CHAPTER XXI 



THE CIRCULATION OF THE BLOOD. BLOOD PRESSURE 

 AND BLOOD- VELOCITY. THE PULSE 



The Flow of the Blood Outside of the Heart. The blood leaves 

 the heart intermittently and not in a regular stream, a quantity 

 being forced out at each systole of the ventricles : before it reaches 

 the capillaries, however, this rhythmic movement is transformed 

 into a steady flow, as may readily be seen by examining under the 

 microscope thin transparent parts of various animals, as the web 

 of a frog's foot, a mouse's ear, or the tail of a small fish. In conse- 

 quence of the steadiness with which the capillaries supply the 

 veins the flow in these is also unaffected, directly, by each beat 

 of the heart ; if a vein be cut the blood wells out uniformly, while 

 from a cut artery the blood spurts out not only with much greater 

 force, but in jets which are much more powerful at regular inter- 

 vals corresponding with the systoles of the ventricles. 



The Circulation of the Blood as seen in the Frog's Web. There 

 is no more fascinating or instructive phenomenon than the circu- 

 lation of the blood as seen with the microscope in the thin mem- 

 brane between the toes of a frog's hind limb. Upon focusing 

 beneath the epidermis a network of minute arteries, veins, and 

 capillaries, with the blood flowing through them, comes into view 

 (Fig. 106). The arteries, a, are readily recognized by the fact 

 that the flow in them is fastest and from larger to smaller branches. 

 The latter are seen ending in capillaries, which form networks, 

 the channels of which are all nearly equal in size. While in the 

 veins arising from the capillaries the flow is from smaller toJarger 

 trunks, and slower than in the arteries, but faster than in the 

 capillaries. 



The reason of the slower flow of the capillaries is that their 

 united area is considerably greater than that of the arteries 

 supplying them, so that the same quantity of blood flowing 

 through them in a given time has a wider channel to flow in and 

 moves slowly. The area of the veins is smaller than that of the 



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