822 A MANUAL OF PHYSIOLOGY 



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. 699) . 



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 paralyzed 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 symptoms of complete facial palsy are 

 very characteristic. The face and forehead on the paralyzed side 

 are smooth, motionless, and devoid of expression. The eye re- 

 mains 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 

 buccinator and the teeth. The labial consonants are not properly 

 pronounced. 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. 



The seventh nerve is not purely motor. From the cells of a ganglion 

 on it corresponding to a spinal ganglion (the geniculate ganglion) 

 afferent fibres arise, which pass in the pars intermedia or nerve of 

 Wrisberg into the pons between the seventh and eighth nerves, and 

 there bifurcate into ascending and descending branches, like other 

 afferent fibres originating in ganglia of the spinal type. The descending 

 branches enter the fasciculus solitarius, and end by arborizing around 



trigeminus, or to degeneration and swelling of the trigeminal fibres in the 

 lingual nerve and consequent interference with the conductivity of the 

 intermingled chorda tympani fibres. Gushing believes 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 posterior 

 third in the ganglion petrosum of the ninth nerve. 



