THE ENCEPHALIC OR CRANIAL NERVES 639 



the medulla oblongata; (b) through its afferent cortical tracts to the cerebral 

 cortex where they evoke sensations. In common with the dorsal roots 

 of the spinal nerves in general, the trigeminal nerve contains both extero- 

 ceptive and proprioceptive fibers. It therefore endows the skin of the 

 face and associated structures with epicritic, protopathic and deep 

 sensibility. 



The function of the efferent fibers is the transmission of nerve impulses 

 from the cells from which they take their origin, to the muscles of mastication, 

 which are excited to activity by them. The afferent nerves are in relation 

 centrally with the nuclei of origin of the efferent nerves, hence the latter 

 can be excited not only voluntarily but reflexly as in the usual acts of masti- 

 cation. The afferent fibers from the mouth doubtless assist in the reflex 

 secretion of saliva. 



Peripheral stimulation of different areas in the distribution of the 

 afferent fibers, e.g., conjunctiva, nasal and oral mucous membranes, teeth, 

 etc., causes a variety of reflex activities in the muscles associated with the 

 eyes, face, the respiratory and cardiac mechanisms, which indicate that the 

 afferent fibers are centrally in relation with a number of motor nerve- 

 centers. 



THE SIXTH NERVE. THE ABDUCENT 



The sixth cranial nerve, the abducent, consists of peripherally coursing 

 axons which serve to bring the nerve-cells from which they arise into rela- 

 tion with the external rectus muscle. 



Origin. The axons arise from a group of cells located in the gray matter 

 beneath the upper half of the floor of the fourth ventricle. From this same 

 nucleus, however, a small group of fibers emerge which passes upward, 

 and after a short course crosses the median plane of the body and enters 

 the trunk of the motor oculi nerve. These fibers are finally distributed 

 to the internal rectus muscle (Duval and Laborde). This muscle thus 

 receives its innervation from two different sources (see Fig. 266). 



Distribution. The nerve-fibers pass forward from their origin through 

 the gray and white matter and emerge through the groove between the med- 

 ulla oblongata and the pons Varolii just external to the anterior pyramid. 

 The nerve then passes through the sphenoid fissure into the orbit cavity, 

 where it is distributed to the external rectus muscle. In its course the nerve 

 receives filaments from the carotid plexus of the sympathetic. 



Cortical Connections. The nucleus of the sixth nerve is in histologic 

 and physiologic connection with the motor area of the cerebral cortex. From 

 nerve-cells in this region axons are given off which enter the pyramidal 

 tract, descend through the internal capsule and cms cerebri, after which 

 they cross to the opposite side, where their end-tufts arborize around the 

 cells of the nucleus already described. 



Properties. Stimulation of the nerve is followed by spasmodic con- 

 traction of the external rectus muscle and external deviation of the eyeball. 

 Division of the nerve is followed by paralysis or relaxation of the muscle. 

 As a result of the unopposed action of the internal rectus the anterior pole 

 of the eyeball is turned toward the middle line (internal strabismus). In 

 consequence of this deviation there is homonymous diplopia. The images 



