VENOUS CIRCULATION. 293 



piration in crying produce a similar turgescence and engorgement 

 of the large veins during expiration, while they are momentarily 

 emptied during the hurried and forcible inspiration. 



In natural and quiet respiration, therefore, the movements of the 

 chest hasten and assist the venous circulation ; but in forced or 

 laborious respiration, they do not assist and may even retard its flow. 



2. The contraction of the voluntarg muscles. The veins which 

 convey the blood through the limbs, and the parietes of the head 

 and trunk, lie among voluntary muscles, which are more or less 

 constantly in a state of alternate contraction and relaxation. At 

 even- contraction these muscles become swollen laterally, and, of 

 course, compress the veins which are situated between them. The 

 blood, driven out from the vein by this pressure, cannot regurgitate 

 toward the capillaries, owing to the valves, already described, which 

 shut back and prevent its reflux. It is accordingly forced onward 

 toward the heart ; and when the muscle relaxes and the vein is 

 liberated from pressure, it again fills up from behind, and the cir- 

 culation goes on as before. This force is a very efficient one in 

 producing the venous circulation ; since the voluntary muscles are 

 more or less active in every position of the body, and the veins 

 constantly liable to be compressed by them. It is on this account 



Fig. 96. Fig. 97. 



VEI.V with valves opeu. VEIN with valves closed: stream of 



blood passing otfby a lateral channel. 



that the veins, in the external parts of the body, communicate so 

 freely with each other by transverse branches ; in order that the 

 current of blood, which is momentarily excluded from one vein by 



