684 



THE NEKVOUS SYSTEM. 



brain, dorsal to the efferent portion of the facial nerve, opposite to the auditory vesicle, 

 and in close association with it. 



Becoming separated from the brain, the cellular mass separates into three portions, of 

 which the intermediate part is associated with the facial nerve and intermediate nerve (as 

 the genicular ganglion), while the medial and lateral parts are converted into the medial 

 (vestibular) and lateral (cochlear) ganglia and the roots of the acoustic nerve. The cells 

 becoming bipolar, their central processes are secondarily connected with the brain on 

 the dorsal (lateral) aspect of the facial nerve ; the peripheral processes proceed to the 

 auditory vesicle, to which they are distributed as the vestibular and cochlear nerves. 

 Numerous cells are carried along with the nerve trunks into relation with the auditory 

 capsule, and constitute the vestibular and cochlear ganglia. 



IX. and X. The glossopharyngeal and vagus nerves are developed from 'the 

 side of the hind-brain, both in the same way, and each by two roots. A collection of 

 cells separates itself from the alar lamina of the hind-brain behind the auditory vesicle to 

 form the ganglionic afferent root. The ganglion of the vagus is much larger than that 

 of the glossopharyngeal. Each ganglion becomes divided into two parts, a proximal and a 

 distal portion, connected together by a commissural band of fibres. The proximal ganglion 

 (superior ganglion of the glossopharyngeal ; j ugular ganglion of the vagus) is secondarily 

 connected by centripetal fibres to the hind-brain. From the distal ganglion (petrous 



LATERAL AREA 



MEDIAL AREA 



(BASAL 

 LAMINA 



f ANTERIOR ROOT 



POSTERIOR ROOT 



. IX.X.XI. _. 



A JD 



FIG. 604. COMPARISON OF ORIGINS OF NERVE ROOTS FROM SPINAL MEDULLA AND HIND-BRAIN (after His). 



A. Spinal medulla ; B. Hind-brain. 



ganglion of the glossopharyngeal ; ganglion nodosum of the vagus) peripheral fibres 

 grow outwards to form the nerve trunk. 



Each nerve is also provided with a small efferent root, consisting of nerve fibres, 

 arising from a collection of neuroblasts in the lateral part of the basal lamina of the hind- 

 brain, and emerging beneath the ganglionic root at the junction of the alar and basal 

 laminae (in series with the fibres of the efferent root of the facial nerve above and of the 

 accessory nerve below). 



XL The accessory nerve arises in two parts one medullary, the other spinal. 

 The medullary (accessory) portion develops as the processes of a series of neuroblasts in 

 the lateral portion of the basal lamina of the hind-brain, which emerge in series with the 

 efferent roots of the glossopharyngeal and vagus nerves. The spinal portion arises 

 as the processes of a group of neuroblasts in the anterior part of the medullary tube 

 (anterior column), which, turning outwards, emerge as a series of roots on the lateral aspect 

 of the spinal medulla. 



XII. The hypoglossal nerve is developed, not in series with the nerves above 

 mentioned, but like the third, fourth, and sixth nerves, from the medial part of the basal 

 lamina of the hind-brain, in the space between the glossopharyngeal and other nerves 

 above, and the first cervical nerve below. It is formed as a series of peripheral processes 

 from a collection of neuroblasts occupying the hind -brain. Froriep's ganglion is a 

 transitory collection of nerve cells developed from the alar lamina of the hind-brain on 

 the dorsal aspect of the nerve, and represents in a rudimentary condition its posterior 

 ganglionic root. The ganglion gives off no branches and soon disappears. 



