638 TEXT-BOOK OF PHYSIOLOGY 



the animal. Various reflexes, e.g., those of mastication, insalivation, degluti- 

 tion, the afferent paths of which are formed in part by the fifth nerve, are 

 often seriously impaired. At the same time the lachrymal secretion dimin- 

 ishes and the pupil contracts. The same results are observed in human 

 beings in whom the nerve has been divided for relief from severe neuralgia. 

 Anesthesia 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 infre- 

 quently 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; and ulceration sets in, which may lead to com- 

 plete destruction of the eyeball. The mucous membrane of the nose be- 

 comes swollen, vascular, and liable to hemorrhage on the slightest irritation. 

 The degenerative changes may lead to a complete loss 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 condition and by the 

 changed vascular supply from division of the vasomotor fibers which join the 

 nerve at or near the ganglion. 



Origin of the Efferent Axons. The efferent axons arise for the 

 most part from nerve-cells located in the gray matter beneath the upper 

 half of the floor of the fourth ventricle. A group of cells known as the 

 superior or accessory nucleus, situated posterior to the corpora quadrigem- 

 ina, gives origin to axons which descend and join the axons from the chief 

 motor nucleus (Fig. 265). 



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 

 inferior maxillary division already described. Its axons are ultimately 

 distributed to the muscles of mastication: viz., the masseter, the temporal, 

 the external and internal pterygoids, the mylohyoid, and the anterior portion 

 of the digastric. A few axons are also distributed to the tensor tympani and 

 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 crus 

 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) 



