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 injected into the chambers of the 

 heart, diitends this combination of hollow muicles ; till by the; 

 ftimulus of detention they contract themfelves; and, pu'hing 

 forwards the blood into the arteries, exert fufficient force to 

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

 fome elafticity, of the very extenfive ramifications of the two great 

 fyftems of the aortal and pulmonary arteries. The power neeef- 

 fary to do this in fo fhort a time rauft be considerable, and has 

 been variouily eftimated by different phyfiologifts. 



The mufcuiar coats of the arterial fyftem are then brought into 

 action by the flimulus of diilention, and propel the blood to the 

 mouths, or through the convolutions, which precede the fecretory 

 apertures of the various glands and capillaries. 



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

 but the vena portarum, which doer, the office of an artery, is dif- 

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

 is by this diilention {Simulated to contract itfeif, and propel the 

 blood to the mouths of the numerous glands, which compofe 

 that vifcus. 



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

 experiments of Dr. Hales on vine-dumps 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 

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

 lacteals and lymphatics always burft the receptaculum chyli. Mr. 

 Cooper adds, "The contractile powers of the ab for bents are 

 proved by thefe experiments to be very flrong 5 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 reliltance than the thoracic 

 duct j yet it is able to bear the preflure of a column of quick- 

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

 ed by the abforbents mull be acknowledged to be greater than 

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

 remembered, that living parts will refill a force, which will read- 

 ily tear them when dead." Medical Refearches. London. 

 1798, p. 1 10. 



Dr. Hales made experiments fimilar to thofe on the flumps 

 of vines above mentioned, by ooenii:? the crural arteries of a 

 hone, a dog, and a fallow deer, by applying mercurial gauges to 

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

 the vine-Hump to be five times greater than the force of the 

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



than 



