134 NERVOUS SYSTEM. 



remains dilated and does not respond to light ; and by the paraly- 

 sis of the ciliary muscle the accommodation of the lens for near 

 vision is prevented. The control of the pupil is not a voluntary 

 one ; but the effect of a strong voluntary effort, exerted through 

 the third nerve, shows itself in contraction of the pupil, as when 

 the eyeball is turned strongly inward and upward. 



Describe and give the function of the patheticus or fourth cra- 

 nial nerve. 



It arises close by the third nerve beneath the aqueduct of Syl- 

 vius, and emerges, after decussation, from the valve of Vieussens. 

 Thence, passing around the crus cerebri, it runs parallel with the mo- 

 tor oculi (third) nerve to the orbit, where it*is supplied to the supe- 

 rior oblique muscle. Its paralysis prevents the muscle from main- 

 taining the horizontal plane of the eyeball. If this paralysis occurs, 

 there is double vision, and the image seen by the affected eye 

 appears oblique and inferior to the image of the other eye. This 

 may be corrected by inclining the head to the opposite side. This 

 nerve is also known as the trochlearis or trochlear nerve. 



What is the course and function of the sixth or abducens nerve ? 

 It arises from a nucleus of gray matter in the floor of the fourth 

 ventricle, and its nucleus is connected with those of the third, fourth, 

 and seventh nerves. It emerges without decussation at the poste- 

 rior border of the pons Varolii, and passes forward to the orbit 

 with the third and fourth nerves. In its course it has many com- 

 munications with the sympathetic nerves, but their significance is 

 unknown. It is supplied to the external rectus muscle of the eye, 

 and its stimulation causes external squint, and paralysis causes 

 internal. 



What is the origin of the trigeminus or fifth nerve ? 



This nerve resembles the spinal nerves in having a motor and a 

 sensory root, the latter possessing a ganglion (Gasserian). The 

 origin of the nerve seems to be in centres, separate for motor and 

 sensory, in the floor of the fourth ventricle. There are fibres whicli 

 join the trunk of the nerve which are derived from the spinal cord 

 and from the cerebellum. It emerges from the pons Varolii as two 

 distinct nerve-roots. The larger of the two, the sensory, soon enters 

 the Gasserian ganglion, the motor root passing beneath without 

 communication. The nerve then breaks up into three branches : 

 of these the first and second are formed entirely from the sensory 



