274 TEXT-BOOK OF PHYSIOLOGY 



heart-beat and the influences which modify it has been obtained from experi- 

 ments made on the isolated heart of the turtle, frog, and allied animals. 



If the thorax of a dog, completely anesthetized, is opened and artificial 

 respiration established, the heart will be observed in active movement inside 

 the pericardium. If this sac is divided and turned aside, the heart will be 

 fully exposed to view. At the normal rate of movement of the heart 

 characteristic of the dog it will be almost impossible to determine either the 

 succession of events or their duration. But by observing the heart under 

 different conditions at different rates of movement and with instrumental 

 aids, physiologists have succeeded not only in analyzing the movements, but 

 in describing their sequence and in estimating their time duration. 



Phenomena Observed. From many observations and experiments it 

 has been determined that the heart at each beat presents two distinct move- 

 ments which alternate with each other in quick succession. One is the 

 movement of contraction, or the systole, by which .the blood contained within 

 its cavities is ejected into the arteries pulmonic artery and aorta; the 

 other is the movement of relaxation, or the diastole, followed by a pause 

 during which the cavities again fill up with blood from the venae cavae and 

 pulmonic veins. 



The contraction of any part of the heart is termed the systole; the relaxa- 

 tion, the diastole. As each side of the heart has two cavities the walls of 

 which contract and relax in succession, it is customary to speak of an auricu- 

 lar systole and diastole, and a ventricular systole and diastole. As the two 

 sides of the heart are in the same anatomic relation to each other, they 

 contract and relax in the same periods of time. 



It has also been ascertained that the contraction of the auricles and 

 ventricles as well as their subsequent relaxations, though occurring with 

 extreme rapidity, do not take place simultaneously but successively; that 

 the contraction process passes over the heart in the form of a wave; that it 

 begins, indeed, at the terminations of the great veins, viz., the vena cava, then 

 passes to and over the auricles, thence to and over the ventricles from base 

 to apex with great rapidity, but occupying in these different regions unequal 

 periods of time; that the relaxation immediately succeeds the contraction, 

 in the same order, and that at the close of the ventricular relaxation there 

 is a period during which the whole heart is in repose, passively filling with 

 blood. 



The immediate cause of the movement of the blood through the vessels 

 is the contraction and relaxation of the muscle-walls of the heart, and more 

 particularly of the walls of the ventricles, each of which plays alternately 

 the part of a force-pump, and possibly to a slight extent of a suction-pump. 

 The motive power is furnished by the heart itself, by the transformation 

 of potential energy, stored up during the period of rest, into kinetic 

 energy i.e., heat and mechanic motion. 



Changes in Position and Form. It is also apparent under the condition 

 of the foregoing observation that the heart during each pulsation undergoes 

 changes of both position and form. In the diastolic condition, during which 

 the heart is in repose, the apex is directed obliquely downward and to the 

 left; the body of the heart is enlarged and its walls relaxed. As th*e 

 systole begins and reaches its maximum, the apex is tilted upward, the 

 entire heart is rotated on its axis from left to right and forced forward by 



