938 ANIMAL PHYSIOLOGY. 
planations of the working of complex organs, based on a single 
experiment, observation, or argument of any kind. 
The view we recommend the student to adopt in the light of 
our present knowledge is, that the first sound is the result of 
several causative factors, prominent among which are the sud- 
dén tension of the auriculo-ventricular valves, and the contrac- 
tion of the cardiac muscle, not leaving out of the account the 
possible and probable influence of the blood itself through 
eddies or otherwise; nor would we ridicule the idea that in 
some cases, at all events, the sound may be modified in quality 
and intensity by the shock given to the chest wall during sys-. 
tole. 
ENnpDo-CARDIAC PRESSURES. 
Bearing in mind the relative extent of the pulmonary and 
systemic portions of the circulation, we should suppose that 
the resistance to be overcome in opening the aortic valves and 
lifting the column of blood that keeps them pressed together, 
would be much greater in the left ventricle than in the right; 
or, in other words, that the intra-ventricular pressure of the 
left side of the heart would greatly exceed that of the right, 
and this is confirmed by actual experiment. 
By means of an instrument known as the maximum and 
minimum manometer, the highest and lowest pressure within 
any chamber of the heart may be learned approximately. As 
a specimen measurement it may be stated that it has been 
found that in a dog the greatest pressure was 140 mm. of mer- 
cury for the left ventricle, for the right only 60, and for the 
right auricle 20. But it is also found—a matter not quite so 
obvious—that a minimum pressure proportionate to the maxi- 
mum may exist in all the chambers of the heart; and the press- 
ure may fall below that of the atmosphere, or be negative. By 
the same method it was found that in a dog the negative pressure 
varied between —52 and —20 mm. of mercury for the left ven- 
tricle and —17 to —16 mm. for the right, with —12 to —7 mm. for 
the right auricle. As will be shown later, part of this diminished 
pressure is due to the effect of the respiratory movements; and, 
indeed, more recent experiments seem to show that ordinarily, 
with the heart beating with its usual rate and force, the nega- 
tive pressure or suck from its own action is comparatively 
slight. The discussion of the cause of this negative pressure, 
like the related subject of the cause of the heart’s diastole, has 
given rise to much difference of opinion. 
