764 A, SUBBA RAU AND V. II. JOHNSON ON THE NERVOUS 



dift'erentiateil to distinguish tliem. It is unfortunate that the 

 photographic ilhistrations, which Goorniaghtigh puts forward in 

 support of his view, are far too indistinct to serve as evidence 

 for it. 



We ai-e tluis led to support the entirely ectodei-nial origin of 

 the sympathetic nervous system, and from the observations 

 iilready given, it will be seen that we do not confine that origin 

 to any one part of the central nervous system. The statement 

 by experimental physiologists that the sympathetic contains both 

 afferent and efferent components, while it does not afford a proof 

 of a diverse origin, yet supports the possibility of such being the 

 case. We have seen that some cells originate very eiiirly from 

 the neural crest, and we are convinced that both dorsal and 

 ventral roots contribute towards the formation of the sympathetic 

 primordia. 



The observations of His (8) and Held (7) support the origin 

 from the dorsal root ganglia. These writers describe the 

 primordia as arising by cells, migrating from the spinal giinglia 

 by way of the spinal nerve, from which they diverge a little 

 beyond the point of their junction, and thence make their way 

 to the dorso-lateral angle of the aorta. The sympathetic cells 

 thus leave the spinal nerves at a point subsequent to the union 

 of dorsal and ventral roots, and so may originate from either of 

 those sources. The assumption that all have arisen from the 

 dorsal root ganglion is based simply on an analogy with Selachian 

 material, and has no foundation on observations of avian embryos. 



The descriptions of Held and His lead one to the supposition 

 that this process is continuous throughout the development of 

 the sympathetic ; this, however, is quite contrary to the results 

 of our investigations. If the sympathetic ganglia were derived 

 directly from the spinal nerve, one would naturally expect the 

 cells composing them to bear a distinct resemblance to those 

 fi'oni wliich they are said to originate ; but this we do not find 

 to be the case. The cytological characters of the dorsal root 

 ganglia, and of the cells of the ventral horn of grey matter, are 

 so different from those of both primary and secondary sympa- 

 thetic ganglia, t')a.t there seems no possibility of one being 

 derived immediately from, the other (see text-(ig. 20). 



Our description, as given in § E, of the morphological deve- 

 lopment of the sympathetic, allows ample opportunity for this 

 differentiation to arise ; for we discover that though the ganglia 

 of the dorsal roots and the definitive sympathetic ganglia lie 

 close to one another, yet the latter have had opportunity to 

 undergo considerable changes in the course of their extensive 

 migi-ation, Jirst, from the neui-al crest, &c., to the aorta, and, 

 secondly/, from the aorta to near the spinal ganglion. 

 , The reasons for this complicated migratory movement h.ave 

 yet to be discovered, but the evidence for it is, we think, suffi- 

 ciently definite. The situation of the definitive sy)ni)athetic 

 in the vertebral canal in birds, was pointed out as long ago as 



