CIRCULATION. 337 



ticity, but this is even less called upon than that of the capillaries ; and, pres- 

 ently, in the larger veins, the moving blood is found to press no harder from 

 within than the atmosphere from without. 



Subsidiary Forces which Assist the Flow in the Veins. There are 

 certain forces which, occasionally or regularly, assist the heart to return the 

 venous blood into itself. Too much stress is often laid upon these ; for it is 

 easy to see by experiment that the heart can maintain the circulation wholly 

 without help. The origins of these subsidiary forces are, first, the contraction 

 of the skeletal muscles in general; second, the continuous traction of the 

 lungs; third, the contraction of the muscles of inspiration. 



The Skeletal Muscles and the Venous Valves. A vein may lie in 

 such relation to a muscle that when the latter contracts the vein is pressed 

 upon, its feeble blood-pressure is overborne, the vein is narrowed, and blood 

 is squeezed out of it. The veins in many parts are rich in valves, competent 

 to prevent regurgitation of the blood while permitting its flow in the physio- 

 logical direction. The pressure of a contracting muscle, therefore, can only 

 squeeze blood out of a vein toward the heart, never in the reverse direction. 

 Muscular contraction, then, may, and often does, assist in the return of the 

 venous blood with a force not even indirectly derived from the heart. But 

 such assistance, although it may be vigorous and at times important, is tran- 

 sient and irregular. Indeed, were a given muscle to remain long in contrac- 

 tion, the continued squeezing of the vein would be an obstruction to the flow 

 through it. 



The Continuous Pull of the Elastic Lungs. The influence of thoracic 

 aspiration upon the movement of the blood in the veins deserves a fuller dis- 

 cussion. The root of the neck is the region where this influence shows itself 

 most clearly, but it may also be verified in the ascending vena cava of an 

 animal in which the abdomen has been opened. The physiology of respira- 

 tion shows that not only in inspiration, but also in expiration, the elastic fibres 

 of the lungs are upon the stretch, and are pulling upon the ribs and intercostal 

 spaces, upon the diaphragm, and upon the heart and the great vessels. This 

 dilating force at all times exerted upon the heart by the lungs is of assistance, 

 as we shall see, in the diastolic expansion of its ventricles. In the same way 

 the elastic pull of the lungs acts upon the venae cavse within the chest, and 

 generates within them, as well as within the right auricle, a force of suction. 

 The eifects upon the venous flow of this continuous aspiration are best known 

 in the system of the ascending vena cava. This suction from within the 

 chest extends to the great veins just without it in the neck. In these, close to 

 the chest, as we have seen, mauometric observation reveals a continuous slightly 

 negative pressure. A little farther from the chest, however, but still within 

 the lower portions of the neck, the intravenous pressure is slightly positive. 

 The elastic pull of the lung, therefore, continuously assists in unloading the 

 terminal part of the venous system, and thus differs markedly from the irreg- 

 ular contractions of the skeletal muscles. 



The Contraction of the Muscles of Inspiration. But some skeletal 



