Peripheral ISTervous System in Human Embryo 297 



it is to be remembered that in man the typical visceral cranial nerve has 

 three central terminations: 



1. Sensory root (tractus solitarius). 



2. Curved ventral motor root (nucleus ambiguus). 



3. Straight dorsal motor root (nucleus vagi dorsalis). 



These three elements may be represented in the different nerves in 

 different proportions. The ninth nerve approaches the mean and all 

 elements are fairly represented. In the vagus the curved ventral motor 

 roots are increased in proportion in the caudal portions and form the 

 spinal accessory. In the facial the sensory root (n. intermedins) is 

 diminutive, while the curved ventral root becomes the main trunk of 

 the nerve. In the trigeminal it is the straight dorsal motor root that 

 forms the principal motor supply, while the curved ventral motor root 

 is either not present or is represented by its mesencephalic root. 



Spinal Nerves. 



At the end of the first month each segmental nerve of the trunk 

 possesses a sharply outlined spinal ganglion, whose constituent cells are 

 in the early stages of differentiation. On examination it can be seen 

 that many of these cells consist of a prominent nucleus surrounded by 

 a thin rim of ill defined body protoplasm. In other cells the body pro- 

 toplasm has increased in the form of a process at one or both ends. 

 Cells of this kind are clustered so that their processes unite to form 

 fibrous strands. These strands in turn fuse into larger bundles and lead 

 toward the two poles of the ganglion. At the proximal pole they be- 

 come grouped into the dorsal roots, which enter the spinal cord in an 

 uninterrupted longitudinal series. In the cord they unite and extend 

 up and down in the marginal zone in the form of a flattened band of 

 fibres which later constitutes one of the dorsal funiculi of the cord. 



The fibres from the distal pole of the ganglion imite in a common 

 bundle which is almost immediately joined by the fibres of the ventral 

 root, the two together forming the main trunk of the nerve. The ventral 

 roots consist of a continuous series of rootlets emerging from the ven- 

 trolateral border of the neural tube, and derived from the neuroblasts 

 of the mantel layer of its basal plate. These neuroblasts form a longi- 

 tudinal column which, as shown in Plate II, is continuous with and of 

 the same character as the nucleus of the hypoglossal nerve. 



