THE BACK. 413 



The she at J i of the optic nerve is derived from what source / 



It is derived from the dura mater coming through the optic foramen and de- 

 laminating ; one layer ensheathes the optic nerve, the other forms the orbital 

 periosteum. (Fig. 95.) 



The third cranial nerve the motor oculi supplies : (i) The ciliary muscle of 

 the eyeball ; (2) the sphincter muscle of the iris ; (3) all the muscles of the eye- 

 ball except the superior oblique and the external rectus. 



This nerve arises beneath the floor of the aqueduct of Sylvius. On removing 

 the brain from the cranium, you see the nerve between the crura cerebri. (Fig. 

 92.) The nerve pierces the dura midway between the anterior and posterior 



FIG. 288. THIRD CRANIAL NERVE, MOTOR OCULI. 



clinoid processes, and enters the cavernous sinus ; in the sinus it lies above and 

 internal to the fourth nerve. (Figs. 8688.) The nerve divides behind the 

 sphenoidal fissure into a superior and an inferior division. (Fig. 86.) These two 

 divisions enter the orbit by the sphenoidal fissure, passing between the two heads 

 of the external rectus muscle. (Fig. 101.) The superior division supplies the 

 superior rectus muscle and the levator palpebrae ; the inferior division has the 

 remaining part of the distribution of the third nerve. 



\VJiat can yon say of the fourth cranial, or patJieticns? 



It is also called the trochlear nerve, from the fact that it supplies the superior 

 oblique muscle of the eyeball only. This nerve has a long course in the cranial 

 cavity. In dissection you see the nerve just under the margin of the anterior 



FIG. 289. FOURTH CRANIAL NERVE, TROCHLEAR. 



free border of the tentorium cerebelli, and soon piercing the dura to the outer 

 side of the posterior clinoid process, to gain the cavernous sinus. (Fig. 98.) 

 This nerve is the smallest of the cranial nerves ; it enters the orbit by the sphe- 

 noidal fissure. The nerve has its 'deep origin in the floor of the aqueduct of 

 Sylvius. 



Describe briefly the fifth cranial or trifacial nerve. 



This nerve is specialized almost exclusively for the prehension and mastication 

 of food directly or indirectly. It will be noted that the relation between this and 

 other nerves depends upon the near or remote association these latter bear to the 

 functions of the fifth nerve. Take notice, then, of the function of the trigeminus, 



