PHYSIOLOGY. 203 



CCLXVIII. The equality of heat in the different parts of 

 the same body seems to require the generating power to be very 

 generally diffused over the whole ; but it does not seem to re- 

 quire its being precisely equal in every part, as the interposition 

 of pretty large vessels in every part of the body, and the speedy 

 communication of the fluids from any one part to every other, 

 will sufficiently account for the equality of heat, though the 

 generating power should be in some measure confined to certain 

 parts only. 



However we take no notice of the suppositions which have 

 been made of the generating power's being confined to certain 

 small portions of the system only. These suppositions give no 

 relief in the general theory, and they are not supported by any 

 particular evidence. The breathing animals are the warm- 

 est; but that they are warmer because they breathe, is not 

 more probable than that they breathe because they are warmer. 



CCLXIX. With respect to this theory, which deduces ani- 

 mal heat from the motion of the blood, we must own that it is 

 attended with several difficulties. It will be difficult to show, 

 in so many animals of different age, size, and temperament, in 

 which the degree of heat is nearly the same, that the motion of 

 the blood, in all its circumstances, is also exactly the same ; or 

 to show, in the different animals in which the degree of heat is 

 considerably different, that the motion of the blood in each is 

 correspondent to the difference of heat. May it not be sup- 

 posed that there is some circumstance in the vital principle of 

 animals which is in common to those of the same class, and of 

 like economy, and which determines the effect of motion upon 

 the vital principle to be the same, though the motion acting 

 upon it may be in different circumstances ? 



CCLXX. In all we have hitherto said of animal fluids, we 

 have considered the common mass of blood as consisting of three 

 parts or three kinds of matter only ; but many more have 

 been supposed to be present in it ; and we shall inquire upon 

 what ground. 



It is common to suppose that the aliment or the chyle formed 

 of it is not perfectly assimilated in passing once only through 

 the lungs ; but that, for some time after such passage, it con- 

 tinues to circulate with the blood under the same form and of 



