SECT. XXIII. i. i. SYSTEM. 207 



pelled towards the heart, as the valves in many veins, and the ab- 

 forbent mouths in them all, will not fufFer it to return. 



The blood, thus forcibly injeled into the chambers of the 

 heart, diftends this combination of hollow mufcles ; till by the 

 ftimuius of difbention they contract themfelves ; and, pufhing 

 forwards the blood into the arteries, exert fufficient force to 

 overcome in lefs than a fecond of time the vis inertiae, and per- 

 haps fome elafticity, of the very extenfive ramifications of the 

 two great fyftems of the aortal and pulmonary arteries. The 

 power neceffary to do this in fo (hort a time muft be confidera- 

 able, and has been varioufly eltimated by different phyfiologifts. 



The mufcular coats of the arterial iyftem are then brought 

 into action by the ftimuius of diftention, and propel the blood 

 to'the mouths, or through the convolutions, which precede the 

 fecretory apertures of the various glands and capillaries. 



In the vefTels of the liver there is no intervention of the heart ; 

 but the vena portaru;n, which does the office of an artery, is dif- 

 tended by the blood poured into it from the mefenteric veins, 

 and is by this diftention ftimulated to contract itfelf, and propel 

 the blood to the mouths of the numerous glands, which com- 

 pofe that vifcus. 



The power of abforption in vegetable roots was (hewn by the 

 experiments of Dr. Hales on vine-ftumps to be equal to the pref- 

 fure of thirty-eight inches of quickfilver. Veg. Staticks, p. 107. 

 and from the experiments of Mr. Cooper, who tied the thoracic 

 dirts of living dogs, it appeared, that the abforbent power of the 

 lacteals and lymphatics always burll the receptaculum chyli. Mr. 

 Cooper adds, " The contractile powers of the abforbents are 

 proved by thefe experiments to be very ftrong ; for it appears, 

 that their action is fufficient to occafion a rupture of their coats. 

 It is true, that the receptaculum chyli, which was the part bro- 

 ken, is thinner and lefs capable of refiftance than the thoracic 

 du w t ; yet it is able to bear the preflure of a column of quick- 

 (ilver more than two feet in height. The force therefore exert- 

 ed by the abforbents muft be acknowledged to be greater than 

 that of fuch a column of mercury ; more efpecially when it is 

 remembered, that living parts will refift a force, which will 

 readily tear them when dead." Medical Refearches. Lon- 

 don. 1798, p, no. 



Dr Hales made experiments (imilar to thofe on the flumps 

 of vines above mentioned, by opening the crural arteries of a 

 horfe, a dog, and a fallow deer, by applying mercurial guages to 

 meaiure the projectile impetus of their blood ; and found that of 

 the vine (lump to be five times greater than the force of the 

 blood in the great crural artery of a horfe, feven times greater 



than 



