THE VITAL PROCESSES 



present at a given time in the body, there is a liability of 

 empty spaces occurring in these tubes. Such spaces would 

 seriously interfere with the circulation, since the heart pres- 

 sure could not then reach all parts of the blood stream. 

 This is prevented by the contracted state, or "tone," of 

 the blood vessels, due to the muscular coat. 



In the second place, the muscular coat serves the pur- 

 pose of regulating the amount of blood which any given 

 organ or part of the body receives. This it does by vary- 

 ing the caliber of the arteries going to the organ in ques- 

 tion. To increase the blood supply, the muscular coat 

 relaxes. The arteries are then dilated by the blood pres- 

 sure from within so as to let through a larger quantity of 

 blood. To diminish the supply, the muscle contracts, 

 making the caliber of the arteries less, so that less blood 

 can flow to this part of the body. Since the need of 

 organs for blood varies with their activity, the muscular 

 coat serves in this way a very necessary purpose. 



Capillaries. The capillaries consist of a network of 



minute blood ves- 

 sels which connect 

 the terminations 

 of the smallest 

 arteries with the 

 beginnings of the 

 smallest veins 

 (Fig. 21). They 

 FIG. 21. Diagram of net work of capillaries have an average 



between a very small artery and a very small vein. A - f , 



Shading indicates the change of color of the blood diameter of ^88 



as it passes through the capillaries. S. Places be- than One tWO-thoU- 



tween capillaries occupied by the cells. sandth of an j nch 



(12 p) and an average length of less than one twenty-fifth 

 of an inch (i millimeter). Their walls consist of a single 



bet 



