142 THE CIRCULATION OF THE BLOOD 



fibres, which are usually slightly contracted, relax and 

 thus permit the arterioles to become larger and more 

 blood to pass. If, on the contrary, the tissue's activity 



Vein 



Connective tissue 

 Muscular coat 

 Elastic coat 

 Endothelial 

 Artery 



FIG. 83. Artery and vein, showing structure of walls. 



is diminished, they contract so that only the small 

 amount of blood necessary to supply the resting tissues 

 is allowed to pass through. These muscle fibres, under 

 the control of their nerves, thus automatically adjust the 

 supply of blood to the needs of the tissues. 



Branching of arteries. The arteries divide into 

 smaller and smaller branches and their number be- 

 comes very great. Eventually, they are so small and 

 numerous that they can penetrate nearly every particle 

 of tissue in the body. 1 As the arteries become smaller, 

 their walls also become thinner and the elastic tissue 



1 The only exceptions to this are cartilage and certain transpa- 

 rent parts of the eye. In these, sufficient nourishment reaches the 

 very inactive cells by means of the lymph which soaks through 

 from cell to cell. 



