206 CENTRAL NERVOUS SYSTEM 



II. The optic nerve conducts impulses from the retina to 

 the pulvinar, the corpora quadrigemina, and the external genic- 

 ulate bodies. In man the fibers for the greater part cross in 

 the chiasma. From the primary centers they are continued to 

 the occipital cortex of the same side, passing through the 

 occipital end of the internal capsule. The location of the 

 centres is probably in the cuneus and surrounding parts. The 

 calcarine fissure has been indicated by some as the most 

 important ending. 



III. The motor oculi is a purely motor nerve. Section para- 

 lyzes the elevator of the upper eyelid, giving rise to ptosis; 

 paralysis of the muscles of the eyeball results in inability to 

 move the eye up, down, or inward, while the unopposed action 

 of the external rectus produces external strabismus; paralysis 

 of the muscle of the iris causes the pupil to remain dilated, so 

 that it does not respond to light; paralysis of the ciliary muscle 

 prevents accommodation. The control of the pupil through a 

 strong voluntary effort exerted through the third nerve shows 

 itself in a contraction, when the eyeball is turned strongly 

 upward and inward. 



IV. The patheticus supplies the superior oblique muscle. 

 Its section results in double vision, and the image seen by the 

 affected eye appears obliquely and below that of the other eye. 

 This may be corrected by inclining the head to the opposite 

 side. 



V. The trigeminus nerve breaks up into three branches; of 

 these, the first and second are entirely sensory, while the third 

 is motor. Section of the motor root of the nerve results in a 

 paralysis of the muscles of mastication. Destruction of the 

 sensory root results in complete anesthesia of the skin of the 

 face and the mucous membrane of the mouth. The anesthesia 

 of the conjunctiva, of the nostrils, and of the lips prevents the 

 reflex self-protection which belongs to the parts, and they be- 

 come easily injured. The nerve cells are located in the Gas- 

 serian ganglion. The peripheral axons extend to the skin, 

 while the central axons upon reaching the bulb divide into a 

 shorter branch, which extends cephalad, and a longer branch, 

 which extends caudad, both connecting with cells in the sub- 

 stantia gelatinosa. One set of neurons is thought to pass direct 

 to the cerebellum. 



