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XX. A Letter to Martin Folkes >Efq\ Prefident 
of the Royal Society, from Cromwell Mor- 
timer, M. 2). Seer, of the fame y concerning 
the natural Heat of Animals, 
SIR, June 20. 1745. 
Read July 4OINCE the complete and full Demon- 
ij ftration of the Circulation of the Blood 
in Animals by our illuftrious Countryman the great 
Dr. Harvey, the Generality of medical. Writers have 
attributed the natural Heat of Animals to the Mo- 
tion of the Blood in the Blood-veffels, or rather to 
an Attrition of all the Fluids in the Animal ariling 
from it ; which Fluids, from the later Difcoverics by 
Injections and Microfcopes, are found to move in co- 
nical Canals communicating one with another near the 
Apices, or where the Arteries are the narrowed, foon 
afterwards growing wider and wider, when the fame 
continued Canals obtain the Name of Veins, and con- 
vey back the Fluids they contain to the Heart. 
They aferibe Heat in an Animal to ftrong and fre- 
quent Contractions of the Heart and Arteries ; which. 
Heat* will be the greater, the more denfe the Humours 
are, the more ftrongly they are propell’d, and the. 
greater the Refiftances are, near the Ends of the Ar- 
teries. From this Suppofition they conclude, that 
the Heat arifes from Attrition } that, by a violent 
Agitation of the Particles of the Blood and Humours 
againft one another, and efpecially by the Attrition 
of them againft the Sides of the containing Blood* 
vefielSj 
* Boerhaave Inftit. . §. 968, 
