RESPIRATION. 121 



among them is found to be very great. Reptiles, which consume 



k. little oxygen, have a temperature only a few degrees above the 



it medium in which they live, and the same may be said of fishes, 



with the remarkable exception of the Cetacea (whale, porpoise, &c.) 



which have a high temperature, but consume much oxygen, as they 



breathe air by lungs. 



But it may be said, if the heat of the body is produced chiefly by 

 respiration, why have the lungs not the temperature of a furnace 7 

 Several reasons may be assigned why this should not be so ; and, 

 1st, it must be recollected, that, even though a considerable portion 

 of heat may be supposed to be produced in the lungs, there is also 

 a constant and most rapid influx of cold blood, from the exposed 

 parts of the body, into these, to carry off the heat as soon as it is 

 generated.* Besides, it appears pretty certain that the blood com- 

 ing from the lungs is really a degree or two hotter than in other 

 parts of the body. 2dly, Dr. Crawford in his celebrated Theory of 

 Animal Heat, has very ingeniously attempted to obviate this diffi- 

 culty by maintaining that the capacity for heat (as chemists call it) 

 is greater in arterial than in venous blood ; that as this enlargement 

 of capacity takes place at the same moment, and in the same place, 

 as the heat is generated, a considerable portion of it must be ab- 

 sorbed ; and that this latent heat comes to be given out as the 

 arterial, in its course, is again gradually converted into venous 

 blood. 



The correctness of Dr. Crawford's theory has been doubted by 

 many physiologists, on the ground, 1st, that other chemists have 

 not found the capacities of arterial and venous blood to correspond 

 with his statements ; and, 2d, that it has been, since his time, ren- 

 dered highly probable, by the experiments of Dr. Edwards and 

 others, that a considerable portion, at least, of the carbonic acid 

 produced by respiration, is formed, not in the lungs, but by the 

 blood in its course, and is merely separated or given off at the 

 lungs. Still, many of the most eminent physiologists consider 

 modifications of Dr. Crawford's theory as affording the best explana- 



* An experiment of Mr. Hunter's shows the influence of the circulation in keeping 

 down temperature. A living part (with blood circulating through it, probably near 98 

 degrees) was placed in water at 118 degrees, and had its temperature elevated to 102! 

 degrees, that is, only 3 or 4 degrees above the natural temperature of some parts of the 

 body ; while a dead part, under the same circumstance, rose to 1 14 degrees, or about 

 12 degrees higher. Mr. Hunter imputed this difference to the influence of the vital 

 principle, but our friend Dr. Holland has probably given a more correct explanation, 

 in referring it to the comparatively colder fluid that circulated in the living part. 



