The obliquity superior is a long and slender muscle, which is sometimes called the trochlearis muscle from the fact of it being reflected through ! a trochlea or pulley. It arises from the fibrous sheath of the optic nerve, and from the inner part of the optic foramen between the superior and internal recti muscles ; it passes forwards along the internal superior angle of the orbit, in the form of a rounded fleshy belly, to which succeeds a rounded tendon, which after passing through the pulley beneath the internal angular process is diimcted backwards, outwards, and downwards, passing beneath the superior rectus muscle to be inserted by a thin aponeurosis into' the sclerotic coat between the superior and exter nal rectus, rather behind the anterior half of the globe. This pulley or trochlea, through which .passes the tendon of the superior oblique, is a small cartiliginous ring, inserted by means of fibrous tissue into a depression beneath the internal angular process ; it is lined by a syno vial membrane. The orbital surface of the . superior oblique is in contact with the perios teum ; the fourth nerve passes into this surface about its centre; the relations of its ocular surface are the same as those of the superior rectus.
The superior rectus and superior oblique muscles may now be cut through and turned aside, and after removing carefully some fat and cellular tissue the following parts are brought into view :—the internal and external recti muscles, the optic, the third, and the nasal branch of the ophthalmic nerves, the lenticular ganglion between the optic nerve and external rectus muscle, and the ophthalmic artery and vein.
The third nerve before entering the orbit divides into two portions ; a superior smaller, and an inferior larger; it enters the orbit through the sphenoidal fissure between the two heads of the external rectus muscle ; the inferior division then passes beneath the globe of the eye and must be examined in a subsequent stage of the dissection. The superior division is now visible; it passes to the under surface of the superior rectus muscle, to which it sends numerous fila ments ; some filaments also pass on the inner side of the superior rectus and enter the deep surface of the levator palpebrre superioris ; these are the only 'muscles supplied by this divisien of the third nerve.
The nasal nerve is in size the second branch of the first division of the fifth. It enters the orbit through the sphenoidal fissure, passing between the two heads of the external rectus muscle, in company w'ith the third and sixth nerves, being external to the former, and be .tween its two divisions, and internal and some 1what superior to the latter. Having entered the rbit, it passes forwards and inwards towards the nternal wall, crossing over the optic nerve etween it and the superior rectus muscle; it as also above it the levator palpebne and supe iior oblique muscles, and the superior division pf the third nerve ; it passes out of the orbit ihrough the anterior internal orbital hole in company with the anterior ethmoidal artery.
Within the orbit it sends off lenticular, ciliary, and infra-trochlear branches. The lenticular branch is given off on the outer side of the optic nerve; it anastomoses with the superior division of the third nerve, and joins the posterior supe rior angle of the lenticular ganglion. The ciliary branches are two or three in number ; given off above the optic nerve, they pass for wards and pierce the posterior part of the sclerotic. The infra-trochlear branch is given off near the inner wall of the orbit; it passes out beneath the pulley of the superior oblique muscle, and sends branches to the superior eye lid, the lachrymal sac, and integuments of the nose ; within the orbit there is an anastomosis be tween this and the supm-trochl ear or frontal nerve.
The lenticaldr or ciliary ganglion is situated in the posterior and outer part of the orbit between the optic nerve and the external rectus muscle; it is very small, and of a somewhat square form. Its superior posterior angle is joined by.the lenticular branch of the nasal nerve, which constitutes the long root of the ganglion ; its inferior posterior angle receives a branch from the inferior division of the third nerve ; this forms the short root of the ganglion. To its posterior part is also connected one filament from the cavernous plexus, and occasionally one from, the spheno-palatine ga.nglion. From the anterior part of the ganglion a number of delicate ciliary nerves pass off ; they are divided into two sets, one set coming from the superior anterior, and the other from the inferior anterior angle of the ganglion ; the former are the more numerous; in all they are from twelve to sixteen in number; they pass forwards and pierce the sclerotic near the optic nerve.
The optic nerve passes forwards from the optic hole to the inner and back part of the eyeball, which it enters to terminate in the re tina. It is invested by a sheath of fibrous membrane, which is continuous behind with the dura mater, and in front with the sclerotic ; the outer surface of this sheath posteriorly gives attachment tp the recti muscles, which surround the optic nerve as/ll emerges from the optic hole. The optic nerve is crossed above by the nasal nerve and the ophthalmic artery, below by the branch of the inferior division of the third nerve, which supplies the internal rectus muscle; it is surrounded by numerous delicate ciliary nerves and arteries.