CEREBRAL NERVES 



127 



Cerebral nerves. Exclusive of the olfactory and optic, which must be 

 included in a different category from the others, and will be treated of with the sense- 

 organs to which they belong, the cerebral nerves divide themselves into a group 

 of pure motor nerves, the hypoglossus, abducens, trochlearis, and oculo-motorius, 

 and a group of mixed sensory and motor nerves, the vagus and accessorius 

 (vagus complex), the glossopharyngeus, the acousticus, facialis, and trigeminus. 



The motor roots in both groups spring from the basal lamina of the neural 

 tube, but the nuclei of origin appear in two series, a mesial and a lateral. The 

 distinction is well marked as far forwards as the isthmus, but in that portion of the 

 tube and in the mesencephalon the separation into two distinct ranges disappears 

 to a considerable extent. The mesial column, which is a continuation upwards of 

 the ventral nerve column of the cord, gives origin in the region behind the auditory 



FIG. 171, A and B. SECTIONS ACBOSS THE HIND-BRAIN OP A HUMAN EMBRYO 10 MM. 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 parts 

 continuous with a bundle of longitudinally coursing fibres placed at the periphery of the alar lamina, 

 and corresponding in situation to the commencing dorsal white columns shown in fig. 137. 



vesicle to the hypoglossal nerve (fig. 171). Its roots are in series with those of the 

 ventral roots of the cervical nerves, and it is to be regarded as representing several 

 (three or four) segmental i.e. trunk or spinal nerves fused into one stem. This 

 idea is strengthened by the fact that a ganglion-rudiment (Froriep's ganglion), 

 typical but rudimentary, is occasionally present in connexion with one or more of 

 the roots. These hypoglossal nerves are connected with the occipital myotomes, 

 and are occipito- spinal nerves in Fiirbringer's sense i.e. nerves properly belonging 

 to a part of the trunk which has become included in the hinder part of the head. 

 In the pre-otic region the abducens springs from the mesial column (fig. 172), and 

 its roots are in the same series as those of the hypoglossal. It is regarded therefore 

 by nearly all observers as the ventral (somatic) root of a segmental nerve. The 

 trochlearis and oculo-motorius, however, have been variously interpreted, some 

 observers regarding them as ventral mesial, others as lateral, motor roots ; they 



