THE TRANSPORTING SYSTEM OF THE BODY 265 



pressure to increase often to a dangerous point. The im- 

 mense number of capillaries into which the arteries divide 

 distributes and lessens the pulsations of the heart and when 

 the blood reaches the veins it shows no pulsations. The 

 blood flows through the veins largely by reason of the pressure 

 from the heart, though the veins have valves at frequent 

 intervals which oblige the blood to flow in only one direction. 

 Each movement of the muscles, therefore, aids in compressing 

 the veins and forcing the blood onward. 



225. Regulation of the Blood Stream. Vigorous exercise 

 increases the heart beat and more blood reaches all parts of 

 the body in a given time, but the blood does not ordinarily 

 flow in unvarying quantity to each tissue and organ. In- 

 stead, the flow is automatically controlled by the nervous 

 system in such a way that the parts needing the greatest 

 supply shall receive it. After a meal, a large part of the blood 

 is sent to the digestive organs to provide them with the 

 materials for work. When one is studying, a larger amount 

 than usual is sent to the brain. In exercise, the muscles 

 receive an increased supply. From these facts we can under- 

 stand why one should not exercise vigorously immediately 

 after a hearty meal. It also explains why a light meal before 

 retiring may induce sleep by calling the blood from the brain 

 to the digestive organs. The regulation of the blood supply is 

 effected by nerves which cause the blood vessels to increase 

 or diminish in size. In blushing, the capillaries are dilated and 

 a larger amount of blood is sent to the skin. Fear and some 

 other emotions cause the capillaries to contract and make the 

 skin pale. Heat also causes the capillaries to expand and cold 

 causes them to contract. When injuries or the attacks of 

 bacteria cause an unusual flow of blood to any part, we speak 

 of it as congestion. 



226. Bleeding. If a blood vessel is injured, the blood, ow- 

 ing to the pressure upon it, begins to run out. If a vein_is 



