THE VASCULAR MECHANISM. 195 



of the systole, and that the second sound lasts for a brief space at the 

 commencement of diastole. 



The first sound is due to two causes, namely, (1) the vibrations set 

 up by the closure of the tricuspid and mitral valves, and (2) the 

 contraction of the muscular wall of the ventricles. When the tricuspid 

 or mitral valves become diseased so that they fail to close, the first 

 sound is largely replaced by a " blowing " noise, known as a murmur 

 or bruit. That the contraction of the heart muscle contributes to the 

 first sound is shown by the fact that, during the contraction of the 

 bloodless, excised heart, a faint sound can be heard with the stethoscope. 

 The duration of the first sound almost to the end of systole furnishes 

 additional evidence that its origin is partly muscular. The relative 

 importance of the valvular and muscular factors is still a matter of 

 discussion. The part of the first sound due to the muscular con- 

 traction is not peculiar to the heart, since a similar sound is produced 

 by any note of low pitch, and may be heard on listening to a contract- 

 ing voluntary muscle. 



The second sound is due entirely to vibrations set up in the semilunar 

 valves by their sudden closure at the end of systole, and is replaced by a 

 murmur if these valves are diseased, or if, in an animal, they are hooked 

 back and prevented from closing. The first sound is most distinctly 

 heard near the apex beat ; the closure of the aortic valves is best heard 

 in the second right intercostal space close to the sternum, and the 

 closure of the pulmonary semilunar valves in the second left inter- 

 costal space. 



(5) Endocardiac Pressure. The pressure within the auricles and 

 ventricles rises during systole and falls in diastole, and the variations 

 in pressure are closely bound up with the other events taking place 

 during the cardiac cycle. The changes in pressure occur so rapidly 

 that a slowly moving fluid, such as mercury, fails to record them ac- 

 curately, although a maximum and minimum mercury manometer may 

 be employed to ascertain the highest and lowest pressure occurring 

 during a cardiac cycle. 



In the early observations of Chauveau and Marey a cardiac sound, 

 consisting of a long rigid tube having at its lower end a bulb of very 

 thin rubber supported on a metal framework, was passed along the 

 jugular vein into the right ventricle, or along the carotid artery into 

 the left ventricle, of a horse. The upper end of the sound was attached 

 to a Marey 's tambour (fig. 66), which consists of a shallow metal cup 

 having a small lateral opening and covered by a thin rubber membrane 

 on which rests a light lever. The whole apparatus contains air, and 

 any rise of pressure in the ventricle compresses the rubber bulb, thereby 



