144 



THE VASCULAE SYSTEM 



posture the column of blood in the vessels of the limbs can be 

 broken up into segments by muscular action, and the blood per- 

 mitted to circulate by alternate and appropriate contraction and 

 relaxation of the muscles. A most important and hitherto almost 

 unexplored reflex mechanism is here engaged, a mechanism in 

 which the skeletal muscles and the muscular wall of the arteries 

 each take a part, and in which sensory " receptors " are engaged, 

 tuned in sensitivity to capillary blood pressure. 



To sum up then, the function of the heart is to drive the blood 

 into the capillary bed, the function of the arterioles is to regulate 

 the distribution and switch the current on to one or other part 

 of the capillary bed and limit the pooling of the blood under the 

 stress of gravity ; the function of the skeletal muscles and res- 

 piratory pump, aided by the valves in the veins, is to drive the 

 blood back from the capillary bed to the heart, to prevent hypo- 

 static congestion, and to act as a pump to the vascular system 

 of no less importance than the heart. The respiratory pump even 

 may drive the blood through the right heart and lungs. 



THE ACTION OF THE RESPIRATORY PUMP 



The writer's argument that the circulation is largely maintained 

 by bodily movement and varies continuously with every change 

 of posture and muscular contraction, and is liable to an extent 

 which defies analysis in the living active animal, is borne out by 

 recent researches, of T. Lewis on the effect of respiration on the 

 blood pressure of man. 



To determine this on man the sphygmograph can be used, but it 

 must not be fixed by a band encircling the wrist,, because this entails 

 a serious fallacy due to the swelling of the volume of the arm. A 

 sphygmograph applied with a band acts like a plethysmograph. 

 T. Lewis fixed the sphygmograph by a suspension method and con- 

 trolled the results with a sphygmomanometer. The effect of pure 

 intercostal or pure diaphragmatic breathing in a trained subject 

 he found to be as follows : 



