OF NATURAL HISTORY. 57 



aorta defcends along the inlide of the back-bone, and detach-. 

 es numerous ramifications to nourifh the vifcera and inferlo? 

 extremities. After ferving the moft remote extremities of 

 the body, the arteries are converted into veins, which, in 

 their return toward the heart, gradually unite into larger 

 branches, till the whole terminate in one great trunk called 

 the vena cava, which difcharges itfelf into the right ventriclo 

 of the heart, and completes the circulation. 



Befide the heart, the thorax or cheft contains the lung?, 

 or organs of refpiration. They are divided into five lobes, 

 three of which lie on the right, and two on the left fide of 

 the thorax. The fubftance of the lungs is chiefly compofed 

 of infinite ramifications of the trachea or windpipe, which, 

 after gradually becoming more and more minute, terminate 

 in little cells or veficles, which have a free communication 

 with one another. At each infpiration, thefe pipes and cells 

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

 In this manner, a circulation of air, which is neceflary to the 

 exiftence of men and other animals, is conflantly kept up as 

 long as life remains. 



The inftruments and procefs of digefilon fall next to be 

 confidered. The ftomach is a membranous and m.ufculnr 

 bag furnifhed with two orifices : By the one it has a com- 

 munication with the ocfophagus, or gullet, and by the other 

 with the bowels, which begin at the ftomach and terminate 

 at the anus. In the ftomach and inteftines there are im- 

 menfe numbers of minute veflels called laHeals, the mouths 

 of which are conftantly open for the reception of the nutri- 

 tious particles. iVfter being molfiened and lubricated by the 

 faliva, the food is received into the ftomach, where it is ftill 

 farther diluted by the gaftric juice, which has the power of 

 diirolving every kind of animal and vegetable fubflancc. 

 When the food has remained fome time in the ftomach, it 

 is reduced to a grayifh pulp, mixed with fome chylous or 



