46 report — 1842. 



spiratory process, and some have even gone so far as to suppose that they 

 are capable of generating heat. When the pons Varolii is cut through in a 

 dog, or when a sudden blow is inflicted on the back of the head, the dog 

 continues to respire, perhaps more quickly than before, but its body cools as 

 rapidly as if sudden death had occurred. Exactly similar effects ensue on 

 the cutting of the spinal cord, or the par vagum. These experiments have 

 been supposed to prove that animal heat is due to nervous influence, and not 

 to combustion ; but this singular view has arisen from the erroneous con- 

 ception that the combustion proceeds in the blood itself. Nothing could be 

 more erroneous. As will afterwards be shown, the compounds which are 

 consumed by the oxygen of the air are produced by the viscera, and these 

 being paralyzed in the experiments alluded to, are unable to furnish com- 

 pounds for combustion, so that the heat disappears. 



Others, oh the contrary, have ascribed animal heat to the contraction of 

 the muscles, just as heat is evolved when caoutchouc is allowed to contract 

 from a state of extension. Some have gone so far as to ascribe part of the 

 heat to the mechanical motions of the body, as if these motions could exist 

 without an expenditure of force consumed in producing them. Let us in- 

 quire in what manner this heat is produced. 



We kindle a fire under the boiler of a steam-engine and generate steam, 

 which steam may be applied to a machine destined for producing friction. 

 By this friction heat is disengaged, but it is quite impossible that the heat 

 thus obtained can ever be greater than that employed to heat the boiler. We 

 employ a galvanic current to produce heat ; but the amount of heat obtained 

 is never greater than we might have by the combustion of the zinc used in 

 producing the current. 



The contraction of muscles produces heat ; but the force necessary for the 

 contraction has manifested itself through the organs of motion, in which it 

 has been excited by chemical changes. The ultimate cause of the heat pro- 

 duced is therefore to be found in these chemical changes. 



Professor Liebig then proceeds to prove that the heat evolved by the com- 

 bustion of carbon in the body is sufficient to account for all the phenomena 

 of animal heat. He shows that the 14 oz. of carbon which are daily con- 

 verted into carbonic acid in an adult, disengage no less than 1 97*477° of heat; 

 a quantity Avhich would convert 24 lbs. of water, at the temperature of the 

 body, into vapour. And if we assume that the quantity of water vaporized 

 through the skin and lungs amounts to 3 lbs., then we have still 146.380° of 

 heat to sustain the temperature of the body. And when we take into calcu- 

 lation the heat evolved by the hydrogen of the food, and the small specific 

 heat possessed by the organs generally, no doubt can be entertained that the 

 heat evolved in the process of combustion, to which the food is subjected in 

 the body, is amply sufficient to explain the constant temperature of the body. 



From what has preceded, it is obvious that the amount of carbon con- 

 sumed in food ought to depend on the climate, density of air, and occupa- 

 tion of the individual. A man will require less carbon when pursuing a 

 sedentary occupation, than when he is engaged in active exercise. 



Professor Liebig having thus discussed the source of animal heat, proceeds 

 next to consider what are the ingredients in the food which may be properly 

 considered to be nutritious. Physiologists conceive that the various organs 

 in the body have originally been formed from blood ; if this be admitted, it 

 is obvious that those substances only can be considered as nutritious which 

 are susceptible of being transformed into blood. 



When blood is allowed to stand it coagulates and separates into a watery 

 fluid called serum, and into the clot, which consists principally of fibrin. 



