Chap. 23.] Valves. 239 



and are therefore called valvulas tricufpides j thofe of 

 the pofterior ventricle terminate in two points, and 

 from being compared to a mitre, are called valvular 

 mitrales. In each of the great arteries, which pro- 

 ceed from the ventricles, the aorta and pulmonary- 

 artery, are feated three valves turned from the ven- 

 tricles, and are called femilunares. All thefe valves 

 are elongations of the internal membrane of thfc 

 part to which they belong. They are clofely con- 

 nected on that fide from which the current of blood 

 proceeds, and their other extremity is loofe. When 

 the blood, therefore, proceeds in its proper courfe, 

 they are prefied clofe to the fide of the veffel, and 

 occafion no impediment ; but when it is about to re- 

 turn in the contrary direction, they are raifed from the 

 fide of the veflel, and meeting in the middle of its 

 .cavity, {hut up the channel. The internal furface of 

 the ventricles is extremely uneven, from a number of 

 flefliy columns which rife from its infide, and fome of 

 which terminate by tendinous extremities in the valves 

 of the heart, which they fupport, and enable to per- 

 form their office more effectually. 



Befides the connection, however, between the au- 

 ricles and ventricles of the heart, each auricle com- 

 municates with a large vein, and each ventricle with 

 a large artery, The ufe of the auricle is to receive 

 the blood from the vein, and to difcharge it into the 

 cavity of the ventricle. The ventricle receives the 

 blood from the auricle, and drives it forcibly into the 

 artery. By a repetition of thofe actions is performed 

 the circulation of the blood, which is the fubject of 

 another chapter, in which I (hall take occafion.to 

 mention fome remarkable varieties in the hearts of 

 different races of animals. 



The 



