DISEASES OF THE CIRCULATORY SYSTEM. 181 



are audible in health. They may be best heard by extension 

 of the left fore-leg, and by pulling forward the humerus as 

 far as possible. 



They differ in character, and have been compared to the 

 two syllables 'lab' and ' dup,' the first being longer and heavier 

 (lab), the second, which follows quickly after a short interval, 

 being shorter and sharper (dup). After the second sound 

 follows another longer interval. 



The first sound is complex, and although in part due 

 to the closure of the auriculo-ventricular valves, other 

 factors appear to be necessary for its normal production, 

 such as a healthy condition of the muscular tissue of the 

 heart and of the blood. It is heard during the first part of 

 the contraction or systole of the heart. 



The second sound is chiefly due to the closure of the 

 semilunar valves in the aorta and pulmonary arteries, and 

 corresponds in time to the first part of the diastole, or period 

 of dilatation. 



In investigating the heart's action in disease, -we have to 

 consider firstly the two sounds, and secondly the two 

 intervals by which the sounds are separated. In health 

 there is no sound audible during the intervals. 



The natural sounds may be modified in disease, and fresh 

 sounds may be heard. These fresh sounds are termed 

 murmurs, and although they may replace the normal sounds, 

 they always do something more, i.e., they alicays occupy 

 part of the interval as ivell. The natural sounds may be 

 altered in intensity, in duration, in rhythm, in pitch (high- 

 pitched and clear, or low-pitched and muffled), and finally in 

 some cases they are absent altogether. 



Murmurs may be heard when both sounds are present, 

 or when one or both of the sounds is absent. The terms 

 used to describe murmurs are soft, hloiving, loud, faint, harsh, 

 grating and rasping. 



