THE CARDIAC IMPULSE 57 



close the semilunar valves, and su prevent the return of 

 blood to the heart, and, on the other hand, to distend the 

 next portion of the a6rta, di-i\ing an extra quantity of 

 blood into it. And this second portion, in a similar 

 way, distends the next, and this again the next, and so 

 on, right through the whole arterial system. Thus the 

 impulse given by the ventricle travels like a wave ah.ng 

 the arteries distending them as it goes, and ultimately 

 forcing the blood through the capillaries into the veins, 

 and so on to the heart again. 



Several of the practical results of the working of the 

 heart and arteries just described now become intelligible. 



9. The Cardiac Impulse.— If a finger be placed on the 

 chest over the space between the fifth and sixth ribs on 

 the left side, about one inch below the left nipple and 

 slightly towards the sternum, a certain throbbing move- 

 ment is perceptible, which is known as the "cardiac 

 impulse." It is the result of the heart-beat making it- 

 self felt through the wall of the chest at this point, at the 

 moment of the systole of the ventricles. Even when the 

 heart is at rest the apex, in a standing position, lies close 

 under and in contact with this part of the chest-wall. 

 When the systole takes place the muscular substance of 

 the ventricles becomes suddenly hard and tense, as do all 

 muscles when they contract. At the same time the apex of 

 the heart, as the result of the peculiar movements already 

 described (p. 52), is brought into still firmer contact with 

 the chest-wall. The cardiac impulse is the outcome of this 

 sudden hardening of the ventricular walls, aided by their 

 closer contact with the wall of the chest at the moment 

 when the hardening takes place. It is nut due, as is so 

 frequently stated, to the heart " striking " or " tapping " 

 against the chest-wall. 



10. The Sounds of the Heart.— If the ear be applied 

 over the heart, certain sounds are heard, which recur with 

 great regularity, at intervals corresponding with those 

 between every two beats. First comes a longish dull 



