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A TEXTBOOK OF PHYSIOLOGY 



the muscles of expression of the face. Paralysis of the nerve leads 

 to a characteristic " facies " an expressionless, vacant look. In 

 addition, the stapedius muscle of the ear and certain muscles of the 

 scalp are also supplied. 



Certain afferent fibres belonging to the nerve of Wrisberg also 

 run in the seventh nerve. Their cell-station is in the geniculate 

 ganglion. The fibres passing inwards from the ganglion divide into 

 ascending and descending branches, the latter connecting with the 

 ninth nucleus. Peripherally, the fibres pass into the large superficial 

 petrosal nerve and the chorda tympani nerve, and thence to the 

 fifth nerve, furnishing the sensation of taste to the anterior two-thirds 

 of the tongue. From the nerve of Wrisberg also come secretory 

 fibres which go to supply the submaxillary and sublingual glands 

 through the chorda tympani nerve. 



The sixth nerve arises from a group of cells situated on either side 

 of the middle line just below the floor of the upper part of the fourth 

 ventricle (Fig. 428). The fibres form the motor nerve to the external 

 rectus muscle of the eyeball. From the sixth nucleus other fibres ascend 

 in the posterior longitudinal bundle, to emerge with the third nerve 

 and supply the internal rectus muscle. The oculo-niotor nerves, 

 sixth, fourth, and third, contain muscle-sense fibres, the ganglion 

 cells of which are to be found in the nerve trunks. 



The fifth or trigeminal nerve has three nuclei in the medulla: 

 one connected with the central connections of the sensory cells of the 

 Gasserian ganglion the principal sensory nucleus of the fifth ; and two 

 connected with the motor functions of the nerve the chief and the 

 accessory motor nuclei (Fig. 429). The afferent fibres of the nerve 

 enter the pons, and bifurcate into ascending and descending branches, 

 The ascending pass to the principal sensory nucleus, which lies just 

 laterally to the principal motor nucleus. The cells of this nucleus 

 give rise to fibres most of which cross the middle line and pass in 

 the mesial fillet, to end in the optic thalamus. Some fibres ascend 

 in the mesial fillet of the same side. The descending fibres form a well- 

 marked tract, which descend in the reticular formation (the descending 

 branch of the fifth) into the cervical part of the spinal cord. In its 

 course it forms connections with the motor nuclei of the pons and 

 medulla. It lies in close association with the substantia gelatinosa. 

 Rolandi. 



The nerve acts as the nerve of common sensation to the fact-. 

 eyeball, nose, and mouth. The fibres connected with the gustatory 

 nerve-endings in the anterior two-thirds of the tongue run in this 

 nerve. 



The motor fibres, which form but a small part of the nerve, arise 

 chiefly from the principal motor nucleus, which is situated laterally 

 below the floor of the fourth ventricle (Fig. 429). Some arise from the 

 accessory nucleus, which is situated higher in the pons, and in part in 

 the mid-brain (see Fig. 429). The fibres are motor to the muscles of 

 mastication, the tensor palati, the tensor tympani. and the anterior 

 belly of the digastric muscles. 



