142 



THE VASCULAR MECHANISM. 



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 toward 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 capil- 

 laries 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 pres- 

 sure which the blood is exercising on the walls of the capillaries and minute 

 arteries and veins may be approximately determined. In the frog's web 

 this has been found to be equal to about 7 to 11 mm. of mercury. In the 

 mammal the capillary blood-pressure is naturally higher than this and may 

 be put down at from 20 to 30 mm. It is, therefore, considerable, being 

 greater than that in the veins, though less than that in the arteries. 



3. There is thus a continued decline of blood-pressure from the root of 

 the aorta, through the arteries, capillaries, and veins to the right auricle. 

 We find, however, on examination that the most marked fall of pressure 



takes place between the small arteries 



FlG - 41 - on the one side of the capillaries and 



the small veins on the other, the 

 curve of pressure being somewhat 

 of the form given in Fig. 41, which 

 is simply intended to show this fact 

 graphically and has not been con- 

 structed by exact measurements. 



4. In the arteries this mean pres- 

 sure is marked by oscillations corre- 

 sponding to the heart beats, each 

 oscillation consisting of a rise (in- 

 crease of pressure above the mean) 

 corresponding to the systole of the 

 ventricle, followed by 'a fall (de- 

 Diagram of Blood-pressure: A, arteries ; p, pe- crease of pressure below the mean) 

 > caplllaries> and corresponding to the diastole of the 



ventricle. 



5. These oscillations, which we may speak of as the pulse, are largest 

 and most conspicuous in the large arteries near the heart, diminish from the 

 heart toward the capillaries, and are, under ordinary circumstances, wholly 

 absent from the veins along the whole extent from the capillaries to the 

 heart. 



Obviously a great change takes place in that portion of the circulation 

 which comprises the capillaries, the minute arteries leading to and the 

 minute veins leading away from the capillaries, and which we may speak 

 of as the " peripheral region." It is here that a great drop of pressure 

 takes place ; it is here also that the pulse disappears. 



106. If the web of a frog's foot be examined with a microscope, the 

 blood, as judged of by the movements of the corpuscles, is seen to be passing 

 in a continuous stream from the small arteries through the capillaries to the 

 veins. The velocity is greater in the arteries than in the veins, and greater 

 in both than in the capillaries. In the arteries faint pulsations, synchronous 

 with the heart's beat, are frequently visible ; but these disappear in the 

 capillaries, in which the flow is even, 'that is, not broken by pulsations, and 



