THE CIRCULATION OF THE BLOOD AND LYMPH 



Various explanations of the third sound have been given, but, as 

 the authors who have studied it are not even agreed as to whether it 

 is produced at the auriculo-ventricular orifices or at the aortic and 

 pulmonary orifices, it would not be useful to discuss them at present. 

 The Cardiac Impulse. A surface-movement is seen, or an impulse 

 felt, at every cardiac contraction in various situations where the 

 heart or arteries approach the surface. The pulsation, or impulse, 

 of the heart, often styled the apex-beat, is usually most distinct to 

 sight and touch in a small area lying in the fifth left intercostal 

 space, between the mammary and the parasternal line,* and gener- 

 ally, in an adult, about an inch and a half to the sternal side of the 

 former. It is due to the systolic hardening of the ventricles, which 

 are here in contact with the chest-wall, the contact being at the 

 same time rendered closer by their change of shape, and by a slight 



movement of rotation of 

 the heart from left to right 

 during the contraction 

 (Practical Exercises, 

 p. 207). When the left 

 ventricle is in contact with 

 the chest at the position of 

 the apex-beat, as is usually 

 the case, an important 

 element in the impulse is 

 the actual forward thrust 

 of the apex. When the 

 apex-beat corresponds in 

 position with the right 

 ventricle, there is no 

 actual forward movement, 

 although the hardening of 

 the ventricle may be felt as a thrust by the ringer. Even in health 

 the position of the impulse varies somewhat with the position of 

 the body and the respiratory movements. In children it is usually 

 situated in the fourth intercostal space. In disease its displacement 

 is an important diagnostic sign, and may be very marked, especially 

 in cases of effusion of fluid into the pleural cavity. It is sometimes, 

 though not invariably, a little lower in the standing than in the 

 sitting position, and shifts an inch or two to the left or right when 

 the person lies on the corresponding side. 



Various instruments, called cardiographs, have been devised for 

 magnifying and recording the movements produced by the cardiac 

 impulse. Marey's cardiograph (Fig. 27) consists essentially of a small 



* The mammary line is an imaginary vertical line supposed to be drawn 

 on the chest through the middle point of the clavicle. It usually, but not 

 necessarily, passes through the nipple. The parasternal line is the vertical 

 line lying midway between the mammary line and the corresponding border 

 of the sternum. 



Fig. 27. Diagram of Marey's Cardiograph. 



