86 ANATOMY FOR NURSES. [CHAP. VIII. 



arterial openings are provided with a set of valves. These valves, 

 called semilunar valves, consist of three semicircular flaps, each 

 flap being attached by its convex border to the inside of the 

 artery where it joins the ventricle, while its other border projects 

 into the interior of the vessel. The flaps of these valves form a 

 complete barrier, when closed, to the passage of the blood from 

 the arteries into the heart, but offer no resistance to the flow 

 from the heart into the arteries. 



The beat of the heart. So long as life lasts, the muscular 

 tissue of the heart contracts and relaxes unceasingly. We may 

 call the heart a muscular pump, the force of whose strokes is 

 supplied by the contraction of muscular fibres, the strokes 

 being repeated so many times a minute. It is constructed and 

 furnished with valves in such a way that, at each stroke, it 

 drives a certain quantity of blood with a certain force and a 

 certain rapidity from the ventricles into the arteries, receiving, 

 during the stroke, and the interval between that stroke and 

 the next, the same quantity of blood from the veins into 

 the auricles. 



The contractions of the heart are rhythmical ; that is to say, 

 they occur in a certain order. First, there is a simultaneous 

 contraction of the walls of both auricles ; immediately following 

 this, a simultaneous contraction of both ventricles ; then comes 

 a pause, or period of rest, after which the auricles and ven- 

 tricles contract again in the same order as before, and their 

 contractions are followed by the same pause as before. The 

 state of contraction of the heart is called the systole ; the state 

 of relaxation and dilatation, its diastole. 



If the chest of an animal be opened and artificial respiration 

 kept up, the heart may be watched beating, and a complete beat 

 of the whole heart may be observed to take place as follows : 



The great veins are seen, while full of blood, to contract in 

 the neighbourhood of the heart, the wave of contraction running 

 on towards the auricles, increasing in intensity as it goes. 

 Arrived at the auricles, which are now full of blood, the wave 

 of contraction passes on to them, and they contract suddenly 

 and quickly. During this contraction, the walls of the auricles 

 press towards the auriculo-ventricular orifices, and the blood 

 passes over the tricuspid and mitral valves into the ventricles. 

 The ventricles fill rapidly, and as soon as the auricular systole 



