280 THE CIRCULATION 



bined area of their cross-sections owing to their multitude is perhaps 

 several hundred times that of the aorta. In small tubes, however, 

 friction is enormous and the heart has this friction to overcome as a 

 resistance. This raises the aortic blood-pressure to the 150 millimeters 

 or more of mercury already noted. Thus, while the blood forced into 

 the aorta at each beat pushes some of the contained blood onward into 

 the smaller arteries, distending them in turn, some of the newly arrived 

 blood distends the aorta directly. With a pulse-rate of 75 there is an 

 interval of half-a-second between ventricular systoles. During this 

 half -second the flow in the aorta would tend to slacken and that in 

 the arterioles and capillaries to stop were it not for the energy put 

 into the elastic arterial walls by their forcible distention, this distention 

 in turn being dependent on, the high resistance of the minute capillaries. 

 But immediately the systole stops this energy of passive elastic recoil 

 becomes kinetic and presses hard upon the sides of the cylinder of blood 

 in the aorta, etc. Toward the heart the aortic valves (closed promptly 

 after the ventricular systole ended by the back-pressure beyond them) 

 shuts off absolutely the blood's escape, and the only way for the blood- 

 mass to flow is toward the capillaries. Thus, while ventricular systole 

 lasts 0.3 second, this strong recoil of the arterial wall pushes on the 

 blood during the 0.5 second which elapses before another systole bursts 

 open the aortic valves again and pushes out another ventricleful of 

 blood to cause another pulse throughout the whole arterial system. (See 

 the circulation-schema experiments in the Appendix.) It is to the high 

 resistance in the capillaries and to the elasticity of the arteries that the 

 constancy of the flow in the capillaries is due. When an artery is cut, 

 intermittent spurting of blood is seen, for the taking up of the inter- 

 mittence is not complete until the capillaries are reached. From veins, 

 on the other hand, except in abnormal cases of overdilated capillaries, 

 allowing the pulse to pass through them, the flow is uniform and gentle. 



The muscle of the arterial wall is used to maintain the tonus of the 

 circulation locally and to adapt the latter to the general systemic condi- 

 tions. By its automatic or reflex contraction and relaxation the caliber 

 of the artery is varied to suit the manifold conditions. It is not evident 

 that its contraction ever occurs suddenly enough to make it an aid to the 

 movement of the blood through its vessels. There is, however, no actual 

 evidence that this sudden vaso-constriction does not occur, especially 

 under abnormal conditions in the heart or the arteries. (See p. 298.) 



THORACIC SUCTION, or the "aspiration of the thorax," is the most 

 powerful of the three forces which act upon and assist the venous side of 

 the circulation. The physical principles underlying this suction on the 

 veins entering the boundaries of the chest have been gone over in our 

 discussion of respiration (page 116). 



The thorax is an adjustable box closed save for the trachea passing 

 upward toward the nostrils and for the blood- and lymph-vessels extending 

 from it both upward and downward. This box enlarges in every direc- 

 tion at each inspiration. Much of the increased space so made within the 



