784 



ORBIT. 



noidal fissure. Entering the optic hole it 

 divides into two portions, one forming 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 removed, 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 fourth 

 nerve. Immediately under the fourth nerve is 

 the superior oblique muscle ; beneath the fron- 

 tal nerve are the levator palpebrse 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 between the muscles 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. 



The luclin/mal 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, from 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 opening. It then passes 

 forwards and inwards immediately under the 

 periosteum, crossing over the origin of the leva- 

 tor palpebrse and superior rectus muscles, and 

 it is distributed to the orbital surface of the 

 superior oblique muscle. 



Thej'rontal nerve, 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 palpebree, and soon 

 divides into two branches, internal and external 

 frontal, or supra-trochlear, and supra-orbital. 

 The supra-orbital is the larger branch ; it passes 

 out through the supra-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 palpc- 

 bral filaments, which are very numerous and are 

 distributed in the substance of the upper eyelid. 



The supra-trochlear nerve passes out of the 

 orbit between the supra-orbital notch and the 

 pulley of the superior oblique; it gives ofT 

 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 laclin/mal 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 mater. It runs along the su- 

 perior border of the external rectus muscle, 

 immediately under the periosteum ; 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 malar branch, which passes through a 

 canal in the malar 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 

 which are placed most superficially in the upper 

 part of the orbit, and which are visible in this 

 stage of the dissection, viz. the levator palpebrse, 

 the superior rectus, and the superior oblique. 



The levator pulpebrte super ioris arises ten- 

 dinous from the inferior surface of the lesser 

 wing of the sphenoid bone above the optic 

 foramen, 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- 

 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 palpebrse ; in 

 doing so a small branch of the third nerve is 

 seen to enter its inferior surface. 



The rectus superior arises from the upper 

 part of the fibrous sheath of the optic nerve, and 

 from the outer and upper pait of the margin of 

 the optic foramen ; the fleshy fibres from this 

 point of origin pass forwards and outwards in 

 the direction of the axis of the orbit ; the muscle 

 becomes broader and thinner anteriorly, and 

 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. This 

 muscle is covered above by the levator pal- 

 pebrae, and by the nerves which cross the levator 

 palpebrse; below it is in relation with the nasal, 



* For a more minute account of the distribution 

 of these branches of the fifth nerve, see article 

 FIFTH NERVE. 



