THE CIRCULATION OF THE BLOOD AND LYMPH 79 



second sound is best heard (over the junction of the second 

 right costal cartilage and sternum) is that at which any 

 change produced by disease of the aortic valves is most 

 easily recognised. The sound is conducted up from the 

 valves along the aorta, which comes nearest to the surface 

 at this point. Changes connected 

 with disease of the pulmonary 

 valves are most readily detected 

 over the second left intercostal 

 space near the edge of the 

 sternum, for here the pulmonary FIG. 18. DIAGRAM OF MAREY'S 

 artery most nearly approaches CARDIOGRAPH. 



,11 -M A, knob attached to flexible mem- 



the Chest-Wall. brane tied over end of metal box 



first snnnrl i<= * circtnliV ' _ the knob is P laced over the a P ex ' 



systonc - - beat . c is the folded edge of the 



that is, it OCCUrS during: the ven- membrane ; B is the tube communi- 



cating with a recording tambour. 



tncular systole ; the second is 



'diastolic,' beginning at the commencement of the diastole. 



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

 impulse felt, at every cardiac contraction in various situa- 

 tions 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 generally, 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 Exer- 

 cises, p. 182). Even in health the position of the impulse 

 varies somewhat with the position of the body and the res- 

 piratory 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, especi- 

 ally in cases of effusion of fluid into the pleural cavity. 



Various instruments, called cardiographs, have been devised 



* T-Lfc mammary line is an imaginary vertical line supposed to be 

 drawxi 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 corre- 

 sponding border of the sternum. 



