THE FROG AND VERTEBRATES IN GENERAL 197 



auriculo-ventricular aperture and into the left side of the ventricle. 

 There is more or less chance of mixing the venous and arterial 

 blood in the one-chambered ventricle, but this is minimized by 

 the structure of the ventricle. 



The contraction of the ventricle now takes place in such a way 

 that the venous blood from the right side is first forced out through 

 the conus arteriosus. This latter vessel is so arranged that the 

 venous blood is forced into the pulmocutaneous arteries which run 

 to the lungs and the skin. The oxygenated blood from the left 

 side of the ventricle is next forced out, and this blood passes from 

 the conus arteriosus into either the carotid or systemic arteries 

 which, with their branches, supply all the other regions of the body. 



Both the arteries and the veins break up in the tissues into a 

 dense network of microscopic, connecting capillaries. A certain 

 amount of the plasma of the blood together with some of the white 

 corpuscles are able to pass through the thin walls of the capillaries 

 into the lymph spaces and thus come into direct contact with the 

 tissues. The portion of the blood which leaves the closed vascular 

 system through the walls of the capillaries is the lymph. The 

 greater portion of the blood, instead of passing through the walls 

 of the capillaries, continues its more rapid course through the 

 capillaries and then into a vein leading back to the heart. 



Course of Circulation in the Mammal. The general course 

 of the circulation in Mammals is the same as in the Frog, but the 

 complete separation of the right and left sides of the heart has 

 established a pulmonary circulation which is independent of the 

 systemic circulation. Blood from all parts of the body, except the 

 lungs, is received directly into the right auricle from the superior 

 and inferior vena cava. The contraction of the auricle forces 

 the blood through the tricuspid valves into the right ventricle, 

 from where it passes through the pulmonary artery into the lungs 

 for oxygenation. Leaving the lungs, the blood returns to the left 

 auricle through the pulmonary veins and thus completes the pul- 

 monary circulation. (W. f. 121, C.) 



The contraction of the left auricle drives the oxygenated blood 

 through the mitral valve and into the left auricle. The strong 

 contractions of the latter drive the blood, under considerable 

 pressure, through the aorta and to the tissues of the body. In the 

 latter it passes through the capillaries, then into a connecting vein 

 and back to the heart, and thus completes the systemic circulation. 



