DISSECTION OF THE ORBIT 339 



the medial wall of the cavity above the medial rectus. At 

 the anterior part of the orbit it ends in a slender tendon, 

 which enters the trochlea and at once changes its direction, 

 proceeding postero-laterally, upon the upper surface of the 

 eyeball, under cover of the superior rectus. Beyond the 

 lateral edge of the superior rectus the tendon expands some- 

 what, and is inserted into the sclera midway between the 

 entrance of the optic nerve and the cornea. 



The trochlea or pulley through which the tendon passes is 

 a small nbro-cartilaginous ring, which is attached by fibrous 

 tissue to the trochlear fossa a depression in the frontal bone 

 close to the medial angular process. The pulley is lined with 

 a synovial sheath which facilitates the movement of the 

 tendon, and from its lateral margin it gives a fibrous invest- 

 ment to the tendon. 



Dissection. Divide the superior rectus midway between its origin and 

 its insertion, and reflect the cut ends. On raising the posterior part of 

 the muscle the superior division of the oculo-motor nerve is brought into 

 view, as it sinks into the deep or ocular surface of the muscle. It 

 sends a twig to the levator palpebrae superioris. The removal of some 

 fat will bring the optic nerve more fully into view. At the posterior part of 

 the orbit three structures will be seen crossing the optic nerve viz., (i) the 

 naso-ciliary nerve ; (2) the ophthalmic artery ; and (3) the superior 

 ophthalmic vein. These should be carefully cleaned and their branches 

 followed out. From the naso-ciliary nerve one or two delicate thread- 

 like branches the long ciliary nerves will be found passing along the 

 optic nerve to reach the eyeball. The short ciliary nerves, much more 

 numerous, accompany the long ciliary branches, and can readily be dis- 

 engaged from the fat which surrounds the optic nerve. A strong member 

 of this group should be selected and followed posteriorly ; it will lead the 

 dissector to the ciliary ganglion. This is a minute body which is situated 

 upon the lateral side of the optic nerve in the posterior part of the orbit. 

 With a little patience and care the roots which the naso-ciliary nerve and 

 inferior division of the oculo-motor nerve give to this ganglion can be 

 isolated, and perhaps even the sympathetic root from the internal carotid 

 plexus will be found. 



Nervus Opticus. The optic nerve enters the orbit through 

 the optic foramen. It carries with it a strong loose sheath 

 of dura mater, and also more delicate investments from 

 the arachnoid and pia mater. The ophthalmic artery, which 

 accompanies it, lies on its infero-lateral aspect. Within the 

 orbit the nerve inclines antero - laterally, and at the same 

 time somewhat downwards, to the back of the eyeball, where 

 it pierces the sclera a short distance to the medial side of 

 its centre. The dissector has noted already that the 

 ophthalmic artery and vein and the naso-ciliary nerve cross 



