232 THE H UMAN BOD T. 



10. Carefully cut the auricles away from the other sheep's heart t 

 taking great care not to injure the ventricles or the auriculo-ventric- 

 ular valves. Then holding the ventricles, apex down, in one hand, 

 pour water in a stream into them from a pitcher -held about a foot 

 above them. As the ventricles fill, the flaps of the mitral and tricus- 

 pid valves will be seen to float up and close the auriculo-venlricular 

 orifice, illustrating their movement as the ventricle fills during its 

 diastole in the natural working of the heart. 



11. The manner in which the elasticity of the arteries and the fric- 

 tion resistance to flow in the capillaries together serve to turn a 

 rhythmic into a steady flow may be readily demonstrated as follows: 



Take an elastic bag such as is commonly sold with enema appara 

 tus in drug-stores, and having an entry and exit tube provided with 

 valves In the exit tube place a piece of glass tubing six feet long. 

 Put the entry tube of the bag in a basin of water. On pumping, an 

 intermittent flow of water, corresponding to the strokes of the pump, 

 will be obtained from the glass tube. Connect a very fine glass 

 nozzle with the end of the long tube; on pumping, less water can 

 be forced through, and the outflow is still rhythmic. 



12. Replace the glass tube by a rubber tube of the black, highly 

 elastic kind : on pumping we get again a rhythmic outflow. Now 

 connect your narrow nozzle to the end of the rubber tube, and pump: 

 the outflow will be nearly constant, because the rubber tube not being 

 able to empty itself as fast as the water is pumped into it, becomes 

 stretched, and in the interval between two strokes of the pump it 

 keeps on squeezing out the extra water accumulated in it. The 

 longer and more elastic the tube, the quicker and stronger the stroke 

 of the pump, and the narrower the exit, the more steady will be the 

 outflow. In the body the heart keeps the arteries very tightly 

 stretched all the time, and they keep up accordingly a steady flow 

 into the capillaries. The experiment shows that to get such a steady 

 flow two things are necessary : (1) that the tubes fed directly by the 

 heart shall be highly elastic, and (2) that there shall be considerable 

 resistance to the exit from their outflow ends 



