The iVerior and internal angle formed by the meeting of the inferior and internal walls presents a continuous horizontal suture, which in front connects the maxillary bone with the lachrymal, behind this the maxillary with the ethmoid, and still more posteriorly the palate bone with the ethmoid.
The base or circuinfirence is of an irregular quadrilateral form with curved sides and rounded angles; it inclines obliquely from Within outwards. It is formed above by the supra-orbital arch of the frontal bone, on the outer side by the external angular process of the frontal and by part of the orbital border of the malar ; below by the continuation of the orbital border of the malar, and by the corres ponding orbital border of the superior maxillary bone ; on the inner side it is completed by the nasal process of the superior maxillary, and the internal angular process of the frontal bone. At the junction of the middle with the inner third of the supra-orbital arch is the supra-orbital notch or foramen, which transmits the frontal nerve and vessels. There are three sutures in the margin of the orbit, one between the fron tal and malar, a second betvveen the frontal and superior maxillary, and a third between the malar and superior maxillaiy bones. At the junction of the lower with the inner border of the orbit is a small tubercle, the lachryinal tubercle, which is sometimes pointed out as a guide in the operation for fistula lachrymalis, but it is seldom very prominent even in the bare bone, and it could scarcely be detected through the tumefaction consequent on ob struction of the lachrymal duct. The lachry mal groove is immediately behind the internal margin of the-orbit.
In the apex of the orbit is the optic foramen situated between the two roots of the lesser wing of,the sphenoid bone; the direction of the optic hole is backwards and inwards to wards the centre of the sella Turcica. The in ferior root of the lesser win.- of the sphenoid, which separates the optic hOle from the sphe noidal fissure,presents anteriorly a small tuber cle which gives origin to the common tendon of the intevnal, external, and inferior recti m uscles.
Dissection of 6: orbit.—Having removed the skull-cap and brain, the roof of the orbit may be taken away by two vertical cuts with a saw, the inner cut extending- from a point just external to the internal angular proces§, backwards along the roof to the optic foramen, the outer cut extending from a point just in ternal to the external angular process, also back wards to the optic foramen. In making these
cuts care must lie taken to avoid injuring in front on the inner side the pulley and tendon of the superior oblique muscle, on the outer side the lachrymal gland with its vessels and nerves, and posteriorly the optic nerve and ophthalmic artery passing through- the optic hole. Having removed the bony part of the roof the periosteum is exposed, and must be examined before proceeding farther.
The periosteuin of the orbit appears to be a continuation of the dura mater ; it passes in through the optic hole, and through the sphe noidal fissure. Entering the optic hole it divides into two portions, one fonning a tu bular sheath for the optic nerve, and becoming continuous with the sclerotica, the other form ing the proper periosteum. Where it enters the sphenoidal fissure it also forms a sheath for the vessels and nerves which pass through that opening. At the anterior margin of the orbit this fibrous membrane divides into two por tions, one becoming continuous with the pal pebral ligament or fascia, the other with the periosteum of the forehead.
The periosteum may now be retnoved, and the following parts are seen immediately be neath it : in the middle line the frontal branch of the ophthalmic division of the fifth nerve, on the outer side the lachrymal branch of the same nerve, and on the inner side is the founla nerve. Immediately under the fourth nerve is the superior oblique muscle ; beneath the fron tal nerve are the levator palpebrx and superior rectus, and below the lachrymal nerve is the external rectus muscle ; beneath the external angular process is the lachrymal gland. Some branches of the ophthalmic artery are seen in this part of the orbit. A considerable quantity of soft fat exists in the orbit, filling, up the intervals betvreen the mweles and other parts ; some of this must be removed before a clear view can be obtained of the parts which we have enumerated as being visible in this stage of the dissection.