THE CIRCULATION OF THE BLOOD 



149 



blood in the intrathoracic veins flow on more rapidly to the 

 heart, whence it is pumped into the arteries. In a word, 

 deep breathing greatly promotes a good circulation. 



2. Intermittent compression of the veins. The other second- 

 ary factor of the circulation is intermittent compression of the 

 veins, and in ordinary life this is brought about in two ways : 

 (1) Whenever a muscle contracts it thickens and hardens; 

 the veins and capilla- 

 ries which are between 

 the fibers and fiber bun- 

 dles, or in the con- 

 nective tissue between 

 two contracting mus- 

 cles, will thus have the 

 blood squeezed out of 

 them into the large 

 veins ; when the mus- 

 cle relaxes, the empty 

 veins and capillaries 

 will readily fill from 

 the arteries, since the valves of the veins will prevent any 

 backward flow of the blood from the larger veins. Alternate 

 contractions and relaxations of muscles therefore aid the 

 flow of blood through this so-called " pumping " action on 

 the veins. (2) A similar pumping action on the veins is ex- 

 erted by alternate flexions (bendings) and extensions at any 

 joint. In general, flexions force the blood out of the veins, 

 while extensions allow them to fill. When we remember how 

 largely most of our usual muscular actions consist of alter- 

 nate flexions and extensions of joints and alternate contrac- 

 tions and relaxations of muscles (for example, in walking 

 and running), we can at once appreciate how greatly mus- 

 cular activities must aid the circulation. When to the effect 

 of these we add the suction action of the deepened breathing 

 movements, the effect upon the circulation becomes very great. 



FIG. 73. The pocket valves in the veins 



On the right is shown the external appearance 

 of the vein at the valves when the latter are 

 closed ; on the left, a vein slit lengthwise and 

 opened ; in the middle, a longitudinal section 

 of a vein 



