SOURCE OF ANIMAL HEAT. RESPIRATION. 



15 



deficient exercise; just as violent exercise, 

 which implies an increased supply of food, 

 is incompatible with weak digestive organs. 

 In either case the health suffers. 



But the quantity of oxygen inspired is 

 also affected by the temperature and density 

 of the atmosphere. 



The capacity of the chest in an animal is 

 a constant quantity. At every respiration a 

 quantity of air enters, the volume of which 

 may be considered as uniform; but its 

 weight, and consequently that of the oxygen 

 it contains, is not constant. Air is expanded 

 by heat, and contracted by cold, and there- 

 fore equal volumes of hot and cold air con- 

 lain unequal weights of oxygen. In sum- 

 mer, moreover, atmospherical air contains 

 aqueous vapour, while in winter it is dry; 

 the space occupied by vapour in the warm 

 air is filled up by air itself in winter ; that 

 is, it contains, for the same volume, more 

 oxygen in winter than in summer. 



In summer and in winter, at the pole 

 and at the equator, we respire an equal vo- 

 lume of air ; the cold air is warmed during 

 respiration, and acquires the temperature of 

 the body. To introduce into the lungs a 

 given volume of oxygen, less expenditure 

 of force is necessary in winter than in sum- 

 mer ; and for the same expenditure of force, 

 more oxygen is inspired in winter. 



It is obvious, that in an equal number of 

 respirations we consume more oxygen at 

 the level of the sea than on a mount.vin. 

 The quantity both of oxygen inspired and 

 of carbonic acid expired, must, therefore, 

 vary with the height of the barometer. 



The oxygen taken into the system is given 

 out again in the same forms, whether in 

 summer or in winter; hence we expire 

 more carbon in cold weather, and when the 

 barometer is high, than we do in warm 

 weather; and we must consume more or 

 less carbon in our food in the same propor- 

 tion ; in Sweden more than in Sicily; and 

 in our more temperate climate a full eighth 

 more in winter than in summer. 



Even when we consume equal weights 

 of food in cold and warm countries, infinite 

 wisdom has so arranged, that the articles of 

 food in different climates are most unequal 

 in the proportion of carbon they contain. 

 The fruits on which the natives of the 

 south prefer to feed do not in the fresh state 

 contain more than 12 per cent, of carbon, 

 while the bacon and train oil used by the 

 inhabitants of the arctic regions contain 

 from 66 to 80 per cent, of carbon. 



It is no difficult matter, in warm climates, 

 to study moderation in eating, and men can 

 bear hunger for a long time under the equa- 

 tor; but cold and hunger united very soon 

 exhaust the body. 



The mutual action between the elements 

 of the food and the oxygen conveyed by the 

 circulation of the blood to every part of the 

 body is THE SOURCE OF ANIMAL HEAT, 



III. All living creatures, whose existence 

 depends on the absorption of oxygen, pos- 



sess within themselves a source of heat in- 

 dependent of surrounding objects. 



This truth applies to all animals, and ex- 

 tends, besides, to the germination of seeds, 

 to the flowering of plants, and to the matura- 

 tion of fruits. 



It is only in those parts of the body* to 

 which arterial blood, and with it the oxygen 

 absorbed in respiration, is conveyed, that 

 heat is produced. Hair, wool, or feathers, 

 do not possess an elevated temperature. 



This high temperature of the animal 

 body, or, as it may be called, disengagement 

 of heat, is uniformly and under all circum- 

 stances the result of the combination of a 

 combustible substance with oxygen. 



In whatever way carbon may combine 

 with oxygen, the act of combination cannot 

 take place without the disengagement of 

 heat. It is a matter of indifference whether 

 the combination take place rapidly or slowly, 

 at a high or at a low temperature: the 

 amount of heat liberated is a constant 

 quantity. 



The carbon of the food, which is con- 

 verted into carbonic acid within the body, 

 must give out exactly as much heat as if it 

 had been directly burnt in the air or in oxy- 

 gen gas; the only difference is, that the 

 amount of heat produced is diffused over 

 unequal times. In oxygen, the combustion 

 is more rapid, and the heat more intense ; 

 in air it is slower, the temperature is not so 

 high, but it continues longer. 



It is obvious that the amount of heat libe- 

 rated must increase or diminish with the 

 quantity of oxygen introduced in equal 

 times by respiration. Those animals which 

 respire frequently, and consequently, con- 

 sume much oxygen, possess a higher tem- 

 perature than others, which, with a body of 

 equal size to be heated, take into the system 

 less oxygen. The temperature of a child 

 (102) is higher than that of an adult 

 (99-50). T hat ot birds (104 to 105-40) is 

 higher than that of quadrupeds (98-5 to 

 100-4) or than that of fishes or amphibia, 

 whose proper temperature is from 27 to 

 3"6 higher than that of the medium in 

 which they live. All animals, strictly 

 speaking are warm-blooded ; but in those 

 only which possess lungs is the temperature 

 of the body quite independent of the sur- 

 rounding medium. (5) 



The most trustworthy observations prove 

 that in all climates, in the temperate zones 

 as well as at the equator or the poles, the 

 temperature of the body in man, and in 

 what are commonly called warm-blooded 

 animals, is invariably the same ; yet how 

 different are the circumstances under which 

 they live ! 



The animal body is a heated mass, which 

 bears the same relation to surrounding ob- 

 jects as any other heated mass. It receives 

 heat when the surrounding objects are hotter, 

 it loses heat when they are colder than 

 itself. 



We know that the rapidity of cooling in 



