440 HEALTH AND DISEASE 



blood forwards, a part is expended in dilating the arteries. As soon as 

 the contraction is completed, relaxation immediately follows, and the 

 heart passes into the state of commencing dilatation. The great vessels 

 into which the blood has been forced now retract, and the first effect is 

 to close the semilunar valves, and thus to prevent the return of the 

 blood into the ventricles, whilst the next is to compel the onward move- 

 ment of the 'blood in the vessels; the elastic reaction of the stretched 

 walls restoring to the circulation during the diastole the force temporarily 

 borrowed from the heart. The wave which is produced by the injection 

 of blood into the vessels when the heart contracts is the pulse, but the 

 closure of the semilunar valves is so sudden, and follows so immediately 

 upon the contraction, that a reflex wave from the valves succeeds the 

 main systolic wave and forms a part of the pulse. This is known as 

 the dicrotic wave. 



If the ear be applied to the chest two sounds may be heard to accom- 

 pany the action of the heart; the first is dull and prolonged, the second 

 is shorter, sharper, and ends abruptly. The first sound owes its origin 

 mainly to the sudden tension of the auriculo- ventricular valves in the right 

 and left hearts, but is almost certainly intensified by the muscular sound 

 of the contracting ventricles. The second sound is exclusively due to the 

 sudden tension of the aortic sigmoid, and pulmonary semilunar valves 

 which guard the orifices of the aorta and pulmonary artery respectively. 

 Both sounds are therefore valvular, and any rent, or inequality, or imper- 

 fection in the valves, permitting the blood to flow in the wrong direction, 

 or causing friction, as in heart disease, causes alteration in the characters 

 of the sounds easily recognized by the practised ear. The frequency of 

 the beats of the heart is increased by exercise, by food, and by mental 

 emotions. They are more frequent in the morning than at night, in the 

 young than in the old, and in the female as compared with the male. 



The Nerves of the Heart. The heart continues to beat in an 

 orderly and regular manner even when quite removed from the body, 

 and in the process of development the speck which represents it in the 

 young begins to beat rhythmically long before any nerves are formed. 

 These circumstances show that its action is to a large extent independent 

 of the great centres of the nervous system, whilst on the other hand the 

 readiness with which the heart responds to disturbing conditions of the 

 general system, in regard alike to the frequency and the strength of its 

 beats, clearly indicates that it is under the control of certain nerves which 

 can be demonstrated by anatomical as well as by physiological evidence 

 to have intimate relations with it. The nerves distributed to the heart 

 are derived from the spinal cord and medulla oblongata, as well as from 



