160 



BLOOD FKESSUKE. 



[BOOK i. 



that is to say, the pressure of blood in the vein bearing on the 

 proximal descending limb of the manometer may be less than 



FIG. 28. LUDWIG'S KYMOGRAPH FOR RECORDING ON A CONTINUOUS ROLL, OF PAPER. 



the pressure of the atmosphere on the ascending distal limb, so 

 that when communication is made between the interior of the vein 

 and the manometer, the mercury sinks in the distal and rises in 

 the proximal limb, being sucked up towards the vein. 



The manometer cannot well be applied to the capillaries, but we 

 may measure the blood pressure in the capillaries in an indirect way. 

 It is well known that when any portion of the skin is pressed upon, 

 it becomes pale and bloodless ; this is due to the pressure driving 

 the blood out of the capillaries and minute vessels, and preventing 

 any fresh blood entering into them. By carefully investigating 

 the amount of pressure necessary to prevent the blood entering 

 the capillaries and minute arteries of the web of the frog's foot, or 

 of the skin beneath the nail or elsewhere in man, the internal 

 pressure which the blood is exercising on the walls of the capil- 

 laries and minute arteries and veins may be approximately deter- 

 mined. In the frog's web this has been found to be equal to 

 about 7 or 11 mm. mercury. In the mammal, the capillary blood 

 pressure is naturally higher than this, and may be put down at 



