THE CEAJvriAL NERVES. 531 



venienfc to arrange them, in two groups, in accordance with the 

 •distinction laid down above, and to describe the nerves of each 

 group in order, from behind forwards. 



Group A. Nerves arising from groups of neuroblasts in the 

 substance of the brain, in the same way as the motor or ventral 

 roots of the spinal nerves. 



To this group belong the third, fourth, and sixth nerves ; 

 the motor root of the trigeminal nerve ; the facial nerve ; the 

 motor roots of the glossopharyngeal and pneumogastric nerves ; 

 and the spinal accessory and hypoglossal nerves. 



Along the spinal cord, the motor roots all leave the cord at 

 the same horizontal level, the sole exception being at the anterior 

 •end of the cervical region, where the hinder roots of the spinal 

 accessory nerve arise at a level dorsal to that of the motor 

 spinal roots. In ■ the brain there are two series of motor 

 roots, a ventral series and a lateral series; the ventral series 

 including the hypoglossal, the sixth, and perhaps the fourth and 

 third nerves as well ; and the lateral series including the 

 anterior roots of the spinal accessory, and the motor roots of the 

 pneumogastric, glossopharyngeal, facial, and trigeminal nerves. 



The hypoglossal, or twelfth cranial nerve (Fig. 227, xii), 

 arises by a long series of roots, each formed by a bundle of axis 

 cylinders which arise as outgrowths from a group of neuroblasts 

 in the ventro-lateral wall of the medulla oblongata (Pig. 228, xii). 

 The roots commence just in front of the motor root of the first 

 spinal nerve, and in line with this, and extend forwards to the 

 level of the glossopharyngeal nerve and the posterior border of 

 the auditory vesicle. 



The mode of origin, and the position and relations of these 

 roots, strongly suggest a comparison with the ventral or motor 

 spinal roots. 



In sheep embryos Proriep describes a dorsal ganglionic root 

 of the hypoglossal nerve, in addition to the ventral roots, so the 

 •comparison with a spinal nerve or nerves seems quite legitimate. 

 In human embryos at the end of the fourth week, and beginning 

 of the fifth week, His has described a small ganglion, which he 

 names Proriep's ganglion (Pig. 227, fg), lying immediately in 

 front of the first cervical ganglion, N.i, and in line with this. 

 Proriep's ganglion is small and gives off no nerves at all, and at 

 a slightly later stage it disappears altogether ; it appears, how- 



