THE THIRD OR MOTOR OCULI NERVE. 723 



and the Ciliary muscle. It is rather a large nerve, of rounded form and firm 

 texture. 



Its aparent origin is from the inner surface of the crus cerebri, immediately in 

 front of the pons Varoln. The deep origin may be traced through the substantia 

 nigra and tegmentum of the crus to a nucleus situated on either side of the me- 

 dian line beneath the floor of the aqueduct of Sylvius. The nucleus of the third 

 nerve also receives fibres from the sixth nerve of the opposite side. These will 

 be referred to again m the description of the latter nerve. The nucleus of the 

 third nerve considered from a physiological standpoint, can be subdivided into 

 several smaller groups of cells, each group controlling a particular muscle. The 

 nerves to the different muscles appear to take their origin from before backward 

 as follows : Inferior oblique, Inferior rectus, Superior rectus and Levator palpe- 

 brse, Internal rectus ; while from the anterior end of the nucleus the fibres for 

 accommodation and for the Sphincter pupillre take their origin. 



Iiifratrochlear 

 nerve. 



Motor root. 

 Sensory root. 



FIG. 391. Nerves of the orbit. Seen from above. 



Recurrent filament 

 to dura mater. 



On emerging from the brain, the nerve is invested with a sheath of pia mater, 

 and enclosed in a prolongation from the arachnoid. It passes between the 

 superior cerebellar and posterior cerebral arteries, and then pierces the dura 

 mater in front of and external to the posterior clinoid process, passing between 

 the two processes from the free and attached borders of the tentorium, which are 

 prolonged forward to be connected with the anterior and posterior clinoid 

 processes of the sphenoid bone. It passes along the outer wall of the cavernous 

 sinus, above the other orbital nerves, receiving in its course one or two filaments 

 from the cavernous plexus of the sympathetic, and a communicating branch from 

 the first division of the fifth. It then divides into two branches, which enter the 



