CHAPTER XXXII. 



THE VASOMOTOR NERVES AND THEIR PHYSIO- 

 LOGICAL ACTIVITY. 



During the first half of the nineteenth century the physical or 

 mechanical conditions of the circulation were carefully studied and 

 great emphasis was laid upon such properties as the elasticity of 

 the coats of the vessels. The physical adaptability thereby con- 

 ferred upon the vascular tubes was thought to be sufficient for the 

 purposes of the circulation. We now know that many of the blood- 

 vessels are supplied with motor and inhibitory nerve fibers through 

 whose activity the size of the vascular bed and the distribution of 

 blood to the various organs are regulated. We know, also, that 

 without this nervous control the vascular system fails entirely to 

 meet what seems to be the most important condition of a normal 

 circulation, namely, the maintenance of a high arterial pressure. 

 Although a number of physiologists had assumed the existence of 

 nerve fibers capable of acting upon the muscular coats of the blood- 

 vessels, the experimental proof of the existence of such nerves, 

 and the beginning of the modern development of the theory of 

 vasomotor regulation were a part of the brilliant contributions to 

 physiology made by Claude Bernard.* In 1851 Bernard discovered 

 that when the sympathetic nerve is cut in the neck of a rabbit the 

 blood-vessels in the ear on the same side become very much dilated. 

 He and other observers afterward showed that if the peripheral 

 (head) end of the severed nerve is stimulated electrically the ear 

 becomes blanched, owing to a constriction of the blood-vessels. 

 Thus the existence of vasoconstrictor nerve fibers to the blood-vessels 

 was demonstrated. A vast amount of experimental work has been 

 done since to ascertain the exact distribution of these fibers to the 

 various organs and the reflex conditions under which they function 

 normally. Few subjects in physiology are of more practical im- 

 portance to the physician than that of vasomotor regulation; it 

 plays such a large and constant part in the normal activity of the 

 various organs. Bernard was doubly fortunate in being the first 

 to demonstrate the existence of a second class of nerve fibers, which, 

 when stimulated, cause a dilatation of the blood-vessels and which 



*See "Life of Claude Bernard," by Sir Michael Foster, 1899, in the series, 

 "Masters of Medicine." 



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