300 THE CIRCULATION. 



blood is equal to six inches, or 150 millimetres, of mercury, it is 

 evident that this pressure is sufficient to propel the blood through 

 the capillary circulation. 



Beside, the blood is not altogether relieved from the influence of 

 elasticity, after it has left the arteries. For the capillaries them- 

 selves are elastic, notwithstanding the delicate texture of their 

 walls; and even the tissues of the organs which they traverse 

 possess, in many instances, a considerable share of elasticity, owing 

 to the minute elastic fibres which are scattered through their sub- 

 stance. These elastic fibres are found in considerable quantity in 

 the lungs, the spleen, the skin, the lobulated glands, and more or 

 less in the mucous membranes. They are abundant, of course, in 

 the fibrous tissues of the extremities, in the fascia3, the tendons, and 

 the intermuscular substance. 



In the experiment of injecting the vessels of the lower extremity 

 with defibrinated blood, if the injection be stopped, the blood does 

 not instantly cease flowing from the extremity of the femoral vein, 

 but continues for a short time, until the elasticity of the intervening 

 parts is exhausted. 



The same thing may be observed even in the liver. If the end 

 of a water-pipe be inserted into the portal vein, and the liver in- 

 jected with water under the pressure of a hydrant, the liquid will 

 distend the vessels of the organ, and pass out by the hepatic veins. 

 But if the portal vein be suddenly tied or compressed, so as to shut 

 off the pressure from behind, the stream will continue to run, for 

 several seconds afterward, from the hepatic vein, owing to the re- 

 action of the organ itself upon the fluid contained in its vessels. 



As a general rule, also, the capillaries do not suffer any backward 

 pressure from the venous system. On the contrary, as soon as the 

 blood has been delivered into the veins, it is hurried onward toward 

 the heart by the compression of the muscles and the action of the 

 venous valves. The right side of the heart itself continues the same 

 process, by its regular contractions, and by the action of its own 

 valvular apparatus ; so that the blood is constantly lifted away from 

 the capillaries, by the muscular action of the surrounding parts. 



These are the most important of the mechanical influences under 

 which the blood moves through the continuous round of the circu- 

 lation. The heart, by its alternating contractions and relaxations, 

 and by the backward play of its valves, continually urges the blood 

 forward into the arterial system. The arteries, by their dilatable 

 and elastic walls, convert the cardiac pulsations into a uniform and 



