176 



THE SHEPHESD'S MANUAL. 



carries the blood which has circulated through the body to the 

 heart, from which it is forced to the lungs for purification. The 

 lungs are composed of a spongy, highly elastic mass of cells. 

 "When the cavity of the chest is enlarged by the contraction of the 

 diaphragm, this elastic mass swells and fills the cavity. The cells 

 enlarge, and vacuums are formed in them to fill which a stream of 

 air is immediately directed. The air 

 rushes through the nostrils into the 

 trachea or windpipe, and thence into 

 the bronchial tubes, which penetrate 

 the mass of the lungs in all direc- 

 tions, as the branches and twigs of a 

 tree. By these bronchial tubes, air 

 is carried into every portion of the 

 lungs, where it meets the blood 

 brought thither from every extremity 

 of the body by means of the vena cava 

 through the heart. In this contact 

 the blood, loaded with impurities, 

 and deprive;! of the oxygen which 

 is needed for the support of the 

 "body, parts with its load of offensive 

 matter, and takes up from the air 

 whatever oxygen it requires to re- 

 store it to a state of purity, and to 

 enable it to fulfill the functions of 

 circulation once more. The air having performed this office, is 

 expelled from the lungs by the relaxing of the muscles of the 

 diaphragm, and is expired. The expired air is highly charged 

 with carbonic acid and vapor of water, nearly the whole of its 

 oxygen having been absorbed by the blood. It is also charged 

 with other products of the constant decomposition or waste of the 

 tissues of the body, or of unnecessary matters which have entered 

 the circulation through the digestive organs. 



Circulation consists in the constant motion of the blood from 

 the heart through the arteries to the inner and outer surface of 

 the body, and through every tissue of which the body is composed ; 

 thence back by the veins to the heart ; thence to the lungs, where 

 it is purified and fitted to again serve for the nutrition of the body ; 

 it is then sent from the lungs to the ^eart to start again upon its 

 round to the extremities. 



The blood is the most important part in the system of an animal. 

 It consists of a white fluid colored red by very small globules, 



Fig. 68. THE LUNGS EN- 

 VELOPING THE HEART. 



