is the larger branch ; it passes out through the supm-orbital notch or foramen, and divides into ascending frontal branches, usually two in number, which are distributed to the skin of the forehead, and descending palpe brat filaments, which are very numerous and are distributed in ttre substance of the upper eyelid.
The supra-trochlear nerve passes out of the orbit between the supra-orbital notch and tl.e pulley of the supenor oblique; it gives off ascending frontal filaments to the skin of the forehead, and descending palpebral and nasal filaments to the upper eyelid and dorsum of the nose.
The lachrymal nerve is the smallest of the three divisions of the ophthalmic ; it enters the orbit through the sphenoidal fissure external and inferior to the frontal nerve ; in its passage through the sphenoidal fissure it is invested in a sheath of dura meter. It runs along the su perior border of the external rectus muscle, immediately under the periosteurn ; it passes through the lachrymal gland, sending numerous filaments to it, and terminates by sending pal pebral filaments to the upper lid, one of which passes on and is distributed to the skin of the anterior temporal region. In its course it gives
off a molar branch, which passes through a canal in the molar bone and is distributed to the skin on the cheek ; it also sends down one or two filaments which anastomose with the superior maxillary branch of the fifth nerve.* We may now examine the three muscles vvhich are placed most superficially in the upper part of the orbit, and which are visible in this stage of the dissection, viz. the levator palpebrx, the superior rectus and the superior oblique.
The levator palpe' bre superioris arises ten dinous from the inferior surface of the lesser wing of the sphenoid bone above the optic fommen, also from the fibrous sheath of the optic nerve; it passes forwards, and upwards, becoming broader and thinner towards the an terior part of the orbit, where it suddenly curves downwards and ends in a broad thin apo neurosis, which is inserted into the upper bor der of the tarsal cartilage, behind the palpebral ligament. This muscle is of a triangular form, the apex being posterior ; it is crossed by the fourth and frontal nerves, the latter passing for-0 ward above and parallel to it, and separating it) from the periosteum; it covers the superior rectus and eyeball.
In order to expose the superior rectus, cut, through and turn aside the levator txdpebrze ; nil doing so a small branch of the third nerve seen to enter its inferior surface.
The rectos superior arises from the upper, part of the fibrous sheath of the optic nerve, and from the outer and upper pait of the margin ot the optic foramen ; the fleshy fibres from this: point of origin pass forwards and outwards the direction of the axis of the orbit; the muscle becomes broader and thinner anteriorly, autt) ends in a broad aponeurotic expansion, which is inserted into the upper aspect of the sclerotic, a little behind the margin of the cornea. A small synovial bursa is said to exist between the sclerotic and the tendon at its insertion. The) muscle is covered above by the levator pal: pebrm, and by the nerves which cross the levator palpebrm; below it is in relation with the nas4 the third, and the optic nerves, with the ophthalmic artery and the eye-ball.