676 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLVI. 



tympani are efferent fibres which reach it from the facial nucleus 

 via the pars intermedia. 



The eighth nerve (auditory) runs into the hinder margin of the 

 pons by two roots. One winds round the restiform body dorsal to 

 it, and is known as the dorsal or cochlear division ; the other passes 

 ventro-mesially on the other side of the restiform body, and is known 

 as the ventral or vestibular division. 



We will take these two parts separately. The fibres of the 

 cochlear nerve take origin from the bipolar nerve-cells of the spiral 

 ganglion of the cochlea; the peripheral axons ramify among the 

 hair cells of the organ of Corti, and the central axons pass towards 

 the pons ; as they enter they bifurcate, and some pass to and arborise 



tub. 



FIBRES TO NUCL.LEMNISCI 

 &CORPORA QUADR1GEMINA 



\S.O. 



NERVE-ENDINGS 



IN ORGAN OF CORTl 



FIG. 416. Cochlear division of the auditory nerve, r, Restiform body ; K, descending root of the fifth 

 nerve; tub.ac., acoustic tubercle; n.acc., accessory nucleus; s.o., superior olive; n.tr., trapezoid 

 nucleus ; n.VI., nucleus of the sixth nerve ; VI., issuing fibre of sixth nerve. (Schafer.) 



around a collection of nerve-cells situated between the two roots and 

 the restiform body, called the accessory auditory nucleus ; the remain- 

 ing fibres terminate similarly in a collection of cells in the grey matter 

 overlying the restiform body, and extending into the ventricular 

 floor in its widest part. This is called the ganglion of the root, and 

 the mass of grey matter is termed the acoustic tubercle. The auditory 

 path is continued by new axons that arise from these cells. Those 

 from the accessory nucleus enter the trapezium, and pass in it partly 

 to the superior olive and trapezoid nucleus of the same side, but 

 mainly to the corresponding nuclei of the opposite side ; some fibres 

 end here, others traverse the nuclei, and merely give off collaterals to 

 them ; they then turn upwards in the lateral fillet, and so reach the 

 inferior C. quadrigemina. The fibres which arise in the acoustic 



