THE CIRCULATION 



281 



chest is filled by the tidal air. The same "suction" which causes the air to 

 rush in through the trachea inevitably draws also upon the partly filled 

 veins, which extend upward from the abdomen and downward from the 

 neck, and, by tending to pull apart their flaccid walls, sucks inward their 

 liquid contents toward the interior of the chest and the heart. The 

 thoracic space being increased, fluids under pressure rush in to fill it up. 

 This same influence is exerted on the heart itself, thereby assisting diastole 

 of all its chambers, and also on the arteries which enter the thorax. 

 Because, however, of their rigid, distended walls it would be nearly 

 ineffective on the arteries, the balance of the suction thus favoring the 

 circulation. The blood-pressure curve shows plainly that soon after 

 the beginning of inspiration the general blood-pressure begins to rise, and 

 that it reaches its maximum a short time after expiration begins. During 

 inspiration the pulse-rate also tends to increase, this being effected by 

 some nervous excitation, perhaps by an overcoming of the inhibitory 

 influence exerted by the vagus as its augmentor fibers are stimulated in 

 the expanding alveoli, but perhaps, too, in some other way. 



I To 



- Wave .01 



FIG. 148 



Systolic Wave III. 



Traube-He 



Dicrotic Wave IV. 





Exp. \ Insp. [ Exp. \Zero- 

 Pressure 







Exp. \ Insp. \ Exp. I Insp. Exp. Insp. 



A typical blood-pressure tracing made from an artery. (Hall.) 



COMPRESSION BY THE BODY-MUSCLES of the valve-supplied veins 

 is another factor in causing the circulation. The veins are soft and 

 easily compressed, being never normally distended with blood. The 

 muscles when they contract thicken and harden, and not only thereby 

 immediately compress the veins lying beneath, between, or within them, 

 but they thus increase the pressure in the limb or in the vicinity and 

 tend to cause a general compression of the veins of the region. Owing 

 to the presence of valves within the veins, the displacement of the blood 

 contained in these veins can occur only in one direction, and that always 

 toward the heart. As we have seen already, this action is still more 

 effective on the lymphatic system, and partly because the valves are there 

 much more numerous. Of course, the contracting muscles tend to 

 compress the arteries nearly as much as the veins, and so to impede 

 the circulation ; but here (as in the last-described cause of the circulation) 

 the forcible distention and incompressibility of the arteries prevent any 

 action and the balance of effect favors the circulation. In the hollow 

 viscera having muscular movements of their own this influence on the 



