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creases, as, at a distance from the heart, the canals along' which it circtt* 

 lates, increase in number; and lastly, the deviations which the blood 

 meets with in its course, from the trunks into the branches which, 

 coming off sometimes, almost at right angles, divert it from its original 

 direction. 



Several physiologists have called in question this progressive slacken- 

 ing of the flow of arterial blood, and several among them, who reject en- 

 tirely the application of the physical sciences to that of the animal econo- 

 my, have, nevertheless, supported their opinion by a fact taken from hy- 

 draulics. To give any certainty to these calculations, respecting the 

 impediments to the circulation of the blood in the arteries, it would, they 

 say, be necessary, that the arteries should be empty at the instant when 

 they receive the jet of blood sent into them by the contraction of the 

 ventricles. This, however, is not the case ; the arteries are always full, 

 the blood flows along all of them with the same degree of velocity. This 

 system of vessels may be compared to a syringe, from which a number of 

 straight and tortuous tubes should arise; each of these would throw out 

 the fluid with an equal degree of velocity, on applying pressure to the 

 piston. 



In refuting this doctrine, I must take notice of the manifest contradic- 

 tion of pretending to exclude, absolutely, all application of the principles 

 of mechanics tp physiology, and the complete application of these princi- 

 ples to the phenomena of the animal economy. This contradiction, how- 

 ever, is not more surprising than that of authors who exclaim against the 

 abuse of modern nomenclators, and who, nevertheless, eagerly embrace 

 every opportunity of adding to it, by assigning new names to such parts 

 as may have escaped the attention of the new nomenclators. What re- 

 semblance is there, between a forcing pump, whose sides are unyielding, 

 as well as those of the tubes which might arise from it, and the aorta which 

 dilates every time the blood is sent into it; and again, what resemblance 

 is there, between tubes which decrease towards their open extremities, 

 \vhilethespacecontained in the arterial tube constantly enlarges, from 

 the innumerable divisions of the vessels. Since it is admitted, that the 

 coarse of the blood is slower in the capillary vessels, must not this re- 

 sistance, opposed to the blood which fills the series of vessels from the 

 capillaries to the heart, be felt more at a greater distance from that organ, 

 &c.? Without this progressive increase of resistance, as the arterial 

 blood is at a greater distance from the heart, this fluid would flow along 

 the arteries, as it does along the veins without any pulsations ; for, this 

 resistance, which causes the lateral effort of dilatation effected by the 

 blood on the parietes of the arteries, is the principal cause of the pulse, 

 which belongs only to that set of vessels. A very remarkable difference 

 is observable, between the blood which is sent to the toes, and that which 

 goes to the mammae, as I have several times noticed in removing the cu- 

 rious bones of the toes, or in extirpating cancerous breasts : the small ar- 

 teries of these parts are nearly of the same size, but the jet of blood is 

 much more rapid, the blood is sent to a much greater distance, when one 

 of the mammary arteries is divided. 



The re-action of the arteries on the blood which dilates them, depends 

 not only on the great elasticity of their parietes, but likewise on the con- 

 tractility of the muscular coat. Elasticity has a considerable share in the 

 action of the larger trunks, while contractility is almost the sole agent, 

 in producing the action of the minute arteries. If a finger is introduced 



