THE HEART BEAT. 499 



upon the direction of the wave of contraction. Since the character 

 of the electrocardiogram depends so much on the delicacy of the 

 instruments used, arguments based on this evidence have not so 

 far proved very satisfactory, particularly as the later records 

 are quite complex and difficult to interpret (see Fig. 209 and legend). 

 Change in Form of the Ventricle During Systole. The 

 systole of the ventricle diminishes, of course, the cavity within and 

 forces out the blood. Whether the cavity is completely obliterated 

 under ordinary conditions that is, whether the ventricles empty 

 themselves at each beat is not certain. Under what we may 

 designate as unusual conditions such, for instance, as an unusually 

 high pressure in the aorta it seems certain that the ventricle can not 

 empty itself completely or at least can not continue to do so, and the 

 result in such cases is a backing up of blood and a rise of pressure in 

 the left auricle and pulmonary vein. Much attention has been paid 

 to the external change of form of the ventricle during systole. 

 Does it diminish in size i.n all diameters or only in certain diam- 

 eters? The question is one that cannot be answered definitely for 

 all normal conditions, owing to the fact that the form of the heart 

 during diastole varies with the posture of the body. During 

 diastole the heart muscle is quite soft and relaxed, and consequently 

 its shape is influenced by gravity. The exact change of form 

 that it undergoes in passing from diastole* to systole will vary with 

 its shape, whatever that may happen to be, in diastole. During 

 systole the musculature, on the contrary, is hard and resisting and 

 the form of the heart in this phase is probably constant. The 

 change from the variable diastolic to the constant systolic form will 

 naturally be different in different positions. With an excised frog's 

 heart one can show that the ventricle is elongated in passing from 

 diastole to systole or one can show the reverse. If the heart is laid 

 upon its side it flattens in diastole so as to increase in length, 

 and systole causes a shortening. If the heart is held or placed 

 with its apex pointing upward it flattens during diastole so as 

 to shorten the diameter from base to apex and during systole 

 this diameter is lengthened. In ourselves the exact change of 

 shape is probably different in the erect from what it is in the 

 recumbent posture. Speaking generally, the accounts agree in 

 stating that the long diameter of the heart is decreased, base and 

 apex are brought closer together, and the diameter from right to 

 left is also decreased, while the anteroposterior or ventrodorsal 

 diameter is increased. That is, the outline of the base of the heart 

 during diastole is an ellipse with its short diameter in the ventro- 

 dorsal direction. During systole this outline approaches that of a 

 circle.* A more interesting change is described for the apex of the 

 * See Haycraft and Edes, "Journal of Physiology," 12, 426. 



