CHAP, iv.] THE VASCULAR MECHANISM. 



161 



from 15 to 20 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 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. 29, which is simply intended to shew this fact graphically, 

 and has not been constructed by exact measurements. 



FIG. 29. DIAGRAM OP BLOOD PRESSURE. 



A, Arteries. P, Peripheral Region (minute arteries, capillaries and veins). 



V, Veins. 



(4) In the arteries this mean pressure is marked by oscillations 

 corresponding to the heart beats, each oscillation consisting of a 

 rise (increase of pressure above the mean) corresponding to the 

 systole of the ventricle, followed by a fall (decrease of pressure 

 below the mean) 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 towards the capillaries, and are, 

 under ordinary circumstances, wholly absent from the veins along 

 their 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. 



99. If the web of a frog's foot be examined with a micro- 

 scope, 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 



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