THE CAPILLARY CIRCULATION. 349 



nectcd with a mercurial gauge, while the other was inserted into the 

 artery of a recently killed animal. When the syringe, filled with defi- 

 brinated blood, was fixed in this position, the defibrinated blood would 

 press with equal force upon the mercury in the gauge and upon the fluid 

 in the bloodvessels ; and thus the height of the mercurial column indi- 

 cated the amount of pressure required to force the defibrinated blood 

 through the capillaries of the animal, and to make it return by the cor- 

 responding vein. In this way Prof. Sharpey found that, when the free 

 end of the injecting tube was attached to the mesenteric artery of the 

 dog, a pressure of 90 millimetres of mercury caused the blood to pass 

 through the capillaries of the intestine and of the liver ; and that under 

 a pressure of 130 millimetres, it flowed in a full stream from the divided 

 extremity of the vena cava. 



We have obtained similar results by experimenting upon the vessels 

 of the lower extremity. A full grown, healthy dog was killed, and one 

 of the lower extremities immediately injected with defibrinated blood, 

 by the femoral artery, in order to prevent coagulation in the smaller 

 vessels. A syringe with a double flexible nozzle was then filled with 

 defibrinated blood, and one extremity of its injecting tube attached to 

 the femoral artery, the other to the mouthpiece of a cardiometer. By 

 making the injection, it was then found that the defibrinated blood was 

 returned from the femoral vein in a continuous stream under a pressure 

 of 120 millimetres, and that it was discharged very freely under a pres- 

 sure of 130 millimetres. 



Since the arterial pressure upon the blood during life is equal to 150 

 millimetres of mercury, it is evident that this pressure is sufficient to 

 propel the blood through the capillary circulation. 



Furthermore, the blood is not altogether relieved from the influence 

 of elasticity, after leaving the arteries. For the capillaries themselves 

 have a certain degree of elasticity, and are surrounded, in addition, by 

 the tissues of the organs in which they are distributed ; many of which, 

 such as the lungs, spleen, skin, lobulated glands, and mucous membranes, 

 contain elastic fibres more or less abundantly disseminated through their 

 substance. The effect of this physical property, in the vessels and the 

 neighboring parts, may be exhibited in artificial injections of one of the 

 lower limbs through the femoral artery, or of the liver through the 

 portal vein. If, while the parts are distended by the fluid passing 

 through their vessels, the injecting force be suddenly arrested, the move- 

 ment of the current does not at once cease, but the fluid of injection 

 continues to escape for several seconds from the femoral or hepatic vein, 

 owing to the continuous pressure exerted from behind. The elasticity 

 of the surrounding tissues, therefore, supplements, that of the minute 

 bloodvessels, and aids in producing a uniform movement of the capillary 

 circulation. 



Velocity of the Blood in the Capillary Vessels. The motion of the 

 blood in the capillary vessels is much less rapid than in either the 

 arteries or the veins. It may be measured, with a tolerable approach 



