The Course of the Blood, 23 



blood they carry is dark red, and It flows in a steady, regular 

 stream. The arteries divide and subdivide into innumerable 

 branches, diminishing to a size where it takes the strongest 

 microscope to discover them, which are collected again into 

 larger and larger branches to form the veins. 



These smallest divisions are the capillaries^ and it is while 

 in them that the blood alters in color from a bright to a dark 

 red, by undergoing the changes necessary to keep up the life 

 of the tissues through which it flows. 



Thus the blood is returned to the heart dark and dull, 

 and loaded with worn out matter. It enters the right side 

 of the heart, which has two cavities wholly separate from 

 two similar ones on the left side. It is at once driven to 

 the lungs, where it is spread over the delicate thin walls 

 of millions of vesicles and exposed to the air inhaled at each 

 act of breathing ; it throws out carbonic acid gas, absorbs 

 oxygen, and returns to the left side of the heart fresh and 

 bright once more, and ready to recommence its journey. 

 And so it continues its endless round till death stops it for- 

 ever. 



In the human race diseases of the heart and blood-vessp.!s 

 are common, but in the lower animals they are rare. No 

 doubt the erect position of man, which demands heavier 

 labor from the heart, is partly a cause of this; another is his 

 greater nervous excitability, and his intemperance. In both 

 species, inflammatory rheumatism is often followed by a 

 change in the valves and walls of the heart. 



Practically, it is of great importance to be able to distin- 

 guish bleeding from arteries from bleeding from veins by the 

 color of the blood. For venous bleeding will usually cease 

 by simple measures ; but arterial bleeding requires the liga- 

 ture. 



When the amount and quality of the food is largely in 

 excess of the needs of the system for repairing its waste by 



