380 



PHYSIOLOGY 



(originally splanchnic or branchial, now typically somatic), and the motor 

 nucleus of the fifth nerve with its prolongation into the mid-brain. 



(4) Sensory Somatic Nuclei. The chief representative of this group 

 is the great sensory root of the fifth nerve. The fibres of this nerve arise 

 from the Gasserian ganglion, pierce the fibres of the pons Varolii, and run 

 to the dorso- lateral part of the pons, where they divide into ascending and 

 descending fibres. These fibres form a cap to the substantia gelatinosa, 

 the descending branches, which are longer, being conspicuous in sections of 



tub. 



FIBRES TO NUCL.LEMWSCI 

 &CORPORA QUAORIGEMINA 



S.O. 



I'/. 



NERVE-ENDINGS 



IN ORGAN OF CORTI 



FIG. 193. Plan of the course and connections of the fibres forming the cochlea r 



root of the auditory nerve. (SCHAFER.) 



r restiform body; V, descending root of the fifth nerve; tub.ac, tuberculum 

 acusticum ; n.acc, accessory nucleus ; s.o, superior olive ; n.tr, nucleus of trape- 

 zium ; n.VI, nucleus of sixth nerve ; VI, issuing root-fibre of sixth nerve. 



the medulla as low down as the first or second cervical nerve. This nerve 

 gives common sensation to practically the whole of the head. 



It is doubtful in what group we should place the fibres of the eighth 

 nerve. This nerve really consists of two parts very different in function, 

 the cochlear or auditory nerve, and the vestibular or labyrinthine nerve. 

 The fibres of each are derived from ganglion- cells in the internal ear, pass to 

 the medulla at its widest part, and then, dividing into two, terminate in 

 masses of grey matter situated at the extreme lateral part of the floor of the 

 fourth ventricle. 



The branches of the cochlear nerve (Fig. 193) make connection with two 

 collections of cells, the dorsal nucleus, apparently embedded in the fibres of 

 the root itself, and the accessory nucleus, a little triangular mass of grey 

 matter situated in the angle between the cochlear and vestibular nerves. 

 From these nuclei fibres are given off which take two courses. Some, follow- 

 ing the previous course of the cochlear nerve, pass across the surface of the 

 fourth ventricle as the striae medullares or stria acousticce, and then bending 

 inwards pass into the tegmentum of the opposite side. Others pass deeply 

 and form a mass of transverse fibres in the ventral part of the tegmentum, the 



