THE CRANIAL NERVES 897 



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 fre- 

 quently, 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 are 

 partly due to the loss of sensation in the eye, and the consequent risk of 

 damage from without, and the unregarded presence of foreign bodies 

 and accumulation of secretion within the lids (p. 778). 



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 is a purely efferent nerve, and supplies the external rectus muscle 

 of the eyeball. Paralysis of it causes internal squint. 



The motor fibres of the seventh or facial nerve arise from a nucleus 

 in the reticular formation of the medulla oblongata,. and running up 

 some distance into the pons. They supply 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 cerebral cortex with the nucleus 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 para- 

 lyzed on the opposite side. Paralysis of the seventh nerve is more 

 common than that of any other nerve in the body. It is often caused 

 by an inflammatory process in the nerve itself (neuritis). The symp- 

 toms of complete facial palsy 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 results in a grotesque contortion. The 

 mouth appears like a diagonal slit in the face, its angle being drawn 

 up on the sound side, and the patient cannot bring the lips sufficiently 

 close together to be able to blow out a candle or to whistle. Liquids 

 escape from the mouth > and food collects between the paralyzed buc- 

 cinator and the teeth. The labial consonants are not properly pro- 

 nounced. Taste may be lost in the anterior two-thirds of the tongue 

 when the nerve is injured above the exit of the gustatory fibres in the 

 chorda tympani, but not when the lesion is in the nucleus of origin, or 

 anywhere above it. Hearing is sometimes impaired because the 

 auditory and facial nerves, lying close together for part of their course, 

 are apt to suffer together., but perhaps also because the stapedius 

 muscle is supplied by the seventh. 



that the fifth nerve supplies no taste fibres, but that the taste fibres for the 

 anterior two-thirds of the tongue have their cells of origin in the geniculate 

 ganglion of the pars intermedia of the seventh nerve, and those for the pos- 

 terior third in the ganglion petrosum of the ninth nerve. 



57 



