PHYLOGENETIC ORIGIN OF NERVOUS SYSTEM 143 



heart's action thereby induced. This is exactly what is seen 

 also in Addison's disease, where with the progress of the dis- 

 ease of the suprarenals a progressive fall of blood pressure and 

 weakness of the heart are invariable symptoms. This evidence 

 shows that the sympathetic nerves alone, although connected with 

 the central nervous system and not interfered with, are unable to 

 keep up the tone of the vascular system in the absence of adrena- 

 line. Indeed Elliott found that in animals, moribund in conse- 

 quence of removal of the suprarenals, the tissues supplied by 

 sympathetic nerves may even fail to respond to electrical stimu- 

 lation of such nerves, although the nerves of external sensation 

 and those controlling the skeletal muscles are perfectly efficient. 



In the case of the lowest vertebrate, Petromyzon, the seg- 

 mental anterior nerves run separately from and alternate with 

 the segmental posterior nerves, and supply only the segmental 

 somatic muscles. On the other hand segmental posterior nerves 

 accompany the blood vessels and are apparently distributed with 

 the various vascular branches throughout the body. Again a 

 marked acceleration of the heart in the lamprey takes place upon 

 the administration of adrenaline, and also, as my son has found, 

 upon exposure of the spinal cord at the end of the branchial 

 region ; the cord having first been isolated, but left in nervous 

 connexion with the heart. Considering the very small number 

 of sympathetic cells found in the lamprey and the great mass of 

 chromaffine cells, especially on the sinus of the heart, we can only 

 conclude that the motor regulation of the vascular system of this 

 animal is carried out by the chromaffine system more than by the 

 sympathetic system. 



The whole evidence points strongly to the conclusion that the 

 three systems, vascular muscular system, chromaffine system, and 

 sympathetic system, are very closely interdependent. Striking 

 evidence of this is afforded by Amphioxus, where there is no 

 evidence of any vascular muscular system, and at the same time 

 no sign of either sympathetic nerve cells or of any chromaffine 

 tissue. 



The evidence of the lowest vertebrate thus points to the 

 origin of the sympathetic nervous system from nerve cells in the 

 central nervous system of some invertebrate, which are motor to 

 the vascular system and contain adrenaline in their substance. In 

 my book on the " Origin of Vertebrates," I have given my reasons 



