CIRCULATION 23 



coagulation time, so that individuals have bled to death 

 after the extraction of a tooth or the production of sim- 

 ilar small wounds. 



The circulatory apparatus consists of a central pump- 

 ing station, the heart; a set of vessels leading from the 

 heart to all portions of the body, the arteries; an im- 

 mense number of small thin-walled vessels, the capilla- 

 ries, in which the arteries terminate; and a set of ves- 

 sels, the veins, which return the blood to the heart. The 

 schematic drawing shows that arteries constantly dimin- 

 ish in size as they get further from the heart, that the 

 capillaries are the terminations of arteries and begin- 

 ning of veins, and that the veins, by one vein joining an- 

 other, increase in size as they approach the heart. The 

 total area of capillaries is greater than that of the veins 

 and much greater than that of the arteries. If the blood 

 in the arteries differs from that in the veins, it is obvious 

 that there must be a sort of midpoint in the capillary 

 system where arterial changes to venous blood; and if 

 this is true, it is apparent that the arteries carry some- 

 thing to the tissues which they need and that the veins 

 take something away which is either useless or inju- 

 rious. The object of the circulation of the blood is, there- 

 fore, briefly, to feed or irrigate the tissues through the 

 arteries and to drain them through the veins. But the 

 veins can not pour out the blood so charged, because it 

 would waste the blood which can be purified; so the 

 lungs liave been provided to burn up the waste material 

 and change venous back to arterial blood which is fur- 

 ther cleansed by the liver and kidneys. The course of 

 the circulation is then from the heart through the arter- 

 ies and capillaries to the tissues where arterial is 

 changed to venous blood; through venous capillaries 

 and veins to the heart and thence to the lungs where 



