Dr. J. R. Mayer on the Mechanical Equivalent of Heat, 499 



blood drawn from the vein of the arm possessed, almost without 

 exception, a surprisingly bright red colour. 



This phenomenon riveted my earnest attention. Starting 

 from Lavoisier's theory, according to which animal heat is the 

 result of a process of combustion, I regarded the twofold change 

 of colour which the blood undergoes in the capillaries as a sensible 

 sign — as the visible indication — of an oxidation going on in the 

 blood. In order that the human body may be kept at a uniform 

 temperature, the development of heat within it must bear a quan- 

 titative relation to the heat which it loses — a relation, that is, to 

 the temperature of the surrounding medium*; and hence both 

 the production of heat and the process of oxidation, as well as 

 the difference in colour of the two kinds of bloody must be on the 

 whole less in the torrid zones than in colder regions. 



In accordance with this theory, and having regard to the 

 known physiological facts which bear upon the question, the 

 blood must be regarded as a fermenting liquid undergoing slow 

 combustion, whose most important function — that is, sustain- 

 ing the process of combustion — is fulfilled without the consti- 

 tuents of the blood (with the exception, that is, of the products 

 of decomposition) leaving the cavities of the blood-vessels or 

 coming into such relation with the organs that an interchange 

 of matter can take place This may be thus stated in other 

 words : by far the greater part of the assimilated food is burned 

 in the cavities of the blood-vessels themselves, for the pur- 

 pose of producing a physical effect, and a comparatively small 

 quantity only serves the less important end of ultimately enter- 

 ing the substance of the organs themselves, so as to occasion 

 growth and the renewal of the worn-out solid parts. 



If hence it follows that a general balance must be struck in 

 the organism between receipts and expenditure, or between work 

 done and wear and tear, it is unmistakeably one of the most 

 important problems with which the physiologist has to deal, to 

 make himself as thoroughly acquainted as it is possible for him 

 to be with the budget of the object of his examination. The 

 wear and tear consists in the amount of matter consumed ; the 

 work done is the evolution of heat. This latter effect, however, 

 is of two kinds, inasmuch as the animal body evolves heat on 

 the one hand directly in its own interior, and distributes it by 

 communication to the objects immediately surrounding it ; while, 

 on the other hand, it possesses, through its organs of motion, 

 the power of producing heat mechanically by friction or in 

 similar ways, even at distant points. We now require to know 



* Compare also on this head the interesting publication of Bergmann, 

 Ueber die Verh'dltnisse der Warme-Oekonomie der Thiere zu ihrer Grosse, 

 Gottingen, 1848. 



