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PHYSIOLOGY 



120 - 



is in itself sufficient to raise the blood-pressure, and it seems probable 

 that the increased activity of the motor centres actually spreads to the 

 medullary centres which preside over the heart and blood-vessels, 

 and that there is an active constriction of the vessels, especially of the 

 splanchnic area, so that the greater part of the blood in circulation 

 is available for the use of the actively contracting muscles. On this 

 account hard exercise is not easily carried out after a meal, and, if 

 forced, seriously interferes with digestion, by the diversion of the 



current of blood needed for the carry- 

 ing out of this function. 



Increase of carbonic or lactic acid 

 in the blood will affect the vaso-motor 

 centre directly, as Mathison has shown, 

 and therefore will concur in causing a 

 rise of blood pressure. Since the blood 

 vessels of a contracting muscle are 

 probably dilated, all the conditions 

 are present for maintaining as rapid a 

 flow as possible through the parts 

 which are the seat of the most active 

 metabolism. 



In the quickening of the pulse-rate 

 the most important factor is probably 

 the central nervous system, i.e. the 

 cardiac centres in the medulla and 

 upper part of the spinal cord. The 

 acceleration is partly central, by a 

 spread of excitatory impulses from 

 the motor paths, partly reflex from the 

 heart, the afferent impulses resulting 

 from the increased tension in the heart 

 cavities being, so to speak, switched 

 off from the cardio-inhibitory on to 

 Under normal 



circumstances, it will be remembered, 

 a rise of blood pressure is attended 

 with a slowing of the pulse. In 

 exercise the rise of blood pressure is attended with cardiac accelera- 

 tion. Slight acceleration of the heart is observed, after division of 

 all its nervous connections, on tetanising the lower limbs. Mansfeld 

 has shown that probably the chief, if not the only, factor in 

 this case is the rise of temperature in the blood flowing to 

 the heart. When exercise is discontinued the pulse-rate and 

 blood pressure rapidly fall to normal, the return being quicker in 



FIG. 455. Curves showing the in- 

 fluence of exercise on the circula- , 

 tion. The exercise was a six-mile the accelerator centre, 

 run. Ordinates = mm. Hg pres- 

 sure and rate per minute. (0. S. 

 LOWSLEY.) 



