mV THE FUNCTIONS OF THE HEART AND AETE&iES. (^3 



peritnents, can only be employed in overcoming the resist- 

 ance of the miuuter arteries and veins, and these observa- 

 tions tend therefore immediately to confirm the analogy 

 drawn from the experiments on the motion of water. It 

 might indeed be asserted, that the viscidity of the blood ex- 

 ceeds that of water in a much greater ratio than that which 

 is here assigned ; but this is rendered improbable by some 

 experiments of Hales, in which, when the intestines were 

 laid open, on the side opposite to the mesentery, so that 

 many of the smaller arteries were divided, the quantity of 

 warm water which parsed through them with an equal pres- 

 sure, was only about twelve times as great as that of the 

 blood which flows through them in their natural state; and 

 it is probable, that at least three or four times as much of 

 any fluid must have passed through them in their divided, 

 as in their entire state, unless we suppose that the coats of 

 the divided vessels, like many other muscular parts, are ca- 

 pable of being contracted by the contact of wAter. In some 

 other experiments it was founil, that a moderate degree of 

 pressure was capable of causing water to exude so copiously 

 through the exhalant vessels of the intestines, that it passed 

 through the aorta with a velocity of about two inches in a 

 second, although these vessels do not naturally allow anv 

 passage to the blood: on theotheriiand, it sometimes hap- 

 pened, that very little water would pass through such chan- 

 nels as naturally transmitted a much larger quantity of 

 blood: a circumstance which Dr, Hales very judiciouslv" 

 attributes to the oozing of the water into the cellular mem- 

 brane surrounding the vessels, by means of which they were 

 compressed, and their diameters lessened. On the wliole, 

 it is not improbable, that in some cases the resistance, op-- 

 posed to the motion of the blood, may exceed that of water 

 in a ratio somewhat greater than I have assigned ; but tliis 

 must be in the minutest of the vessels, while in the larger 

 arteries the disproportion must be less: so that, however we 

 may view the subject, it appears to be established, tliat 

 the only considerable resistance, which the blood experi- 

 ences, occurs in the extreme capillary arteries, of wliich the 

 diameter scarcely exceeds the hundredth part of an iijch. 



We cannot suppose, that the dimensions of the sangnife- The truth of 



rous 



