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 (§ 276). 
270. The combined actions 
of each auricle and its ventri- 
cle, may be illustrated by an 
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 
_ @ pipe c f, in which there are 
two valves d and é, opening 
in the direction of the arrow. 
_ The portion ¢ of the pipe 
ts 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 6 (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 i 
into a by the closure of the valve d) is propelled through the 
valve ¢ into the pipe 7, 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 is 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 
Pig. 126. 
