WORK PRODUCTION 535 



(348), muscular katabolism is the most important source of 

 heat in the animal body. The degree of tonus and conse- 

 quently the rate of heat production seems to vary at different 

 times and in different bodily conditions. During profound 

 sleep it is much reduced. It is probably increased by all con- 

 ditions which favor the development of a vigorous muscular 

 system. What is ordinarily spoken of as a muscular contrac- 

 tion, therefore, and especially a tetanic contraction, is in a sense 

 an enormous increase of a condition already existing in the 

 muscle. 



Secondary effects of muscular exertion 



The great increase of the muscular katabolism during the 

 performance of work gives rise to important secondary effects, 

 particularly upon the circulation and respiration. It is a 

 familiar fact that in active exercise the heart action is largely 

 increased and the breathing becomes deeper and more rapid, 

 and that ordinarily the limit to muscular exertion is set, not 

 by the power of the muscles themselves but by the ability of 

 the heart and lungs to keep pace with the demands upon them. 



633. Circulation. The circulating blood is the medium by 

 which oxygen is conveyed to the muscles and carbon dioxid 

 and other products of their katabolism removed. The latter 

 function is of special importance because an accumulation in 

 the muscle of the products of its own katabolism speedily re- 

 duces and ultimately suspends its power to contract. In mus- 

 cular exercise, therefore, an increase in the rate of circulation is 

 essential to the continued activity of the muscles. For ex- 

 ample, in experiments by Chauveau and Kaufmann l the 

 ratio between the circulation in the resting as compared with 

 the active muscle in the living animal varied between i : 3.35 

 and i : 6.60. Zuntz and Hagemann, 2 in their investigations 

 upon the work of the heart, found the average amount of blood 

 passing through the heart of a horse per minute to be during 

 rest 29.16 liters and during work 53.03 liters. By this increase 

 in the rate of circulation .through the muscles the carbon di- 

 oxid and other injurious products of the muscular katabolism 



1 Comptes rend., 104, 1126, 1352, 1409. 



2 Landw. Jahrb., 27 (1898), Supp. Ill, 405. 



