148 



THE HUMAN MECHANISM 



since, with the same arterial pressure, more blood will flow 



into an empty vein than into one which is partially filled. 



1. The breathing movements. There are two factors which 



thus tend to empty the veins. The first is the suction exerted 



on the blood within the veins 

 by breathing movements. The 

 exact mechanism by which 

 this is accomplished must 

 be left for consideration in 

 the chapter on respiration. 

 Suffice it to say here that 

 just as the enlargement of 

 the thorax, when we take 

 in a breath, sucks air into 

 the lungs, so it also sucks 

 blood from the large veins 

 outside the thorax into those 

 which lie within it ; because 

 of the thickness of the 

 walls of the arteries the 

 same effect occurs to only 

 a very slight extent in the 

 arterial reservoir. During 

 expiration, on the other 

 hand, the reduction in size 

 of the thorax forces air out 

 of the lungs, arid we might 

 expect that it would simi- 

 larly force blood from the 



veins within the thorax into those without. And this it cer- 

 tainly would do if the veins were not provided with valves 

 which allow the blood to flow only toward the heart. In 

 general, therefore, both inspiration and expiration aid the cir- 

 culation, the former by sucking blood into the thoracic veins 

 and so emptying those outside, the latter by making this 



FIG. 72. Cross sections of portions of 



the wall of a smaller artery (a) and a 



smaller vein () 



A, internal coat; B, middle coat, with 

 muscle fibers ; C, outer coat of connective 

 tissue. The contraction of the circularly 

 disposed muscle fibers narrows the bore 

 of the tube 



