250 HEAD AND NECK 



is lined with a synovial sheath which facilitates the movement 

 of the tendon ; and from its lateral margin it gives a fibrous 

 investment to the tendon. 



The superior oblique is supplied by the trochlear nerve 

 and moves the eyeball so that the centre of the cornea is 

 turned downwards and laterally. 



Dissection. Divide the superior rectus midway between its 

 origin and its insertion, and reflect the cut ends. When the 

 posterior part of the muscle is raised, 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. All three must 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 disengaged from the fat which surrounds the optic nerve. A 

 strong member of the short ciliary group should be selected and 

 followed backwards ; it will lead the dissector to the ciliary 

 ganglion. That 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 

 the ciliary ganglion can be isolated, and perhaps even the 

 sympathetic root from the internal carotid plexus will be found. 

 After the ciliary ganglion and its roots and branches have been 

 defined, clear away the fat which lies lateral to the ganglion, and 

 secure the abducens nerve, which enters the ocular surface of the 

 lateral rectus. Then clean the optic nerve (Fig. 92) . 



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 forwards and 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 

 the centre of its posterior surface. The dissector has noted 

 already that the ophthalmic artery and vein and the naso- 

 ciliary nerve cross above the optic nerve, and that it is 

 closely accompanied by the delicate ciliary nerves and 

 vessels. The optic nerve is slightly longer than the distance 



