. Animal physiology. Physiology, Comparative; Physiology, Comparative. MECHANISM OF THE HEART. 209 are contracting. The series of movements is therefore as fol- lows :—The auricles being full of blood which they have received from the venae cavse 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
. Animal physiology. Physiology, Comparative; Physiology, Comparative. MECHANISM OF THE HEART. 209 are contracting. The series of movements is therefore as fol- lows :—The auricles being full of blood which they have received from the venae cavse 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 (§. 276). 270. The combined actions of each auricle and its ventricle, may be illustrated by an apparatus like that represented in Fig. 120. It consists of two pumps, a and b, of which the pistons move up and down alternately; and these are connected with a pipe, c /, in which there are two valves, d and e9 opening in the direction of the arrow. The portion c of the pipe re- presents the venous trunk, by which the blood enters the heart; the pump a represents the auri- cle, 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 raisi
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