THE ENCEPHALIC OR CRANIAL NERVES 645 



and forward and enters the tympanum through the iter chordae posterius, 



crosses the tympanic membrane between the malleus and incus, leaves 



the tympanum by the iter chordae anterius or canal of Huguier, and 



finally joins the lingual branch of the fifth nerve. Some of its fibers can 



be traced to the mucous membrane of the dorsum of the tongue, others 



to the submaxillary ganglion with which they become associated. 



The determination of the origin, course, and functions of the chorda 



tympani nerve has given rise to many investigations and discussions, and it 



cannot be said that the results thus far attained are as satisfactory as might 



be desired. 



If the chorda tympani nerve is divided there follows a contraction of 

 the blood-vessels in the neighborhood of and a diminution in the secretion 

 from the submaxillary and sublingual glands. Stimulation of the peripheral 

 end of the divided nerve gives rise to a dilatation of the blood-vessels and an 

 increased production and discharge of saliva from these glands (see page 

 153). From these results it is certain that the chorda tympani contains both 

 vaso-dilatator and secretor fibers. Nicotin appliedr-to the submaxillary 

 and sublingual ganglia abolishes the effects of stimulation of the chorda 

 tympani. It does not prevent the same effects when the ganglia themselves 

 are stimulated. It is clear, therefore, that the vaso-dilatator and secretor 

 fibers arborize around the cells of the ganglia and are not distributed directly 

 to the gland structures. It is highly probable that the vaso-dilatator and 

 secretor fibers in the chorda tympani are the continuations of the efferent 

 fibers found in the pars intermedia and that they too have their origin in the 

 nucleus salivatorius. 



If the nerve be divided as it crosses the tympanic cavity or before it unites 

 with the lingual branch of the fifth nerve, there follows a loss of taste in the 

 anterior two-thirds of the tongue on the corresponding side, though the sensi- 

 bility remains unimpaired. For this and other reasons, the chorda tympani 

 has long been regarded in part as the nerve of taste for this region. 



The Taste Fibers. The nerve-fibers subserving the sense of taste are 

 believed to be the peripherally coursing fibers which have their origin in 

 the nerve-cells of the geniculate ganglion and which descending in the 

 aqueduct of Fallopius are continued in the chorda tympani as far forward 

 as the mucous membrane of the anterior two- thirds of the tongue. The 

 nerve impulses developed in the peripheral terminations of this nerve by 

 the action of organic matter in solution are transmitted through the 

 chorda tympani, along the facial nerve as far as the geniculate ganglion. 

 The exact pathway for these afferent or gustatory fibers beyond the genicu- 

 late ganglion has long been a subject of much discussion. According to 

 some observers these fibers enter the great petrosal nerve, pass forward as 

 far as the spheno-palatine ganglion, then into the superior maxillary division 

 of the trigeminal, and so to the brain. According to others, these fibers 

 pass into the pars intermedia, into the pons, where they terminate around 

 the sensor end-nucleus of the glosso-pharyngeal. The evidence for and 

 against either of these two views is most conflicting and insufficient to justify 

 positive statements one way or the other. To the writer the weight of evi- 

 dence seems to favor the view that the gustatory fibers have their origin in the 

 geniculate ganglion; that they pass centrally through the pars intermedia; 

 that they are similar in function to the glosso-pharyngeal; and that they are 

 indeed but aberrant branches of this nerve. 



