146 ANATOMY AND PHYSIOLOGY. 



MECHANISM OF THE CIRCULATION. 



The mechanical action, by which the blood is caused to circulate 

 in the vessels, is easily comprehended. The cavities -of the heart, 

 as already explained, contract and dilate alternately, by the alter- 

 nate shortening and relaxation of the muscular fibres that form their 

 walls ; and the force of their contraction is sufficient to propel the 

 blood through the vessels which proceed from them. The two ven- 

 tricles contract at the same moment ; the auricles contract during 

 the relaxation of the ventricles, and are themselves relaxed whilst 

 the ventricles are contracting. The series of movements is, there- 

 fore, as follows : The auricles bein'g full of blood which they have 

 received from the venae cavaB and pulmonary veins, discharge it by 

 their contraction into the ventricles, which have just, before emptied 

 themselves into the aorta and pulmonary artery, and which now 

 dilate to receive it. When filled by the contraction of the auricles, 

 these contract in their turn, so as to propel their blood into the great 

 vessels proceeding from them ; and whilst they are doing this, the 

 auricles again dilate to receive the blood from the venous system, 

 after which the whole process goes on as before. It is when the 

 ventricles contract, that we feel the beat of the heart, which is 

 caused by the striking of its lower extremity against the walls of 

 the chest ; and it is by the same action that the pulse in the arteries 

 is produced. 



The combined action of each auricle and its ventricle may be illus- 

 trated by an apparatus like that represented in the following figure. 

 It consists of two pumps, a and 6, of which the pistons move up and 

 down alternately ; and these are connected with a pipe, cf, 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 is at the same 

 time raised 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/, 

 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. 



