540 TEXT-BOOK OF PHYSIOLOGY. 



dences of pain on the part of the animal. Various reflexes, e. g. t 

 those of mastication, insalivation, deglutition, the afferent paths of 

 which are formed in part by the fifth nerve, are often seriously im- 

 paired. At the same time the lacrimal secretion diminishes and the 

 pupil contracts. The same results are observed in human beings 

 in whom the nerve has been divided for relief from neuralgia. Anes- 

 thesia or a loss of sensibility may also be caused by pathologic lesions 

 of the nerve-trunks or of the sensor end-nuclei. 



Division of the large root at or near the ganglion of Gasser has 

 not infrequently been followed by an alteration in the nutrition of 

 the eye and nose. In the course of twenty-four hours the eye becomes 

 vascular and inflamed; the cornea becomes opaque; ulceration sets 

 in which may lead to complete destruction of the eyeball. The 

 mucous membrane of the nose becomes swollen, vascular, and liable 

 to hemorrhage on the slightest irritation. The degenerative changes 

 may lead to a complete loss of the sense of smell. These results were 

 formerly attributed to a loss of trophic influence which it was believed 

 the nerve exercised over these structures. Modern experimentation 

 and various surgical procedures have demonstrated that the nutritive 

 disorders are septic in origin, made possible by the anesthetic condi- 

 tion and by the changed vascular supply from division of the vaso- 

 motor fibers which join the nerve at or near the ganglion. 



Origin of the Efferent or Motor Axons. The efferent or 

 peripherally coursing axons serve to bring the nerve- cells from which 

 they arise into relation with the muscles of mastication. They arise 

 from nerve-cells located in the gray matter beneath the upper half 

 of the floor of thfi fourth vpntnVlp. These cells constitute the motor 

 or masticatory nucleus. A group of cells known as the superior or 

 accessory nucleus, situated posterior to the corpora quadrigemina, 

 give origin to axons which descend and join the axons from the chief 

 motor nucleus (Fig. 247). 



Distribution. From their origin the fibers pass forward through 

 the pons and emerge on its lateral aspect, forming the so-called small 

 root of the fifth nerve. This then passes forward beneath the ganglion 

 of Gasser, leaves the cavity of the skull through the foramen ovale, 

 and joins the infprjnr^ m^ijlary division already described. Its 

 axons are ultimately distributed to the m\isclesjofjnastication : viz., 

 the masseter, the temporal, the external and inTeTnaTJIej^gQids, the 

 mylohvoid, and the anterior portion of the digastric. A few axons 

 are also distributed to the tensor tympani muscle (Fig. 250). 



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 in- 

 ternal capsule and the crus cerebri, after which they cross to the 



