DISEASES OF HEAET, BLOOD VESSELS, AND LYMPHATICS. 73 



All the arteries, except the pulmonary artery and its branches, 

 carry bright-red blood, and all the veins, except the pulmonary veins, 

 carry dark-red blood. The impure dark-red blood is collected from 

 the capillary vessels and carried to the right auricle by the veins: it 

 passes down into the right ventricle and thence into the pulmonary 

 artery, and through its branches to the capillaries of the lungs, where 

 the carbonic-acid gas and other impurities are given up to the air in 

 the air cells of the lungs (through the thin walls between the capilla- 

 ries and the air cells), and where it also absorbs from the air the 

 oxygen gas necessary to sustain life, which changes it to the bright- 

 red, pure blood. It passes from the capillaries to the branches of 

 the pulmonary veins, which convey it to the left auricle of the heart ; 

 it then passes through the auriculo-ventricular opening into the left 

 ventricle, the contraction of which forces it through the common 

 aorta into the posterior and anterior aortas, and through all the 

 arteries of the body into the capillaries, where it parts with its 

 oxygen and nutritive elements and where it absorbs carbonic-acid 

 gas and becomes dark colored. (See theoretical diagram of the 

 circulation, PI. VII.) 



The branches of certain arteries in different parts unite again after 

 subdividing. This reuniting is called anastomosing, and assures a 

 quota of blood to a part if one of the anastomosing arteries should be 

 tied in case of hemorrhage, or should be destroyed by accident or 

 operation. 



The various kinds of food, after being digested in the alimentary 

 canal, are absorbed and carried into the blood by the lymphatics, and 

 by the blood to the places where nutrition is required. The blood 

 takes from all parts of the body all that is useless and no longer 

 required, and carries it to the different organs where it is eliminated 

 from the body. It contains within itself all the elements which 

 nourish the body. 



The blood may be considered a fluid holding in solution certain 

 inorganic elements and having certain bodies suspended in it. To 

 facilitate description, the blood may be considered as being made up 

 of the corpuscles and the liquor sanguinis. The corpuscles are of 

 two kinds, the red and the white, the red being the most numerous. 

 The color of the blood is due to the coloring matter in the red cor- 

 puscles. The red corpuscles are the oxygen carriers. Both kinds 

 are very minute bodies, which require the aid of the microscope to 

 recognize them. The liquor sanguinis is composed of water contain- 

 ing in solution salts, albumen, and the elements of fibrin. 



The lymphatics, or absorbents, are the vessels which carry the 

 lymph and chyle in the blood. They begin as capillaries in all parts 



