ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 717 



The chief difference between the sympathetic and the parasym- 

 pathetic systems is that the former sends nerve-fibers to all parts of 

 the body, whilst the latter sends fibers only to certain parts. Accord- 

 ing to Langley, when a tissue has a double innervation the effect pro- 

 duced by one set of fibers is, in most cases, the opposite of the main 

 effect produced by the other set of fibers. Thus; if one set causes 

 mainly contraction, the other causes mainly inhibition. 



Post Root 

 Ganglion 



7 1 i e ve 



(rQ) 1 0/1 on. 



Fig. 292. An Afferent Sympathetic Fiber. 



Adrenalin, when injected, produces all the effects seen from 

 stimulating the sympathetic nerves, but does not produce any of the 

 effect characteristic of stimulation of the parasympathetic nerves. 

 Hence it is a test-agent for the presence of sympathetic fibers. 



The pilomotor nerves come from the cord, from the fourth 



on account of their more regularly metameric distribution) ; in the middle the 

 ganglia of the sympathetic chain. These ganglia give off branches of distribu- 

 tion in which join the cutaneous nerve belonging to the same metamere as the 

 branch itself and the ganglion which has given it off. On the other hand, these 

 ganglia receive branches of medullary origin which arise from segments of the 

 spinal cord situated either higher or lower than the corresponding metamere. 

 Spinal origins condensed in the thoracic region, supplementary regions in the 

 medulla oblongata and the sacral spinal corrl. Symmetrical arrangement with 

 regard to a plane (-TT/) cutting across the middle of the thoracic region. 



