THE COURSE OF THE BLOOD-FLOW. 



19. The Course of the Blood-Flow. 



blood, starting from any chamber of 

 the heart returns there after a short 

 time, and starts from it again. This is 

 why the blood-flow is called a circula- 

 tion. The return is not direct; blood 

 leaving the left side of the heart comes 

 back first to the right, and blood start- 

 ing from the right side returns first to 

 the left. 



How this occurs may be easily un- 

 derstood by examining Fig. 39, which 

 represents, in a general way, the heart 

 and blood-vessels. Starting from the 

 left ventricle,/", blood flows along the 

 aorta, m, and its branches, to all parts 

 except the lungs.* These branches 

 end in the very small and very 

 numerous capillaries, /, of the mus- 

 cles, and skin, and mucous membranes, 

 and so forth. From these capillaries 

 the blood is collected by veins, which 

 unite to make the hollow veins, k> 

 which pour it into the right auricle. 

 From right auricle it 'is sent to right 



Any portion of 



FIG. 39. Diagram of 

 the circulatory organs, 

 to show the course of 

 the blood-flow, d, right 

 auricle of the heart; g; 

 right ventricle; , pul- 

 monary artery and its 

 branches; a, the capil- 

 laries of the lungs; c, 

 the pulmonary veins; t, 

 the left auricle of the 

 heart; f, the left ventri- 

 cle; m, the aorta, di- 

 viding into the smaller 

 arteries; /, the capilla- 

 ries of all the organs 

 except the lungs; A, the 

 veins bringing to the 

 right auricle blood from 

 all parts of the body 

 but the lungs; A, the 

 pericardium. 



ventricle, and thence by the pulmonary artery and its 



19. Why is the blood flow called a circulation ? To which side of the 

 heart does blood which has last left the right ventricle first return ? 

 Starting from the left ventricle, describe the course taken by the 

 blood until it gets back there. How often does the blood come back; 

 to the heart in making a complete circulation ? 



* Some branches of the aorta carry a little blood to the lungs; but for the pur. 

 pose of getting a general idea of the blood-flow this may be neglected, 



