58 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



plete rest is sometimes difficult to obtain and still more difficult to 

 ascertain. Benedict's recording cage (see p. 15, fig. 2) will be ex- 

 tremely serviceable in this respect, but the most reliable results are 

 undoubtedly obtained after immobilization with curari (Zuntz [1876], 

 Frank and F. Voit [1902], Tangl [1911]). The figures obtained by 

 Tangl and his pupils on curarized dogs show a wonderful constancy 

 over long periods as shown by the table quoted on p. 70. In many 

 cases, and especially in lower animals (frogs, fishes, invertebrates), 

 standard conditions with regard to the muscles can be brought about 

 by means of narcosis with urethane (Krogh [1914, i, 2]) which has, 

 like, curari, no specific influence upon the respiratory exchange. 

 Raeder [1915] has recently used urethane with complete success 

 also on mammals (see p. 71). 



The standard metabolism is utilized as the basis for the study of 

 the influence of various factors on the metabolic processes and for com- 

 parisons between different animals, and before we proceed further it is 

 necessary therefore to examine if and how far it can be considered as 

 constant. 



It must always be kept in mind that it is at best only an approxi- 

 mation to the true basal metabolism, the metabolism in the absence of 

 all functional activity. Several important functions, circulation, secre- 

 tion in kidneys and other glands are never interrupted, and in most 

 experiments on standard metabolism the respiratory movements take 

 place also. 



The metabolism of the heart during rest has been estimated by 

 Zuntz and Hagemann [ 1 898] for the horse and by Loewy and v. Schrotter 

 [1902] for man. In both cases it is found to be about 4 per cent, of the 

 total standard metabolism. This figure is based upon an estimate of 

 the work performed (blood pressure x minute volume) and the as- 

 sumption that the mechanical efficiency of the heart muscle is about 

 the same as that of skeletal muscles (30 per cent, according to 

 Zuntz. According to the work of Evans [1912] (on dogs) the me- 

 chanical efficiency is probably much lower (about 10 per cent.) and the 

 metabolism of the heart may easily be as much as 15 per cent, of the 

 total. Variations in the circulation rate or in the blood pressure may 

 therefore easily have quite an appreciable effect upon the standard 

 metabolism. 



The metabolism of the kidney appears to be something like 5 

 per cent, of the total according to experiments by Barcroft and Straub 

 [1911] and by Tangl [1911] on cats and dogs. For other glands 



