THE CIRCULATION OF THE BLOOD 153 



two longest toes, but not tight enough to stop the circulation or 

 hurt the frog. 



Fasten the frog upon the board in such a way that the foot will 

 just come over the glass slide. Pull carefully the thread tied to the 

 toes, so as to spread out the web over the glass. Fasten the threads 

 by drawing them into slits cut in the sides of the board. The board 

 should now be fixed by elastic bands, or by any other convenient 

 means, upon the stage of the microscope. The web should be occa- 

 sionally moistened with water. Care should be taken not to occasion 

 any pain to the frog. The circulation of blood thus shown is a 

 wonderful sight, and never to be forgotten. 



211. The Beat, or Impulse, of the Heart. If the hand 

 be laid flat over the chest wall on the left side, a peculiar 

 throbbing will be felt. This throbbing movement is known 

 as the beat, or impulse, of the heart. It is due to the hard, 

 tense part of the ventricles coming in contact with the 

 chest wall at the moment when the hardening of the ven- 

 tricles takes place. The heart beats are unusually strong 

 during active bodily exertion and under mental excitement. 



212. The Sounds of the Heart. If we place our ear 

 against a friend's chest, over the region of the heart, we 

 hear two distinct sounds. These sounds may be fairly 

 imitated by pronouncing the syllables lub, dup. The first 

 is a dull, muffled sound, known as the "first sound," fol- 

 lowed at once by a shorter and sharper sound, known as 

 the " second sound" of the heart. 1 



213. The Pulse. If the finger be placed over an artery 

 which lies near the surface of the body, like the radial 



1 The precise cause of the first sound is not certainly known, but the 

 second sound is, without doubt, caused by the tension of the semilunar 

 valves of the pulmonary artery and the aorta at the moment when the 

 contraction of the ventricles is completed. If these valves are diseased 

 and do not shut properly, a blowing sound or murmur will be produced. 

 Thus, by listening to the sounds of the heart a doctor may be able to tell 

 whether the valves are out of order. 



