412 PHYSIOLOGY 



descending branches, which are much longer, are collected into one or more 

 bundles which pass downwards in the lateral part 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 mesencephalic 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 

 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 ulceration 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. 



The fifth is also sa.id to be the nerve of taste for the anterior third of the 

 tongue, but it is probable that the taste fibres which run in the fifth are 

 derived from the glossopharyngeal or from the nervus intermedius. 



The eighth nerve and its connections have been discussed already on 

 several occasions. We may here briefly summarise what has already been 

 stated. In describing the eighth nerve it is necessary to consider separately 

 its two divisions, the dorsal or cochlear division and the ventral or vestibular 

 nerve. The fibres of the cochlear nerve originate in the bipolar cells of the 

 spiral ganglion of the cochlea. They carry impulses from the auditory end- 

 organ. On entering the medulla they bifurcate into ascending and descend- 

 ing branches which terminate in two nuclei, the ascending branches in the 

 ventral nucleus, the descending branches in the dorsal nucleus. The ventral 

 or accessory nucleus lies between the cochlear and vestibular divisions ven- 

 trally to the restiform body. The dorsal nucleus, often called the acoustic 

 tubercle, forms a rounded projection on the lateral and dorsal aspects of the 

 restiform body. From these two nuclei new relays of fibres start, pass to the 

 other side, by crossing the median raphe (where they form the trapezium) to 

 run up in the lateral fillet of the opposite side. From the ventral nucleus 

 the fibres pass directly to the opposite side, forming the greater part of the 



