THE CIRCULATION. 161 



not sharp by any means. The arteries as they grow smal- 

 ler and branch more and more, gradually lose the thickness 

 of their walls, until finally without a single sudden break in 



Fig. 80. CAPILLARIES AS SEEN IN THE WEB OF A FROG'S FOOT. 



the transition, but the inner coat remains, when it is 

 somewhat arbitrarily called a capillary. On the other side, 

 the capillaries have added to them little by little extra ele- 

 ments in their walls, and shade off without a sudden transi- 

 tion here into the veins. By the term capillary, therefore, 

 is included that small portion where all the coats save the 

 innermost are absent, and is more a physiological unit than 

 an anatomical one. 



THE PHENOMENA OF THE HEARTS BEAT. 



Everybody is familiar with the fact that throughout life 

 the activity of the heart is shown in what is familiarly called 

 the "heart's beat." This beat may be easily recognized 

 in any of three ways: First, where the apex of the heart 

 touches the chest wall on the front and left side there may 

 be felt at each beat a little push or jar, caused by the pres- 

 sing of the heart against this portion of the chest. The 

 point at which this is most pronounced is between the fifth 

 and sixth ribs. Second, it may be felt in the pulse where 

 the beat of the heart appears as a wave running along the 

 larger arteries of the body. As this is usually more con- 

 venient for observation the phenomena of the heart's beat 

 are usually studied in these pulsations. Third, the beat of 

 the heart may be easily detected by the sounds which occur 

 11 



