ix.] THE NERVOUS SYSTEM, 405 



In the existence and main characters of his sympathetic 

 system man agrees with the members of the Vertebrate sub- 

 kingdom generally ; nevertheless, the sympathetic may be 

 absent, as is alleged to be the case in the Marsipobranchs, 

 and its place may be supplied (as in the Myxine) by an ex- 

 tension of the pneumogastric. 



31. The contraction of muscular fibre in man is said to be 

 accompanied by more or less electrical disturbance. This 

 leads us to observe that certain organs may exist for storing up 

 electricity so as by its discharge to administer severe shocks 

 to creatures in the vicinity of animals possessing and making 

 use of such structures. These ELECTRIC ORGANS are met with 

 only in the class of Fishes, and are constructed of membranous 

 chambers enclosing cellular plates, while special nerves end- 

 ing in minute ramifications are distributed upon one and the 

 same side of each of the plates. The arrangement of these 

 plates may vary, as also the position of the organs and the 

 source of their nervous supply. Thus, in the Torpedo there 

 are two such organs, one on each side, near the head ; while 

 in the Electric Eel there are four, two on each side of the 

 trunk and tail. In the Torpedo the nervous supply comes 

 from the medulla oblongata, while in the Electric Eel it is 

 furnished through the ventral branches of the spinal nerves. 

 In the Torpedo the cellular plates are placed horizontally, in 

 the Electric Eel vertically. 



32. The most GENERALIZED CONCEPTION at present at- 

 tainable of the nervous system of Vertebrates, and therefore 

 of man, may perhaps be expressed as follows : 



There is a cylindrical cerebro-spinal axis with a median 

 cavity ending anteriorly at a lamina terminalis, from each side 

 of which lamina a secondary prolongation is developed. Be- 

 sides nerves of special sense and of parts surrounding such 

 special sense organs, the axis gives off a series of bifurcating 

 branches which skirt each side the cranial arches (splanchna- 

 pophyses), from the trabecula to the last branchial inclusive. 



Behind these bifurcating branches, there is given off a 

 series of spinal nerves, and each of these nerves splits into 

 three branches : (i), dorsal ; (2), abdominal ; (3), sympathetic. 



Certain of the abdominal branches unite to supply limb 

 nerves when limbs are developed. The abdominal sympa- 

 thetic branches, constantly united by commissural filaments, 

 pass down from the spine along the mesentery, and are dis- 

 tributed to the viscera. 



