72 HUMAN PHYSIOLOGY 



similar to those already described as existing in veins. 

 The mouths of the pockets open away from the ventri- 

 cle, and the passage of blood from the ventricle into the 

 blood vessel is therefore unfettered (Fig. 22 6), be- 

 cause the blood rushes past the mouths of the pockets. 

 If, however, any back flow or reflux of the blood takes 

 place, the pockets fill and their edges meet in the 

 centre and oppose a barrier to the regurgitation of 

 the blood whose flow they regulate. 



THE HEART'S BEATS. If we place our ear over 

 the heart of an individual we become conscious, after 

 a little accurate observation, of two distinct sounds. 

 These sounds it will further be observed show a cer- 

 tain rhythm in that they proceed in pairs. The first 

 sound is long and loud, the second is short and sharp. 

 The sounds as heard may be imitated fairly by pro- 

 nouncing the words " lubb " and " d&p." It is of great 

 importance to note the meaning of these sounds, inas- 

 much as in the case of the physician his power of 

 diagnosing heart trouble must depend largely on his 

 familiarity with these sounds in health and in disease. 

 The first sound appears to be caused by the action of 

 the valves between the ventricles, whilst certain 

 authorities are of opinion that the mere contraction 

 of the ventricles also aids in producing it. The 

 second and shorter sound is undoubtedly produced 

 by the closure of the semilunar valves. The mar- 

 vellous feature connected with the work of the valves 

 of the heart is that of their instantaneous action, and 

 moreover in their perfect working, it may be, during 

 a long lifetime. What takes even a limited space of 

 time to describe is accomplished in the heart instan- 

 taneously, these valves acting practically as often as 

 the heart beats. 



