THE CRANIAL NERVES 



925 



ihe principal motor nucleus. The descending branches, turning down- 

 wards into the medulla oblongata, terminate in a long tract of scattered 

 cells, constituting with the fibres the so-called spinal root, and extending 

 from the level of the second cervical nerve through the medulla oblon- 

 gata and the pons, where it is continued into the principal sensory nucleus. 

 The afferent path is continued by the axons 

 of cells of the sensory nuclei (or nuclei of 

 reception) of the nerve, many of which cross 

 the middle line and enter the intermediate 

 fillet of the opposite side, and also the 

 special ascending bundle going to the thala- 

 mus. Some of the axons do not decussate, 

 but ascend in the fillet of the same side. 



'Nu.m.pr.n. V. 



Fig. 371. Scheme of Motor and Sensory Neurons of Trigeminus (Gehuchten).- 

 G. s. G., Gasserian ganglion; Nu. m. m. n. V., nucleus of the descending root; 

 Nu. m. pr. n. V ., chief motor nucleus of the fifth nerve; Rad. desc. mes. n. V.. 

 accessory motor nucleus, sometimes called the descending root; Tr. sp. n. V.. 

 tractus spinalis, or spinal root of the fifth. 



The motor fibres of the fifth nerve supply the muscles of mastication 

 and the tensor tympani. The sensory fibres confer common sensation 

 on the face, conjunctiva, the mucous membranes of the mouth and nose, 

 and the structures contained in them, and, according to Gowers, special 

 sensation, through branches given off to the facial and glosso-pharyngeal 

 nerves, on the organs of taste.* Complete paralysis of the nerve causes 



* It should be stated that some physiologists believe that the glosso-pharyn- 

 geal is the nerve of taste, and that none of the taste fibres go to the sensory 

 nuclei of the fifth nerve. The majority hold that the glosso-pharyngeal 

 supplies the posterior third, and the chorda tympani and lingual the anterior 

 two-thirds of the tongue with gustatory fibres. The removal of the Gasserian 

 ganglion and the adjacent portion of the fifth nerve for severe and persistent 

 neuralgia, has afforded opportunities to test this question. But, unfortunately 

 the results described by various observers do not agree, some finding that 

 taste is unimpaired, others that it is abolished. Gowers states that the gus- 

 tajory sensations may persist for some time after the operation, although 

 ultimately (in two or three weeks) they disappear. It has been shown by 

 Harris that when alcohol is injected into the third division of the fifth nerve 

 at the foramen ovale or into the Gasserian ganglion, immediate loss of taste 

 develops in a very large proportion of cases in the anterior half of the tongue 

 on that side. Taste fibres are therefore certainly present in the fifth nerve 

 at this level. 



