SECT. XXIII. i. OF THE CIRCULATORY SYSTEM. 305 



main for ever clofed, after they have contra&ed 

 themfelves, unlefs forne extraneous power be applied 

 to a^ain diftend them. This extraneous power in 

 refpect to the heart is the current of blood, which is 

 perpetually abforbed by the veins from the various 

 glands and capillaries, and pufhed into the heart by 

 a power probably very fimiiar to that, which raifes 

 the fap in vegetables in the fpring, which, accord- 

 ing to Dr. Hale's experiment on the ftump of a vine, 

 exerted a force equal to a column of water above 

 twenty feet high. This force of the current of 

 blood in the veins is partly produced by their abfor- 

 bent power, exerted at the beginning of every fine 

 ramification ; which may be conceived to be a mouth 

 abforbing blood, as the mouths of the la&eals and 

 lymphatics abforb chyle and lymph. And partly by 

 their intermitted compreflion by the pulfations of 

 their generally concomitant arteries ; by which the 

 blood is perpetually propelled towards the heart, as 

 the valves in many veins, and the abforbent mouths 

 in them all, will not fuffer it to return. 



The blood, thus forcibly injected into the cham- 

 bers of the heart, diftends this combination of hol- 

 low mufcles ; till by the ftimulus of diflention they 

 contract: themfelves ; and, pufhing forwards the 

 blood into the arteries, exert fufficient force T.Q 

 overcome in lefs than a fecond of time the vis iner- 

 tise, and perhaps fome elafticity, of the very extenr 

 five ramifications of the two great fyflems of the 

 aortal and pulmonary arteries. The power neceflary 

 to do this in fo fhort a time muft be confiderable, 

 and has been varioufly eitimated by different phyfio- 

 logifls. 



The mufcular coats of the arterial fyftem are then 

 brought into aclion by the ftimulus of diflention, 

 and propel the blood to the mouths, or through the 



Convolution^ 



