THE CIRCULATION OF THE BLOOD. 935 
; 1. Both auricular and ventricular systole are sudden, but 
the latter is of very much greater duration. 
2. While the chest wall feels the ventricular systole, the au- 
riculo-ventricular valves shield the auricle from its shock. 
3. During diastole in both chambers the pressure rises 
gradually from the inflow of blood; and the auricular contrac- 
tion produces a brief, decided, though but slight rise of press- 
ure 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. 
Tue Carpiac 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 astethoscope. These sounds may 
also be heard, and present the same characters when the heart 
is auscultated through the chest wall; hence the cardiac im- 
pulse 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 “ boom- 
ing” in quality; the second short, sharp, and high-pitched. 
In the exposed heart, the first sound is heard most distinct- 
ly 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 
beneath these spots, they are best propagated to the chest wall 
at these points. 
There are, however, individual differences, owing to a va- 
