34 ANATOMY OF THE HEAD AND NECK. 



the orbit through the optic foramen. This nerve is large 

 and of a white color; it is accompanied by the ophthalmic 

 artery, and invested with a firm sheath from the dura 

 mater; it has no branches, and continues forward to the 

 eyeball, where it expands into the retina. 



The orbital plate of the frontal bone, on the opposite side to that 

 used for the dissection of the muscles and arteries of the orbit, should 

 be broken through with a chisel, in such a way as to make a trian- 

 gular opening into the cavity, the base of which, an inch and a half 

 wide, should be as far forward as possible ; the apex should lay bare 

 the optic foramen, but without injuring the nerves. 



As the third and fourth nerves, and a branch of the fifth, lie in the 

 walls of the cavernous sinus, the dura mater which constitutes it 

 must be dissected away. The fifth nerve should also be cleared from 

 the dura mater, and the Gasserian ganglion which lies upon the apex 

 of the petrous portion of the temporal bone in the middle fossa, and 

 into which the fifth nerve soon expands, should be dissected cleanly, 

 and its three large trunks, the ophthalmic, and the superior and 

 inferior maxillary, traced to their points of exit, the sphenoidal fissure, 

 the foramina ovale and rotundum. 



The THIRD, or MOTOR OCULT NERVE, pierces the dura 

 mater just in front of the posterior clinoid processes; pur- 

 suing its course in the external wall of the cavernous sinus, 

 it enters the orbit through the sphenoidal fissure, passing 

 between the two heads of the external rectus muscle, and 

 divides into two branches. The superior branch supplies 

 the superior rectus muscle and the levator palpebrae. The 

 inferior branch supplies the internal and inferior rectus 

 and the inferior oblique muscles. It will thus be seen that 

 this nerve supplies all the muscles of the eye, except the 

 external rectus and superior oblique, each of which has a 

 special nerve. These branches are very small, and, lying 

 in the midst of fat, require great patience and considerable 

 skill to dissect. 



* The inferior branch of the motor oculi nerve sends a 

 branch of communication to the lenticular ganglion, a 

 small rounded body, the size of a pin's head, placed at the 

 back part of the orbit, between the optic nerve and exter- 

 nal rectus muscle, and commonly on the outer side of the 

 ophthalmic artery. This ganglion gives off the ciliary 

 nerves, which pierce the sclerotic in company with the 

 short ciliary arteries around the optic nerve. 



The FOURTH, or TROCHLEARIS NERVE, is very small, and 

 sometimes puzzling to find. It pierces the dura mater 

 close to the third nerve, and passes along the outer wall of 



