1 66 APPLIED PHYSIOLOGY 



blood are expelled. It occurs about seventy-two times 

 each minute. While the heart beats occur regularly 

 without apparent pause, yet it rests in diastole about 

 one half the time. 



288. Sounds of the heart. Two sounds are produced by each 

 beat, which may be heard by listening with the ear close to the heart. 

 The first sound is the longer and softer, and is caused by the vibration 

 of the contracting muscles. The second sound is shorter and sharper, 

 and is caused by the sudden closing of the semilunar valves. At each 

 systole the portion of the heart touching the wall of the chest may be 

 felt to become suddenly harder, as though it beat against the chest 

 wall. Its movements are transmitted through the chest walls so that 

 they may be plainly seen and felt. Ordinarily a person is not aware of 

 his own heart beats, but when they are very forcible they are plainly 

 felt, and are called palpitation. 



289. Nerves of the heart. A nervous mechanism within 

 the heart itself causes it to contract even after it is sepa- 

 rated from the body. A fish's or turtle's heart will con- 

 tract regularly for hours after being removed from the 

 body. Man's heart is easily affected by outside influ- 

 ences, but, because of its own nervous mechanism, it is 

 not so sensitive as has been supposed. Wounds com- 

 pletely penetrating the ventricle have been sewed up, and 

 recovery has taken place. The action of the heart is regu- 

 lated and adapted to the varying needs of the body through 

 two sets of nerves, one set from the brain and the other 

 set from the spinal cord. In physical exertion the spinal 

 nerves cause it to beat faster and more forcibly. This 

 adaptation is so delicate that rising from a sitting to a stand- 

 ing position perceptibly increases the number of heart beats. 

 Joy, or anger, or excitement of any kind hastens its action, 

 while grief usually retards and weakens it. 



290. Effect of violent exercise. In prolonged and vio- 

 lent physical exercise the heart performs more work than 



