CIRCULATION OF THE BLOOD. 315 



artery is one-third as much. The amount of work done by the 

 ventricles daily in thus forcing blood into the arteries is equal to 

 that which is performed by an individual weighing 75 kilograms 

 in climbing a mountain 806 meters in height. 



As soon as the ventricles cease their contraction the pressure 

 of the blood in the arteries closes the pulmonary and aortic valves, 

 the ventricles begin their diastole, and the pause of the heart 

 commences. As it was at this point that the consideration of the 

 changes which take place was begun, the study of a cardiac cycle, 

 cardiac period, or heart-beat is now completed. If the time 

 occupied by such a period is divided into one hundred parts, it 

 will be found that the auricular systole lasts during nine of the 

 parts, the ventricular systole during thirty, and the pause during 

 sixty-one ; or, in other words, the heart is at rest six-tenths and 

 at work four-tenths of the time. 



Shortening of the Heart. At the time of the systole of the 

 heart (ventricular systole) the organ becomes shorter, yet the apex 

 does not change its place, for the lengthening of the aorta which 

 occurs compensates for the shortening, so that while the apex and 

 base approximate the whole heart is lowered, the result being to 

 keep the apex in its original position with reference to the chest 

 wall. 



Cardiac Impulse. The situation of the heart in the thoracic 

 cavity is such that its apex is against the chest wall at the fifth 

 intercostal space, the space between the fifth and sixth ribs, and 

 about 3 cm. below and 1 cm. within the left nipple. The apex of 

 the heart is the extreme point of the left ventricle. If the finger 

 is placed in this region during the ventricular systole, there will be 

 felt a tap as if something was gently striking it. This tap is 

 known as the apex-beat. It is so called because it was formerly 

 supposed that during the systole the heart was raised up and car- 

 ried forward so as to cause the apex to strike against the chest wall 

 and thus produce the sensation.. A more careful study of the 

 changes which the heart undergoes during systole has, however, 

 demonstrated that the apex of the heart is always in contact with 

 the chest wall, and that this supposed striking does not take place. 

 Indeed, the tap is not due to the apex at all. The term apex- 

 beat is a misnomer : it should rather be called cardiac impulse, 

 the sensation being produced by the anterior surface of the con- 

 tracting ventricles swelling out and hardening. The location at 

 which this impulse is felt most pronouncedly is not over the apex, 

 but higher up. If a long needle was to be introduced deeply 

 here, it would penetrate the left ventricle at a point where 

 the middle and lower thirds unite. The cardiac impulse is not 

 always, even in health, detected at the same place : it changes 

 somewhat with respiration and also with changes in the position 

 of the body. 



