SYMPATHETIC SYSTEM 177 



fishes, some of the spinal nerves may enter the appendage directly 

 without the intervention of a collector or a plexus. The collector 

 disappears in the amphibia. 



THE SYMPATHETIC SYSTEM 



The sympathetic nervous system possesses much physiological 

 and anatomical individuality. It is entirely removed from control 

 of the will and its function is largely the regulatory control, either 

 directly or indirectly, of the various viscera, glands, blood-vessels, 

 and respiratory and reproductive organs. Much of this control is 

 accomplished by its stimulation or inhibition of the smooth muscle 

 cells of the walls of the blood-vessels, thus, by their enlargement or 

 contraction, regulating the amount of blood supplied to any part. 

 (Not all involuntary activities of the body are directly controlled by 

 it.) Besides, it contains sensory structures, but the stimulation of 

 these does not necessarily result in conscious states. 



This close relation of the sympathetic system with the smooth 

 muscles and with the blood-vessels necessitates the extension of parts 

 of the system to all parts of the body, many of the nerve trunks paral- 

 leling the blood-vessels; but the main parts are near the mid-dorsal 

 line of the body cavity and are more or less closely associated with 

 the dorsal aorta. 



The system is connected with the spinal nerves by the visceral 

 rami (p. 174). As the spinal nerves develop by outgrowths from the 

 spinal ganglia and the neuroblasts of the spinal cord, cells are carried 

 down with them from both sources and these are aggregated in a series 

 of metameric sympathetic ganglia near the aorta. As is seen in figs. 

 184, 188, the visceral rami are composed of nerve fibres, both motor 

 and sensory, of the visceral series, and by means of these the sympa- 

 thetic system is in communication with the central nervous system. 

 From this first series, the so-called chain ganglia, the system is ex- 

 tended farther by the development of a prevertebral series of ganglia 

 — sometimes single^- sometimes paired — lying ventral to the aorta 

 (fig. 188) and connected with the chain by metameric nerves. Some 

 of these prevertebral ganglia in the higher groups are of considerable 

 size and have received special names — cardiac, pelvic and hypogastric 

 ganglia and s«lar flext}^ 



In the lampreys the chain ganglia are not connected with each 

 •ther and they are confined to the region of the body cavity. In 

 the elasmobranchs and perennibranch amphibia a network of fibres 



