650 MEDULLA OBLONGATA AND THE CRANIAL NERVES 



eyes. Stimuli brought to bear upon its distant receptors, give rise 

 to a large array of reflex actions, such as inhibition of the respiratory 

 movements, closure of the glottis, slowing of the heart-beat, and secre- 

 tion of the tears and saliva. The trigeminus is also said to convey the 

 sensations of taste from the anterior third of the tongue, but it is more 

 than probable that the taste fibers contained in this nerve, have been 

 derived from the glossopharyngeus or nervus intermedins. The 

 sensory fibers of this nerve arise in the Gasserian ganglion in a manner 

 similar to the fibers of the spinal ganglion. Their peripheral branches 

 pass to the sense-organs, while their central branches divide and are 

 arranged as two roots which end (a) in the sensory nucleus situated 

 laterally to the motor nucleus and (6) in a long nucleus which extends 

 through the entire dorsal portion of the medulla. This arrangement 

 enables the impulses to be relayed from the sensory nucleus of this 

 nerve through the median raphe to the cortex of the cerebrum. In 

 addition, collaterals are sent to all the nuclei of the cranial nerves 

 • arising in 'the medulla, with the exception of the nucleus abducens. 

 It should also be mentioned that this nerve communicates with the 

 ganglion sphenopalatinum and ganglion submaxillare which form the 

 outposts of the sympathetic system of this region. 



6. The abducens nerve originates in a nucleus situated below 

 the colliculus facialis, and emerges from the posterior edge of the pons. 

 It is a motor nerve and innervates the external rectus muscle of the 

 eyeball. Like the third and fourth cranial nerves, it is under the con- 

 trol of the will, but not when made to act synchronously with others to 

 produce those movements of the eyeballs which are necessary in 

 binocular vision and accommodation. 



7. The facial nerve arises from a conspicuous nucleus in the teg- 

 mental region of the pons and leaves the brain at the inferior margin 

 of this structure, somewhat lateral to the point of emergence of the 

 sixth nerve. It is chiefly a motor nerve and supplies the muscles of 

 the face, those of a part of the scalp, and those of the ear, inclusive of 

 its intrinsic muscles. As such it governs the expression of the face. 

 This may be gathered from the fact that its division is soon followed 

 by a distortion and a drawing over of the paralyzed side of the face 

 toward the normal. This deviation which eventually may also in- 

 volve some of the bones, is produced by the tonic pull exerted by the 

 muscles of the normal side. In many cases, however, the paralyzed 

 muscles finally show a condition of contracture which then tends to 

 antagonize this pull so that the face again assumes a more normal 

 appearance. Another muscle which takes part in this paralysis is the 

 orbicularis. The inability to close the space between fhe eyelids 

 exposes the cornea to mechanical and thermal influences which in 

 turn give rise to a copious secretion of lacrimal fluid, and possibly 

 also to inflammatory processes. The paralysis of Horner's muscle 

 prevents the offlow of the tears into the nasal cavity. In view of the 

 fact that the facial nerve als6 innervates the muscles which have to do 



