78 



CRANIAL NERVES. 



sensory root of the trigeminus, which thus forms the white bundle known as the 

 ascending root of the fifth, but which in its actual growth is descending ; and also 

 with the afferent fibres of the glosso-pharyngeal and vagus which grow downwards 

 in the medulla to form the so-called solitary bundle. 1 These are traceable in the 

 adult as far down as the middle and lower cervical region respectively, but in the 

 embryo are at first quite short, and limited to near the place of attachment of the 



XIL 



Fig. 90, A and B. SECTIONS ACROSS THE HIND-BRAIN OF A HUMAN EMBRYO, 10mm. long. (His.) <{\ 



In A, the origin of the spinal accessory and hypoglossal nerves is shown, the fibres of both arising 

 from groups of neuroblasts in the basal lamina of the neural tube. In B, one of the roots of the hypo- 

 glossal is still seen, and in addition the root of the vagus nerve. This is represented as in part arising 

 like that of the spinal accessory in A, from a group of neuroblasts in the basal lamina, and in part from 

 a bundle of longitudinally coursing fibres placed at the periphery of the alar lamina, and corresponding 

 in situation to the commencing posterior white columns shown in fig. 87. 



nerve roots, becoming gradually longer as development proceeds. The solitary bundle 

 is at first superficial, like the ascending root of the fifth, but it subsequently becomes 

 covered in by the bending over of the alar plate, and the formation of nervous sub- 

 stance in this. The ganglion-rudiments from which the ingrowth of these afferent 

 fibres takes place, become the Gasserian ganglion of the fifth, and the jugular 

 ganglia of the glosso-pharyngeal and vagus. 



The auditory nerve-roots appear also to be formed by an ingrowth from the cells 

 of its ganglion-rudiment into the alar plate. Subsequently the ganglion-rudiment 

 becomes subdivided into three parts, one forming an intracranial ganglion, and the 

 others giving rise to the branches of the nerve to the cochlea and vestibule respectively 

 (fig. 90, C). The part belonging to the cochlea (ganglion cochleae) forms ultimately 

 the spiral ganglion ; while the one on the vestibular branch forms the gangliform 

 swelling of Scarpa. From a separated part of the ganglion cochleae the nerve to the 

 posterior semicircular canal passes, as well as that to the macula of the saccule ; 

 from the vestibular ganglion the nerves to the other ampullae and to the utricle, are 

 derived. The geniculated ganglion of the facial is derived from the same ganglionic 



1 These bundles, therefore, are homologous with the rudiments of the posterior white columns 

 which are derived from the ingrowing fibres of the spinal ganglia (v. antea, p. 74). 



