AND PHYSIOLOGY. 



227 



Fig. 226. 

 3 4 5 6 



Auricles. 



Ventricles. 



Diagram showing the relative Time occu- 

 pied by the Contraction of the Auricles and 

 Ventricles, the converging lines from left to 

 right indicating the contraction, and the di- 

 verging ones the dilatation. 



7 a 



portion of time occupied by one pulsation into eight intervals, 

 ■we shall find that the auricles employ only one of these in- 

 tervals in contraction, and the remaining seven by dilatation ; 

 while the time occupied by the 

 contration of the ventricles is 12 

 the same as their dilatation, as 

 may be seen in the diagram. 



400. The Sounds of the 

 Heart— Cause of the First 

 and Second Sound —If the 

 ear be applied over the heart 

 of a healthy person, two 

 sounds will be heard, one 

 of which corresponds in time 

 with the pulsation noticed 

 at the wrist, or any other 

 large artery of the body. 

 These sounds do not corre- 

 spond to each other in intensity or duration, but are some- 

 what indefinitely represented by the sounds given to the mono- 

 syllables " lub" and "dup;" the first a long and heavy 

 sound, and the second a short and light one. The first sound 

 is undoubtedly caused by the contraction of the heart, the 

 rush of blood, and the impulse of the organ against the side 

 of the chest ; while the second is the clicking of the semi- 

 lunar valves as they close at the commencement of the aorta 

 after the ventricular contraction. That the last sound is due 

 to this cause is proved by an experiment performed on a dog, 

 of introducing a hook through the aorta, and holding back one 

 of these valves, when the second sound entirely failed. 



401. Number of Pulsations per Minute. — The number 

 of pulsations of the heart varies considerably at different pe- 

 riods of life. Thus the following table shows the average 

 number of pulsations each minute at different ages : 



Give the relative time occupied by the contraction and dilatation of the auricles and 

 Tentricles. 400. What two sounds are made at each complete pulsation of the heart? 

 Give the causes of the first sound. What is the last sound owing to ? What proof of 

 it? 



