THE CIRCULATION 47 



auricles and ventricles are relaxed, and finally, by contract- 

 ing, to force an excess of blood into the ventricles, thereby 

 distending their walls. The ventricles, having in this way 

 been fully charged by the auricles, now contract and force 

 their contents into the large arteries. 



Sounds of the Heart. Two distinct sounds are given out by the heart 

 as it pumps the blood. One of them is a dull and rather heavy sound, 

 while the other is a short, sharp sound. The short sound follows 

 quickly after the dull sound and the two are fairly imitated by the 

 words "'lubb, diip." While the cause of the first sound is not fully 

 understood, most authorities believe it to be due to the contraction of 

 the heart muscie and the sudden tension on the valve flaps. The second 

 sound is due to the closing of the semilunar valves. These sounds are 

 easily heard by placing an ear against the chest wall. They are of great 

 value to the physician in determining the condition of the heart. 



Arteries and Veins. These form two systems of tubes 

 which reach from the heart to all parts of the body. The 

 arteries receive blood from the heart and distribute it to 

 the capillaries. The veins receive the blood from the 

 capillaries and return it to the heart. The arteries and 

 veins are similar in structure, both having the form of 

 tubes and both having three distinct layers, or coats, in 

 their walls. The corresponding coats in the arteries and 

 veins are made up of similar materials, as follows : 



1. The inner coat consists of a delicate lining of flat 

 cells resting upon a thin layer of connective tissue. The 

 inner coat is continuous with the lining of the heart and 

 provides a smooth surface over which the blood glides 

 with little friction. 



2. The middle coat consists mainly of non-striated, or 

 involuntary, muscular fibers. This coat is quite thin in 

 the veins, but in the arteries it is rather thick and strong. 



3. The outer coat is made up of a variety of connective 



