302 FUNCTIONS OF NUTRITION. 



/ was inserted into the artery of a recently-killed animal. When the 

 [ syringe, filled with defibrinated blood, was fixed in this position, it 

 would press with equal force on the mercury in the gauge and on the 

 fluid in the blood-vessels ; and the height of the mercurial column would 

 indicate the pressure required to carry the blood through the capillaries, 

 and to return it by the corresponding vein. With one end of the in- 

 jecting tube attached to the mesenteric artery of the dog, a pressure 

 of 90 millimetres of mercury caused the blood to pass through the double 

 capillary system of the intestine and the liver ; and under a pressure 

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

 vena cava. 



We have obtained similar results by experimenting on the blood- 

 vessels of the limbs. A full grown healthy dog was killed, and one 

 of the hind legs immediately injected with defibrinated blood, by the 

 femoral artery, to prevent coagulation in the small vessels. A double 

 syringe, filled with defibrinated blood, was then attached by one of its 

 extremities to the femoral artery and by the other to a cardiorneter. 

 On making the injection, the defibrinated blood was returned from the 

 femoral vein in a continuous stream under a pressure of 120 millimetres, 

 and was very freely discharged under a pressure of 130 millimetres. 



Since the arterial pressure during life is equal to 150 millimetres of 

 mercury, it is evidently sufficient to account for the capillary circulation. 



The capillaries have also a certain degree of elasticity ; and they are 

 furthermore surrounded, in many organs, by tissues which are them- 

 selves elastic. The effect of this property, in the vessels and neighbor- 

 ing parts, may be seen in artificial injections, not only of a lower limb 

 through the femoral artery, but also of the liver through the portal 

 vein. If, while the parts are distended by the fluid in their vessels, the 

 injecting force be suddenly arrested, the current does not at once cease, 

 but the fluid of injection continues to escape for some seconds from the 

 femoral or hepatic vein. The elasticity of the surrounding tissues sup- 

 plements that of the blood-vessels, and aids in producing a uniform 

 movement in the capillary circulation. 



Velocity of Blood in the Capillaries. The rate of movement in 

 the capillary circulation may be measured, with some precision, in the 

 microscopic examination of transparent tissues. The results obtained 

 in this way by different observers (Valentin, Weber, and Yolkmanu), 

 show that the rate of movement of the blood through the capillaries 

 is rather less than one millimetre per second. Since the rapidity of 

 the current must be in inverse ratio to the calibre of the vessels through 

 which it moves, it appears that the united calibre of all the capillaries 

 must be not less than 300 times greater than that of the arteries. It 

 does not follow from this that the whole quantity of blood contained 

 in the capillaries at any given time is greater than that in the arteries ; 

 since, although the united calibre of the capillaries is large, their length 

 is very small. The structure of the capillary system is such as to dis- 

 seminate a small quantity of blood over a large space, allowing its 



