234 COMPARATIVE PHYSIOLOGY. 



ally from the inflow of blood ; and the auricular contraction 

 produces a brief, decided, though but slight rise of pressure in 

 the ventricles. 



4. The onset of the ventricular systole is rapid, its maximum 

 pressure suddenly reached, and its duration considerable. 



The relations of these various events, their duration and 

 the corresponding movements of the chest wall, may be learned 

 by a study of the above tracing which the student will find 

 worthy of his close attention. 



THE CARDIAC SOUNDS. 



Two sounds, differing in pitch, duration, and intensity, may 

 be heard over the heart when the chest is opened and the heart 

 listened to by means of a stethoscope. These sounds may also 

 be heard, and present the same characters when the heart is 

 auscultated through the chest wall ; hence the cardiac impulse 

 can take no essential part in their production. 



The sounds are thought to be fairly well represented, so far 

 as the human heart is concerned, by the syllables lub, dup ; the 

 first sound being longer, louder, lower-pitched, and '" booming " 

 in quality, the second short, sharp, and high-pitched. 



In the exposed heart, the ilrst sound is heard most distinctly 

 over the base of the organ or a little below it ; while the sec- 

 ond is communicated most distinctly over the roots of the great 

 vessels — that is to say, both sounds are heard best over the 

 auriculo-ventricular and semilunar valves respectively. When 

 the chest wall intervenes between the heart and the ear, it is 

 found that the second sound is usually heard most distinctly 

 over the second costal cartilage on the right; and the first in 

 the fifth costal interspace where the heart's impulse is also often 

 most distinct. In these situations the arch of the aorta in the 

 one case, and the ventricular walls in the other, are close to the 

 situations referred to during the cardiac systole ; hence it is 

 inferred that, though the sounds do not originate directly be- 

 neath these spots, they are best propagated to the chest wall at 

 these points. Prior to the study of the heart in our domestic 

 animals the student is recommended to investigate the subject 

 on himself by means of a double stethoscope or on another per- 

 son with or without any instruments. 



There are, however, individual differences, owing to a va- 

 riety of causes, which it is not always possible to explain fully 



