THE CAPILLARY CIRCULATION. 299 



a terminal ramification of still smaller and more numerous vessels, 

 and so lose themselves at last in the capillary network. 



By this final increase of the vascular surface, the equalization of 

 the heart's action is completed. There is no longer any intermitting 

 or pulsatile character in the force which acts upon the circulating 

 fluid ; and the blood, accordingly, is delivered from the arteries into 

 the capillaries under a perfectly continuous and uniform pressure. 



This pressure is sufficient to cause the blood to pass with con- 

 siderable rapidity, through the capillary plexus, into the commence- 

 ment of the veins. This fact was first demonstrated by Prof. 

 Sharpey, 1 of London, who employed an injecting syringe with a 

 double nozzle, one extremity of which was connected with a mercu- 

 rial gauge, while the other was inserted into the artery of a recently 

 killed animal. When the syringe, filled with defibrinated blood, 

 was fixed in this position and the vessels of the animal injected, the 

 defibrinated blood would press with equal force upon the mercury 

 in the gauge and upon the fluid in the bloodvessels ; and thus it 

 was easy to ascertain the exact amount of pressure required to force 

 the defibrinated blood through the capillaries of the animal, and to 

 make it return by the corresponding 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 milli- 

 metres 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 also performed a similar experiment on the vessels of 

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

 the lower extremity 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 ran from the femoral vein in a continuous stream 

 under a pressure of 120 millimetres, and that it was discharged very 

 freely under a pressure of 130 millimetres. 



Since, as we have already seen, the arterial pressure upon the 



1 Todd and Bowmann, Physiological Anatomy and Physiology of man, vol. ii. p. 

 350. 



