THE CRANIAL NERVES. 589 



tensor palati muscles. The efferent or peripherally coursing axons thus serve 

 to bring the nerve-cells from which they arise into relation with the muscles 

 of mastication. 



Cortical Connections. The nuclei of origin of the small root are in 

 histologic and physiologic relation with the lower third of the motor area 

 of the cerebral cortex. Nerve-cells in this region give off axons which enter 

 the pyramidal tract, descend through the internal capsule and the cms 

 cerebri, after which they cross to the opposite side. Their end-tufts arbor- 

 ize around the cells of nuclei in the medulla oblongata. 



Properties. Stimulation of the small root gives rise to convulsive move- 

 ments of the muscles of mastication. Division of the nerve is followed by 

 a paralysis of these muscles. Contraction or paralysis of the tensor tympani 

 and tensor palati muscles would also be observed under the same conditions. 



Functions. The function of the afferent fibers of the fifth nerve is 

 the transmission of nerve impulses from its peripheral distribution to (a) 

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

 cortex where they evoke sensations. The nerve therefore endows all the 

 parts to which is it distributed with 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. 



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. It is quite prob- 

 able that a few fibers in each nerve-trunk come from the nucleus on the 

 opposite side of the middle line. 



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 (Fig. 276). 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 



