CRANIAL NERVES. 823 



motor root of the fifth, the seventh, the motor root of the vagus, 

 the glosso-pharyngeal, and the hypoglossal decussate partially. The 

 pathetic decussates completely in the valve of Vieussens. The last- 

 named nerve springs from the oculomotor nucleus united with that 

 of the pathetic. These portions of gray matter are a direct part 

 of the anterior horn of the spinal cord lying beneath the aqueduct 

 of Sylvius. 



In Chapters XVII and XIX were considered the olfactory, or 

 first pair of cranial nerves, and the optic, or second pair; so that in 

 this chapter there will be taken up, first, the motor oculi, or third 

 pair of cranial nerves. 



THIRD PAIR, OR MOTOR OCULI NERVE. 



This nerve arises from a nucleus situated between the corpora 

 quadrigemina and beneath the floor of the aqueduct of Sylvius. 

 Beneath the posterior end of the anterior corpus quadrigeminum 

 this nucleus becomes continuous with the nucleus of the trochlearis 

 or patheticus. The oculomotor nuclei consist (1) of a group of 

 cells concerned in accommodation; (2) those concerned in the reflex 

 action of the iris to light; (3) the innervation of all the muscles of 

 the eye except the external rectus and superior oblique. The 

 neuraxons of these cells pass by and through the red nucleus and 

 emerge at the inner side of the cerebral crura, to pass through the 

 interpeduncular space along the outer boundary of the cavernous 

 sinus ; they then enter the sphenoidal fissure, and go to the muscles 

 of the eyeball, except the external rectus and superior oblique. It 

 also gives fibers to the ciliary muscle and the sphincter of the pupil 

 and a branch to the elevators of the upper lid. 



The posterior longitudinal bundle is also connected with the 

 nuclei of the third, fourth, and sixth nerves. The oculomotor 

 nucleus also has a connection with the optic neurons in the anterior 

 corpora quadrigemina. In the cavernous sinus it receives filaments 

 coming from the carotid branches of the great sympathetic and a 

 branch from the ophthalmic of the trigeminus. 



Functions. From a functional point of view, it may be said that 

 the motor oculi is devoted exclusively, in conjunction with the fourth 

 and sixth pairs of nerves, to making the sight perfect. With these 

 nerves it concurs to regulate the varied movements which allow the 

 eye to act as a telescope upon a support that is furnished with 

 numerous articulations. By means of these muscles and nerves of 

 the orbit the individual is enabled to remove the visual field from 



