CONNECTIONS AND FUNCTIONS OF CRANIAL NERVES 465 



organs and the afferents of the third, fourth, and sixth nerve pairs 

 themselves. 



The fifth nerve, or trigeminus, resembles a spinal nerve in that 

 it has a motor as well as a. sensory root. The motor root is much 

 the smaller of the two. The fibres of the sensory root take their 

 origin in the cells of the Gasserian ganglion, which is in all respects 

 similar to the ganglion of a posterior spinal nerve-root. The sensory 

 root represents the somatic afferent part of all the motor cranial 

 nerves from the third to the hypoglossal and has a correspondingly 

 wide field of ending in the brain stem. The afferent fibres of the 

 fifth nerve, as they enter the pons, bifurcate, like a spinal afferent 

 nerve, into ascending and descending branches. The ascending- 

 branches are short and pass to an upper sensory nucleus, situattd 

 below the lateral part of the fourth ventricle in the upper part of the 

 pons. The descending branches, which are much longer, are collected 

 into one or more bundles which pass downwards in the lateral pait 

 of the reticular formation, accompanied by the downward extension 

 of the sensory nucleus known as the substantia gelatinosa. The 

 descending root can be traced down in the upper part of the cervical 

 cord, its fibres in this region forming a cap to the gelatinous substance 

 of Rolando. From the cells of the sensory nucleus fibres pass towards 

 the median raphe, crossing to the other side to take part in the 

 formation of the tract of the fillet (the trigemino-thalamic tract). 

 The efferent fibres forming the motor root arise from two nuclei. 

 The chief motor nucleus consists of large pigmented multipolar cells 

 situated just below the surface of the lateral margin of the fourth 

 ventricle at the upper part of the pons. The accessory or mesence- 

 phalic nucleus is composed of large unipolar cells, situated in the 

 central grey matter along the lateral aspect of the anterior end of the 

 fourth ventricle, and in a corresponding position in the mid-brain as 

 far as the upper border of the inferior corpora quadrigemina. 



The fifth nerve is the motor nerve for the muscles of mastication, 

 and for the tensor tympani and tensor palati muscles. It is the 

 sensory nerve for the whole of the face (including eyeball, mouth, 

 and nose). It also contains dilator fibres to blood-vessels derived 

 from the chorda tympani, and is said to have trophic functions. The 

 latter conclusion is from the fact that section of the fifth nerve in the 

 skull is followed by ulceration and sloughing of the cornea, and 

 finally by destructive changes involving the whole eyeball. Since, 

 however, these results may be prevented by carefully shielding the eye 

 from all dust and deleterious influences, it is probable that the ulcera- 

 tion is merely a secondary consequence of the anaesthesia. The cornea 

 being anaesthetic, foreign objects that fall on its surface are allowed 

 to remain there, and so give rise to injurious changes and ulceration. 



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