THE CIRCULATION OF THE BLOOD. 257 



of life, but the food cannot be changed into blood before the 

 air has acted on it by one of its principles, oxygen. Now, 

 if these two principles are not introduced into the system in 

 the same place, but in separate organs, it is evident they 

 cannot be employed in nutrition till they are brought toge- 

 ther ; which is done in the lungs ; the blood is there fitted 

 to carry nourishment and life to every part of the system. 

 Every animal then, that has a local respiration, must also 

 have a circulation. 



5. The organs of the circulation are the heart, arteries, 

 the veins, and the capillary vessels. The arteries and veins, 

 or the arterial and venous systems as they are called, have 

 been compared to two trees, the one scarlet and the other 

 purple, whose trunks are united at the heart, and whose 

 branches are connected at their extremities, thus forming a 

 regular circle. The blood is then forced out of the heart 

 by the contraction of that organ, into the arteries, by which 

 it is distributed throughout the body ; from whence it is re- 

 turned by the veins. Between the ends of the arteries and 

 the commencement of the veins, are the small hair-like ves- 

 sels called capillaries. 



6. (The heart is a hollow organ of a muscular and fibrous 

 structure, and somewhat conical in shape. It is placed in 

 the fore part of the cavity of the chest, inclined to the left 

 side. It rests on the midriff or diaphragm, which is the 

 muscle that separates the chest from the abdomen ; and it is 

 supported also at its base which is uppermost, by means of 

 the large blood-vessels connected with itA JThe heart is sur- 

 rounded by a strong membranous bag N , called the pericar- 

 dium, by which it is, in a measure, protected. This bag is 

 the seat of dropsy of the heart, and is generally found to 

 contain more or less water after death J 



7. (tn the human species, and throughout the class mam- 

 malia) the heart is a double organ ; consisting in fact of two 

 single hearts, each of which gives motion to a different kind 

 of blood. One of these, the right heart, receives the dark 



'W 22* 



