MECHANISM OF THE HEART. 233 
against the walls of the chest; and it is by the same action 
that the pulse in the arteries is produced (§ 27 6\ 
270. The combined actions 
of each auricle and its ventri¬ 
cle, may be illustrated by an 
apparatus like that repre¬ 
sented in fig. 126. It con¬ 
sists of two pumps, a and 
6, of which the pistons move 
up and down alternately; 
and these are connected with 
a pipe c f, in which there are 
two valves d and e, opening 
in the direction of the arrow. 
The portion c of the pipe 
represents the venous trunk 
by which the blood enters 
the heart; the pump a represents the auricle, and the raising 
of its piston enables the fluid to enter and fill it. When its 
piston is lowered, its fluid is forced through the valve d into 
the pump b {which represents the ventricle), whose piston 
rises at the same time to receive it; and when this piston is 
lowered in its turn, the fluid (being prevented from returning 
into a by the closure of the valve d) is propelled through the 
valve e into the pipe f which may represent an arterial tube ; 
whilst at the same time a fresh supply of blood is received 
into the pump a by the raising of its piston. 
271. The number of contractions of the heart ordinarily 
taking place in an adult man, is from 60 to 70 per minute. 
It is usually rather greater in women; and in children it is 
far higher, being from 130 to 140 in the new-born infant, and 
gradually diminishing during the period of infancy and child¬ 
hood. It is rather greater in the standing than in the sitting 
posture, and in sitting than in lying down : it is increased by 
exercise, especially by ascending a steep hill or going upstairs, 
and also by any mental emotion. It is important to remember 
these facts, in reference to the management of those who are 
suffering under diseases of the heart or of the lungs, which 
prevent the ready passage of the blood through these organs ; 
for if more blood be brought to the heart by the great veins, 
than it can propel through the pulmonary arteries, a feeling of 
