OF NATURAL HISTORY. 47 



"blood. By the contradions and dilatations of this mufcle, the blood 

 is alternately thrown out of, and received into, i:s feveral cavities. 

 When the heart contsadts, the blood is propelled from the right 

 ventricle into the lungs through the pulmonary arteries, which, like 

 all the other arteries, are furnifhed with valves that play eafily for- 

 ward, but admit not the blood to regurgitate toward the heart. The 

 blood, after circulating through the lungs, returns into the left ven- 

 tricle of the heart by the pulmonary vein. At the fame inftant, the 

 left ventricle drives the blood into the aorta, a large artery which 

 fends off branches to fupply the head and arms. Another large 

 branch of the aorta defcends along the infide of the back-bone, and 

 detaches numerous ramifications to nourifh the vifcera and inferior 

 extremities. After fervlng the moft remote extremities of the bo- 

 dy, the arteries are converted into veins, which, in their return to- 

 ward the heart, gradually unite into larger branches, till the whole 

 terminate in one great trunk called the 'uena ca%>a, which difcharo-es 

 itfelf into the right ventricle of the heart, and completes the circu- 

 lation. 



Befide the heart, the thorax or cheft contains the lungs, or organs 

 of refpiration. They are divided into five lobes, three of which He 

 on the right, and two on the left fide of the thorax. The fubflance 

 of the lungs is chiefly compofed of infinite ramifications of the tra- 

 chea or windpipe, which, after gradually becoming more and more 

 minute, terminate in little cells or veficles, which have a free com- 

 munication with one another. At each infplration, thefe pipes and 

 cells are filled with air, which is again ^difcharged by refpiration. In 

 this manner, a circulation of air, which is necefiary to the exiftence 

 of men and other animals, is conflantly kept up as long as life re- 



mams. 



The 



