614 
ON ANIMAL HEAT. 
there always remained, however, a portion which could not be 
referred to that source, but which can now, I consider, be fully 
accounted for by the mechanical action of the arteries. The pre- 
cise quantity given off* during each beat of an artery it would be 
exceedingly difficult, perhaps impossible, to discover ; but, if we 
admit the development of only a very small quantity, it necessa- 
rily follows, from the circumstance of the action of the arteries 
being in incessant operation during life, that the heat must quickly 
accumulate to a great extent, and that the body, unless cooled by 
the functions of the skin and lungs, would in a short space of 
time become preternaturally hot. 
“ The following physiological and pathological facts appear to 
corroborate the view 1 have taken of the mechanical source of 
heat: — 1st. The minute distribution of the arteries to every part 
of the system ensures a general and equal distribution of heat. 
2dly. The ossification of the arteries in old age, by diminishing 
their elasticity, is a probable cause of the diminution of animal 
heat at the close of life. 3dly. The increased warmth of the body 
from exercise appears to be more readily explicable upon the prin- 
ciple of increased force in the arteries, rather than in increased 
vigour in the functions of the lungs ; inasmuch as the immediate 
effect of exercise is evidently to embarrass the breathing, as shewn 
by the hurried respiration. 4thly. In many diseases of the 
lungs, where its functions are all at fault, at a time when the 
arteries are beating with increased violence, the heat of the body 
is found to be above the usual standard. 5thly. Medicines 
which diminish the contractility and elasticity of the arteries 
almost invariably reduce the heat of the body. 6thly. The heat 
of local inflammations, in cases where the constitution does not 
sympathise to any extent, cannot be easily referred to any other 
source, as the arteries immediately in the neighbourhood of the 
affected part are throbbing with violence at a time when the ca- 
pillaries (which are supposed to play so large a share in the che- 
mical theory of heat) are generally considered to have their action 
entirely arrested. Many facts of a similar nature could be enu- 
merated ; but enough, I think, has been stated to establish the 
truth of the theory in question. 
u Of the nature of the mechanical force I have been investi- 
gating little can be said : it may possibly be a kind of intermo- 
lecular friction. It is clearly, however, of a different nature from 
ordinary friction, and which has also been considered a source of 
animal heat ; but I think erroneously so: for, on examining the 
mechanism of the human body, we find that everywhere the most 
efficient means of defence have been provided against its effects, 
as seen in the various synovial, mucous, and serous membranes, 
