312 MANUAL OF PHYSIOLOGY. 



as much work in twenty-four hours as would be required to lift 

 his body 1248 yards into the air, i.e., nearly ten times as high as 

 the top of St. Paul's Cathedral. 



CONTROLLING MECHANISMS OF THE BLOODVESSELS. 

 Vaso- Motor Nerves. 



That the arteries possessed, as well as elastic resiliency, vital 

 contractility, which regulated the amount of blood flowing to any 

 given part, was well known to John Hunter. 



The muscle cells have also been long since clearly demonstrated 

 in the middle coats of the arteries, but nothing was known of the 

 nervous channels which bore the stimulus to the vessels, or the 

 nerve centres which regulated their contraction, until compara- 

 tively recent times. 



The first knowledge concerning special nervous arrangements 

 for the control of the muscular wall of the vessel was given to 

 us by Claude Bernard, in his notable experiment of cutting the 

 sympathetic nerve in the neck, which was always followed by an 

 increase in temperature of that side of the head, and a great ex- 

 pansion and over-filling of the arteries. 



It was further observed that stimulation of the superior gan- 

 glion of the sympathetic brought about an opposite result, namely, 

 a loss of temperature and contraction of the vessels on the same 

 side as the stimulus was applied. If the stimulus was much 

 increased, the vessels contracted much more than the normal 

 amount, but on cessation of the stimulus they became greatly 

 dilated above the normal point and the temperature rose again, 

 but after a time the effect of the stimulus gradually passed off. 

 From this it was concluded that the sympathetic in the neck con. 

 veyed to the muscles in the bloodvessels impulses which caused a 

 certain amount of habitual contraction of the vessel wall, which 

 was called tonic contraction, corresponding to what was already 

 recognized as arterial tone. When the nerve was divided this 

 tone disappeared, but when gently stimulated it reappeared, and 

 when more strongly stimulated an exaggerated contraction set in 

 causing complete occlusion of many of the vessels. 



