Chap. 2.] Circulation through, the Lungs. 303 



.the lungs, and from the lungs, the pulmonary veins 

 bring it back to the pofterior auricle j from the pos- 

 terior auricle it paries into the pofterior ventricle ; from 

 the pofterior ventricle it is carried to every part of the 

 body, by means of the aorta and its branches, and 

 thence is again brought back by the venae cavas to the 

 anterior auricle, whence it proceeded. In this manaer, 

 throughout life, the blood is conftandy performing 

 two circles ; a leffer between the heart, and the lungs, 

 and a larger between the heart and the reit of the 

 -body. 



The two auricles and ventricles are of equal capacity, 

 and correfpond in their contractions. From thefe cir- 

 cumftances it is evident, that the fame quantity of 

 blood pafTes through the lungs in a given time, as 

 through all the reft of the body, and, confequently, 

 that the circulation muft be much more rapid in the 

 lungs than in other parrs. It is fuppofed that about 

 two ounces of blood are thrown from each ventricle of 

 the heart at every contraction. 



The heart, however, though the moft remarkable, 

 "is not the only organ of circulation ; fince every veflel 

 through which the blood paffes aflifts, by its contrac- 

 tile powers, to propel its contents. The fudden con- 

 . tractions of the heart, by which the blood is thrown 

 into the arteries, occafion their pulfation, which is moft 

 violent in the large tjunks, gradually becomes lefs re- 

 markable as they ramify and recede from the heart, 

 and is not at all perceptible in the veins, which receive 

 their blood from the arteries. The contraction of the 

 ventricles, by which the blood is propelled from the 

 heart, is called the fyftole ; the dilatation, by which the 

 blood is received into them, the diaftpk, 



The ftrufture of the heart in the tortoife and fome 



other 



