THE CEANIAL NEKVES. 535 



To this group belong the sensory roots of the pneumogastric 

 and glossopharyngeal nerves ; the auditory nerve ; the sensory 

 root of the trigeminal nerve ; and probably the olfactory nerve 

 as well. 



In the head there are four primary ganglion masses, those 

 of the fifth, eighth, ninth, and tenth cranial nerves. Whether 

 these are connected in the early stages, to form a continuous 

 neural ridge along each side, has not yet been ascertained ; 

 neither has the precise mode in which the permanent connection 

 of these ganglia with the brain is acquired been determined. 



The four ganglionic masses are clearly visible at the end of 

 the third week. During the fourth week they become gradually 

 divided up, each giving rise to two or more ganglia, which*, by 

 further elongation of the connecting nerve strands, move apart 

 to a greater or less distance from one another. 



The sensory root of the pneumogastric, or tenth cranial nerve, 

 is from the first in close relation with the motor root, being at- 

 tached to the brain immediately dorsal to this latter (Figs. 227 

 and 228, x.s). The ganglion is at first single, but by the end of 

 the fourth week it becomes divided into a proximal and smaller 

 part, the ganglion of the root ; and a distal, larger, and fusiform 

 part, the ganglion of the trunk (Fig. 227). In the later stages 

 these two ganglia move some distance apart, owing to lengthening 

 of the nerve trunk between them. The ganglion of the root is 

 connected with the distal, or petrous, ganglion of the glosso- 

 pharyngeal nerve by an oblique commissural band, well seen in 

 Fig. 227 : whether this is a persistent remnant of an originally 

 continuous neural ridge has not been determined. 



By the end of the fourth week, the superior and inferior 

 laryngeal nerves are present, and also a large branch extending 

 down the oesophagus towards the stomach. 



The sensory root of the glossopharyngeal, or ninth cranial 

 nerve, is very similar to that of the pneumogastric, but of smaller 

 size. The ganglion early divides into a proximal 'jugular' 

 portion, and a distal * petrous ' portion (Fig. 227). The nerve 

 itself is straight in the early stages, but becomes curved forwards 

 at its ventral end (Fig. 227) as the first branchial arch, with 

 which it is specially associated, is carried forwards along the 

 inner side of the hyoid arch (cf. Fig. 240). 



The auditory nerve. In the case of the auditory ganglion of 



