236 THE PHYSIOLOGY OF MUSCULAR WORK 



resistance enormously ; the increase in the volume of the blood 

 driven out of the heart raises the blood pressure quickly to a great 

 height, because the blood already in the arteries is prevented from 

 escaping. 



The changes in the blood pressure are effected very rapidly, 

 as is shown by the following examples taken from a series of ob- 

 servations ( 29 ) upon two men, one well-trained, and the other un- 

 trained. The exercise lasted only thirty seconds, and consisted in 

 running down and up stairs. 



The blood pressure does not remain at a high level during the 

 prolongation of exercise, but falls gradually and may become 

 lower than it was before the beginning of the work. Such a con- 

 dition has been observed during a march of seven miles ; the fall 

 in the blood pressure was associated with a dilatation of the 

 cutaneous blood vessels, which was shown by the increased colour 

 and temperature of the skin. 



The causes of the quickening of the heart beat during muscular 

 work are to be sought, as already mentioned, in nervous regulation, 

 effected reflexly or directly by the products of muscular activity, 

 and in the action of those metabolites and the rise of temperature 

 upon the heart itself. It might be expected that the control of 

 the blood vessels is similarly effected, for the heart is developed 

 from blood vessels. The evidence from experiments upon the 

 flow of blood through muscles is conflicting. GaskelPs ( 30 ) work 

 led him to the conclusion that vaso-dilator fibres are stimulated 

 reflexly when muscles contract, but Bayliss ( 31 ) obtained only 

 slight dilatation on stimulation of the dorsal nerve roots. Lactic 

 acid was found by Gaskell to produce a dilatation of the arteries 

 of a frog, but Osborne and Vincent could not obtain a similar 

 result in mammals ; carbon dioxide dilates the blood vessels in 



