138 CARDIAC IMPULSE. [BOOK i. 



this impulse, 'cardiac impulse,' will be found to be synchronous with 

 the systole of the ventricle. In man, the cardiac impulse may be 

 most distinctly felt in the fifth costal interspace, about an inch 

 below and a little to the median side of the left nipple. The same 

 impulse may be felt in an animal by making an incision through 

 the diaphragm from the abdomen, and placing the finger between 

 the chest-wall and the apex. It then can be distinctly recognized 

 as the result of the hardening of the ventricle during the systole. 

 And the impulse which is felt on the outside of the chest is the 

 same hardening of the stationary portion of the ventricle in contact 

 with the chest-wall, transmitted through the chest-wall to the 

 finger. In its flaccid state, during diastole, the apex is (in a 

 standing position at least) at this point in contact with the chest- 

 wall, lying between it and the tolerably resistant diaphragm. 

 During the systole, while being brought even closer to the chest- 

 wall, by the movement to the front and to the right of which 

 we have already spoken, it suddenly grows tense and hard. The 

 ventricles, in executing their systole, have to contract against 

 resistance. They have to produce within their cavities, tensions 

 greater than those in the aorta and pulmonary arteries, respectively. 

 This is, in fact, the object of the systole. Hence, during the swift 

 systole, the ventricular portion of the heart becomes suddenly 

 tense, just as a bladder full of fluid would become tense and hard 

 when forcibly squeezed. The sudden onset of this hardness gives 

 an impulse or shock both to the chest-wall and to the diaphragm, 

 which may be felt readily both on the chest- wall, and also through 

 the diaphragm when the abdomen is opened, and the finger 

 inserted. If the modification of the sphygmograph (see section 

 on Pulse), called the cardiograph, be placed on the spot where the 

 impulse is felt most strongly, the lever is seen to be raised during 

 the systole of the ventricles, and to fall again as the systole passes 

 away, very much as if it were placed on the heart directly. A 

 tracing may thus be obtained, of which we shall have to speak 

 more 'fully immediately. If the button of the lever be placed, 

 not on the exact spot of the impulse, but at a little distance 

 from it, the lever will be depressed during the systole. While 

 at the spot of impulse itself the contact of the ventricle 

 is increased during systole, away from the -spot the ventricle 

 retires from the chest-wall (by the diminution of its right-to-left 

 diameter), and hence, by the mediastinal attachments of the peri- 

 cardium, draws the chest-wall after it. 



Endo-cardiac events. In order to study more fully the 

 changes going on in the heart during the cardiac cycle, it becomes 

 necessary to know something of what is taking place in the interior 

 of the cavities of the heart. Chauveau and Marey, by introducing 

 into the right auricle and ventricle respectively of the horse, through 

 the jugular vein, small elastic bags, each communicating with a 



