224 THE HUMAN BODY. 



creases its rate temporarily and so does excitement; a sick 

 person's pulse should not therefore be felt when he is ner- 

 vous or excited (as the physician knows when he tries first 

 to get his patient calm and confident). In children the 

 pulse is quicker than in adults, and in old age slower than 

 in middle life. 



X^. The Flow of the Blood in the Capillaries and Veins. 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 with a microscope thin 

 transparent parts of various animals, as the web of a frog's 

 foot, a bat's wing, or the tail of a small fish. In consequence 

 of the steadiness with which the capillaries supply the veins 

 the flow in these latter is also unaffected directly by each 

 beat of the heart; if a vein be cut the blood wells out uni- 

 formly, while a cut artery spurts out with much more force, 

 and in jets which are more powerful at regular intervals 

 corresponding with the contractions of the ventricles. 



The Circulation of the Blood as seen in the Frog's 

 Web. There is no more fascinating or instructive spectacle 

 than the circulation of the blood as seen with the micro- 

 scope in the thin membrane between the toes of a frog's 

 hind limb. Upon focusing beneath the outer layer of the 

 skin a network of minute arteries, veins, and capillaries, 



How by exercise? Why should an invalid's pulse not be felt 

 when he is excited? How does age affect the pulse rate? 



In what manner does the blood leave the heart? How is its flow 

 altered before reaching the capillaries? How may this be observed.? 

 Is the flow in the veins rhythmic or steady? Why? How does the 

 bleeding from a cut artery "differ from that of a cut vein? 



What comes into view on examining a frog's web with a micro- 

 scope? 



