THE CIRCULATION OF THE BLOOD. 287 
The veins with their valves, their superficial position and 
thinner walls, make up a set of conditions favoring the onflow 
of the blood, especially under muscular exercise. 
In the mammal the circulatory system, by reason of its con- 
nections with the digestive, réspiratory, and lymphatic systems, 
and in a lesser degree with all parts of the body, especially the 
glandular organs, maintains at once the usefulness and the fit- 
ness of the blood. 
The arterioles, by virtue of their highly developed muscular 
coat, are enabled to regulate the blood-supply to every part, in 
obedience to the nervous system. 
The blood exercises a certain pressure on the walls of all 
parts of the vascular system, which is greatest in the heart it- 
self, high in the arteries, lower in the capillaries, and lowest in 
the veins, in the largest of which it may be less than the atmos- 
pheric pressure, or negative. The heart in the mammal consists 
of four perfectly separated chambers, each upper and each 
lower pair working synchronously, intermixture of arterial 
and venous blood being prevented by septa and interference in 
working by valves. The heart is a force-pump chiefly, but, to 
some extent, a suction-pump also, though its power as such 
purely from its own action and independent of the respiratory 
movements of the chest is slight under ordinary circumstances. 
In consequence of the lesser resistance in the pulmonary divis- 
ion of the circulation, the blood-pressure within the heart is 
much less in the right than in the left ventricle—a fact in har- 
mony with and causative of the greater thickness of the walls 
of the latter; for in the foetus, in which the conditions are dif- 
ferent, this distinction does not hold. 
The ventricles usually completely empty themselves of 
blood and maintain their systolic contraction even after this 
has been effected. The contraction of the heart, which really 
begins in the great veins near their junction with the auriclés 
(that do not fully empty themselves), is at once followed up by 
the auricular and ventricular contraction, the whole constitu- 
ting one long peristaltic wave. Then follows the cardiac pause, 
which is of longer duration than the entire systole. 
When the heart contracts it hardens, owing to closing on a 
non-compressible fluid dammed back within its walls by resist- 
ance a fronte. At the same time the hand placed on the chest- 
walls.over the heart is sensible of the cardiac impulse, owing 
to what has just been mentioned. The systole of the chambers 
of the heart gives rise to a first and a second sound, so called, 
