496 



TEXT-BOOK OF EMBRYOLOGY. 



into ganglion cells, some being found there at birth (Streeter) . This would in- 

 dicate the presence of a small and hitherto undetected afferent element in the 

 spinal accessory nerve, which is usually regarded as purely efferent. The spinal 

 accessory nerves are thus identical with the vagus in their early development 

 and consist at first of a homologous series of efferent roots and ganglia. This 



/x-x-x/ gang, crest. 



Qpthal dlv. 



Sup. max d 

 N.maSticatorfus. 

 Infmax.di 



D.I. 



FIG. 434. From a reconstruction of the peripheral nerves in a human embryo of 



4 weeks (6.9 mm.). Streeter. 



JII-XII, III to XII cranial nerves; C.I, D. I.., L.I., 5.7., ist cervical, ist dorsal, ist lumbar, and 

 ist sacral nerves, respectively; I, 2, 3, branchial arches; Ot. v., auditory vesicle; IX-X-XI 

 gang, crest, ganglionic or neural crest of IX, X and XI cranial nerves. Fiber masses are 

 represented by fine lines, ganglion cell masses by dots. 



indicates that the spinal accessory might be regarded as a specialized part 

 of the vagus extending caudally into the cord (Streeter) (see p. 464).* 



From this point on, the further development of the efferent fibers of the X 

 and XI nerves and of the peripheral processes of their ganglia is the further 



* According to another view (Bremer), the spinal accessory nuclei and roots are to be regarded as 

 representing a specialization of lateral nuclei of the ventral gray column of the cord whose root fibers 

 pass in the dorsal branches of the spinal nerves to the dorsal trunk musculature (p. 487, ccmp. Fig. 

 404) . According to this view, the muscles innervated by the XI would be somatic. The possible 

 homology of the lateral efferent nuclei and roots of the medulla with those dorsal root fibers of the 

 cord which arise from cells in the ventral gray column (p. 487 and Fig. 430) may be mentioned in 

 this connection. 



