128 NERVOUS SYSTEM. 



The distribution of the third nerve is very simple. As 

 it passes into .the orbit by the sphenoidal fissure, it divides 

 into two branches. The superior, which is the smaller, 

 passes to the superior rectus muscle of the eye, and certain 

 of its filaments are continued to the leva tor palpebrse supe- 

 rioris. The inferior division breaks up into three branches. 

 The internal branch passes to the internal rectus muscle ; 

 the inferior branch, to the inferior rectus ; the external 

 branch, the largest of the three, is distributed to the inferior 

 oblique muscle, and, in its course, sends a short and thick 

 filament to the lenticular, or ophthalmic ganglion of the 

 .sympathetic. It is this branch which is supposed, through 

 the short ciliary nerves passing from the lenticular ganglion, 

 to furnish the mofor influence to the iris. 



In its course, this nerve receives a few very delicate fila- 

 ments from the cavernous plexus of the sympathetic and a 

 branch also from the ophthalmic division of the trifacial. 



Properties and Functions of the Motor Oculi Communis. 

 Irritation applied to the root of the third nerve in a living 

 animal produces contraction of the f muscles to which it is dis- 

 tributed, but no pain. If the irritation, however, be applied 

 a little farther on, in the course of the nerve, there are evi- 

 dences of sensibility, which is readily explained by its com- 

 munications with the ophthalmic branch of the trifacial. At 

 its root, therefore, this nerve is exclusively motor, and its 

 functions are connected entirely with the actic-n of muscles. 

 These facts have been experimentally demonstrated by Lon- 

 get * and by Chauveau. 2 



Most of the important facts bearing upon the functions 

 of the motor oculi are clearly demonstrable by dividing the 

 nerve in a living animal, and are illustrated by cases of its 



1 LONGET, Traite de pliysiologie, Paris, 1869, tome iii., p. 554. 



8 CHAUVEAU, lieckerches physiologiques sur Vorig'me apparente et sur Vorigine 

 rfalle des nerfs moteurs craniens. Journal de la physiologic, Paris, 1862, tome 

 *., p. 274. 



