MECHANISM OF THE HEART. 



233 



Fig. 126. 



against the walls of the chest ; and it is by the same action 

 that the pulse in the arteries is produced ( 276V 



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 

 b, of which the pistons move 

 np and down alternately ; 

 and these are connected with 

 a pipe c /, 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 



