Sympathetic nervous system.
The discovery that the spinal and cranial nerves together with their ganglia were formed from the epiblast was shortly afterwards extended to the sympathetic nervous system, which has now been shewn to arise in connection with the spinal and cranial nerves. The earliest observations on this subject were those contained in my Monograph on Elasmobranch Fishes (p. 173), while Schenk and Birdsell (No. [361]) have since arrived at the same result for Aves and Mammalia.
Fig. 274. Longitudinal vertical section through part of the body wall of an Elasmobranch embryo shewing part of two spinal nerves and the sympathetic ganglia belonging to them.
ar. anterior root; pr. posterior root; sy.g. sympathetic ganglion; mp. part of muscle-plate.
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 sympathetic 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 subsequently united into a continuous cord on each side.
The observations of Schenk and Birdsell on the Mammalia seem to indicate that the main parts of the sympathetic 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 suprarenal bodies, which are dealt with in another chapter.
