722 A MANUAL OF PHYSIOLOGY 



tion 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 Cowers, special sensation, through branches given off to the facial and 

 glosso-pharyngeal nerves, on the organs of taste.* Complete paralysis 

 of the nerve causes loss of movement in the muscles of mastication, 

 sometimes impaired hearing, and loss of common sensation in the 

 area supplied by it. Loss or impairment of taste in the corresponding 

 half of the tongue is also often seen in disease involving the sensory 

 root, although not in affections of the trunk of the nerve, since the 

 taste fibres leave it near its origin (Cowers). Both taste and touch 

 are lost in the monkey in the anterior two-thirds of the tongue after 

 intracranial section of the trigeminus (Sherrington). 



Vaso-motor changes are occasionally, and * trophic ' changes 

 frequently, observed in disease of the fifth nerve. The trophic 

 disturbance is most conspicuous in the eyeball (ulceration of the 

 cornea, going on, it may be, to complete disorganization of the eye). 

 These effects seem to be partly due to the loss of sensation in the 

 eye, and the consequent risk of damage from without, and the un- 

 regarded presence of foreign bodies and accumulation of secretion 

 within the lids. 



The sixth or abducens nerve takes origin from a nucleus in the 

 floor of the fourth ventricle at the level of the posterior portion of the 

 pons. It supplies the external rectus muscle of the eyeball. Paralysis 

 of it causes internal squint. 



The seventh or facial nerve arises from a nucleus in the reticular 

 formation of the medulla oblongata, and running up some distance 

 into the pons. It supplies the muscles of the face ; and when these 

 are greatly developed, as in the trunk of the elephant, the nerve 

 reaches very large proportions. Since the fibres which connect the 

 nucleus with the cerebral cortex decussate about the middle of the 

 pons, a lesion above this level which causes hemiplegia paralyzes 

 the face on the same side as the rest of the body, i.e., on the side 

 opposite the lesion. But the paralysis is confined to the muscles of 

 the lower portion of the face, and affects especially the muscles about 

 the mouth. Sometimes the pyramidal tract and the facial nerve, or 

 nucleus, are involved in a common lesion. In this case paralysis of 

 the face is on the side of the lesion, and is total, while the rest 

 of the body is paralyzed on the opposite side. Complete facial 

 paralysis is often caused by an inflammatory process in the nerve 

 itself (neuritis). The symptoms of complete facial paralysis are very 

 characteristic. The face and forehead on the paralyzed side are 

 smooth, motionless, and devoid of expression. The eye remains 

 open even in sleep, owing to paralysis of the orbicularis palpebrarum, 

 A smile becomes a grimace. An attempt to wink with both eyes 



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

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

 the sen>ory nuclei of the fifth nerve. Others suppose that the glosso- 

 pharyngeal supplies the posterior third, and the chorda tympani and 

 lingual the anterior two-thirds of the tongue with gustatory fibres. 



