l8o THE CIRCULATION OF THE BLOOD 



The cause of the second sound is more simple and definite than that of 

 the first. It is entirely due to the vibration consequent on the sudden closure 

 of the semilunar valves when they are pressed down across the orifices of 

 the aorta and pulmonary artery. The influence of these valves in producing 

 the sound was first demonstrated by Hope who experimented with the hearts 

 of calves. In these experiments two delicate curved needles were inserted, 

 one into the aorta and another into the pulmonary artery below the line of 

 attachment of the semilunar valves. After being carried upward about 

 half an inch the needles were brought out again through the coats of the 

 respective vessels, so that in each vessel one valve was held back against 

 the arterial walls. Upon applying the stethoscope to the vessels it was 

 found that after such an operation the second sound had ceased to be audible. 



Tube to communicate 

 with the tambour 



Tympanum Ivory Tape to attach 



knob instrument to the chest 



FIG. 155. Cardiograph. (Sanderson's.) 



Disease of these valves, when sufficient to interfere with their efficient action, 

 also demonstrates the same fact by modifying the second sound or destroying 

 its distinctness. 



The Cardiac Impulse. The heart may be felt to beat with a slight 

 shock or impulse against the walls of the chest at the level of the fifth inter- 

 costal space on the left side. Its extent and character vary in different 

 individuals, a factor of considerable clinical significance, and therefore es- 

 pecially discussed in works on clinical diagnosis. The cause of the cardiac 

 impulse has been variously described. It will be remembered that during 

 the period which precedes the ventricular systole the relaxed heart rests 

 quietly in the pericardial cavity and with its apex exerting no pressure 

 against the wall of the chest. When the ventricles contract, their walls 

 suddenly become firm and tense. Being firmly attached to the base the effect 

 of the movement is to press the hardened ventricle against the chest wall. 

 The discharge of the contents of the ventricle into the curved aorta intensi- 



