CAUSE OF CONTRACTILITY. 017 



that this contraction is hastened by an elevation 

 of temperature. Yet it seems never to have oc- 

 curred to this great man, that caloric is the active 

 principle in the blood ; nor that if the tempera- 

 ture of the whole earth were reduced to 32 for 

 any considerable time, there could not be a vestige 

 of life on its surface. 



Corresponding with the large amount of caloric 

 derived from the atmosphere by the respiration 

 of birds, the pulsations of the heart are stronger 

 and more frequent than in mammalia, ceteris 

 paribus, varying from one hundred and ten, to 

 one hundred and fifty, and even two hundred 

 per minute, in the smaller and more active spe- 

 cies. So rapid is their circulation, that when the 

 vessels of the neck are divided, nearly all their 

 blood escapes in about half a minute ; but re- 

 quires from one and a half to two minutes, in 

 the dog, sheep, deer, hog, and other mammalia 

 of the same size : whereas in man, whose mean 

 temperature is several degrees lower, the pulsa- 

 tions of the heart are less vigorous and frequent. 

 For it has been found to discharge about two 

 ounces of blood from the ventricles at each beat, 

 more or less, according to the energy of the con- 

 stitution, size of the organs, &c. ; so that if it con- 

 tract seventy times per minute, it follows, that 

 from twenty to thirty pounds of blood, or all 

 which the body contains, must circulate through- 

 out the lungs and general system in from two to 



