RESPIRATION. 87 



the lungs of a common-sized man, immediately after an in- 

 spiration, has been calculated to be about two hundred and 

 eighty cubic inches, and about forty inches are drawn in and 

 thrown out at each inspiration and expiration ; so that the 

 whole mass of air is not changed at every breath, but a large 

 proportion remains constantly present, and distends the lungs. 



' If the air which has been respired be examined, a change 

 will be found to have taken place in its composition. A part 

 of its oxygen has disappeared, and in its place is found about 

 the same bulk of carbonic acid or fixed air. There is also a 

 considerable quantity of watery vapor. This change is un- 

 doubtedly connected with the effect produced upon the color 

 of the blood in respiration ; and many have endeavored to 

 give some account of the mode in which it takes place. But 

 it is a process which we shall probably never be able fully to 

 understand. A similar change is produced upon the air re- 

 spired by all animals of whatever class. 



' Respiration has been supposed to be the cause of animal 

 heat. Various opinions have been advanced to account for 

 the manner in which it maintains the temperature of our 

 bodies. None of them, however, seem perfectly satisfactory. 

 It appears undoubtedly to have some very close connection 

 with respiration, and dependence upon it ; for the degree of 

 heat in animals is generally proportioned to the vigor and 

 quantity of respiration. The temperature of birds is higher 

 than that of man, and they consume a greater quantity of air. 

 Reptiles and fishes have cold blood, and the amount of respi- 

 ration in them is comparatively small. The same remark is 

 true of all cold-blooded animals. But we are not yet ac- 

 quainted with the exact nature of the connection between 

 respiration and animal heat.* 



'Respiration commences immediately after birth,' and at 

 the same time a change is produced in the course of the cir- 



contairis a greater proportion of it than atmospheric air, are capable of supporting 

 life for a considerable period, but finally prove fatal. The latter ia celebrated for its 

 intoxicating and exhilarating effects, when respired.' 



* ' Animal heat has been supposed to arise from a chemical action taking place in 

 the lungs at the time of the change of venous blood into arterial, in the same way 

 that heat is produced by many other chemical operations ; but if this were true, the 

 lungs ought to be hotter than any other part of the body, which is not the case. It 

 has been also supposed to arise from a greater capacity for caloric in the arterial, 

 than in the venous blood, in consequence of which, heat would be developed when 

 the change from the former to the latter takes place in the capillary vessels of the 

 body. This is more probable, but still hardly satisfactory. Neither of these hy 

 potheses accounts for the independent temperature of eggs, which resist cold so long 

 as they retain their life ; nor for the power in men of resisting very high degrees of 

 beat, with hardly any increase of the temperature of their bodies.' 



