NERVOUS SYSTEM OF THE VERTEBRATA. 



467 



cranial nerves. The earliest observations on this subject were 

 those contained in my MonograpJi on ElasuwbrancJi Fislies 

 (p. 173), while Schenk and Birdsell (No. 361) have since 

 arrived at the same result for Aves and Mammalia. 



In my account of the development of these ganglia, it is 

 stated that they were first met with as small masses situated at 

 the ends of short branches of the spinal nerves (fig. 275 sy.g]. 

 More recent investigations have shewn me that the sympathetic 

 ganglia are at first simply swellings on the main branches of the 

 spinal nerves some way below the ganglia. Their situation 

 may be understood from fig. 274, sy.g, 

 which belongs however to a somewhat 

 later stage. Subsequently the sympath- 

 etic ganglia become removed from the 

 main stem of their respective nerves, 

 remaining however connected with those 

 stems by a short branch (fig. 275, sy.g). 

 I have been unable to find a longitudinal 

 commissure connecting them in their 

 early stages ; and I presume that they 

 are at first independent, and become sub- 

 sequently united into a continuous cord 

 on each side. 



The observations of Schenk and 

 Birdsell on the Mammalia seem to in- 

 dicate that the main parts of the sym- 

 pathetic system arise in continuity with 

 the posterior spinal ganglia : they also shew that in the neck 

 and other parts the sympathetic cords arise as a continuous 

 ganglionic chain. The observations on the topographical 

 features of the development of the sympathetic system in 

 higher types are however as yet very imperfect. 



The later history of the sympathetic ganglia is intimately 

 bound up with that of the so-called supra-renal bodies, which 

 are dealt with in another chapter. 



FIG. 274. LONGITUDI- 

 NAL VERTICAL SECTION 

 THROUGH PART OF THE 

 BODY WALL OF AN ELASMO- 

 BRANCH EMBRYO SHEWING 

 PART OF TWO SPINAL NERVES 

 AND THESYMPATHETICGAN- 

 GLIA BELONGING TO THEM. 



ar. anterior root ; pr. pos- 

 terior root ; sy.g. sympathetic 

 ganglion ; nip. part of mus- 

 cle-plate. 



302 



