766 



THE XERVOrS SYSTEM 



The accessory nucleus (lateral or ventral nucleus) is a group of cells chiefly 

 contained witliin the lateral root, but also occupying the angular interspace between 

 the lateral and the mesial roots. It presents marked affinities to a spinal nerve- 

 ganglion, particularly in the character of the nerve-cells in its anterior (sui)erior) 

 part. Some of the fibres which spring from it pass tlirough tlie trapezium to the 

 tillet of the opposite side, l)y means of which they reacli the posterior C[Uadrigemi- 

 nate body, whilst others terminate in the upper olive of the same and the op])osite 

 sides. 



Lateral root. — The lateral root arises chiefly from the cells of the accessory 

 nucleus, but a certain number of fibres are believed to pass by that nucleus with- 

 out interruption. Both sets of fibres travel in a tolerably compact bundle around 

 the outer side of the restiform body. A certain number take origin in the cells of 

 the tuberculum acusticum, the others cross the floor of the fourth ventricle, as striae 

 mcdullares, and, having crossed the middle line, take origin partly in the so-called 

 nucleus funiculi teretis of the opposite side, and pass partly into the conductor 

 sonorus. The origin of the conductor sonorus is unknown; its course has l)een 

 noticed in the description of the fourth ventricle (page 719). 



Mesial root. — This root passes to the inner side of the restiform body and has 



Fig. 450. — Transverse Section of the Pons, passino TiiRorciu the most 

 Distal of the Stri.e Meduli.ares. (Kiause. ) 



STRIA 



MEDULLARIS 



NUCLEUS OF X— 



HYPOGLOSSAL ,is 



VENA CENTRALIS 



OLIVARY NUCLEUS 



NUCLEUS PONTIS 



CONTINUA TION OF 

 PYRAMIDAL BODY 



CHIEF NUCLEUS OF 

 A UDITOR Y NER VE 



RESTIFORM BODY 



SUBSTANTIA 

 GEL A TINOSA 



A CCESSOR Y NUCLEUS 



MESIAL ROOT 



no connection with the ventral nucleus; most of its fibres terminate in the small- 

 celled, chief, or dorsal nucleus, l)ut some probably pass directly to the cerel)ellum. 



There appears to be but little doubt that the superior temporal convolution is 

 the cortical centre for hearing (Ferrier). According to Obersteiner, the probable 

 links between the nerves of hearing and the cortical acoustic field are as follow: 

 lateral root, accessory nucleus, superior olive, lateral fillet, posterior quadrigeminal 

 l^ody, internal geniculate body, temporal lobe. 



The two roots unite at the ventral l)order of the restiform Ixxiy, and the nerve 

 thus formed, proceeding upwards, outwards, and forwards, enters the internal audi- 

 tory meatus, near the outer end of which it divides into an anterior or upj)er, and a 

 posterior or lower l)ranch. In its course througli the meatus, it is accom})anied by 

 the facial nerve, the pars intermedia, and the internal auditory artery. These 

 structures are all contained in a common sheath of arachnoid. \\'ithin the meatus 

 the auditory nerve becomes flattened, and it is curved in such a maimer as to form 

 a half-cylinder, open above. The pars intermedia lies in the concavity of this half- 

 cylinder, and above this is the facial nerve, the three nerves being connected by a 

 lax connective tissue. This connective tissue has often been mistaken for nervous 

 comnnmications. (Testut. ) 



