THE CIRCULATION OF THE BLOOD. 



235 



in each case, but owing doubtless in great part to variations in 

 anatomical relations. 



The Causes of the Sounds of the Heart. — There is general 

 agreement in the view that the second sound is owing to the 

 closure of the semilunar valves of the aortic and pulmonary 

 vessels ; the former, owing to their greater tension in conse- 

 quence of the higher blood-pressure in the aorta, taking much 

 the larger share in the production of the sound, as may be 

 ascertained by listening over these vessels in the exposed heart. 

 When these valves are hooked back, the second sound disap- 

 pears, so that there can be no doubt that they bear some impor- 

 tant relation to the causation of the sound. 



In regard to the first sound of the heart the greatest diversity 

 of opinion has prevailed and still continues to exist. The fol-. 

 lowing among other views have been advocated by physiolo- 



1. The first sound is caused by the tension and vibration of 

 the auriculo-ventricular valves. 



2. The first sound is owing to the contractions of the large 

 mass of muscle composing the ventricles. 



3. The sound is directly traceable to eddies in the blood. 



Fig. aOl. 



Fig. 202. 



Fig. 201. — Microscopic appearance of fibers from the heart. The cross-strise, divisions 



(branching), and junctures are visible (Landois). 

 Fig. 202.— Muscular fiber-cells from the heart. (1 x 425.) a, line of juncture between 



two cells; b. c, branching cells. 



But, looking at the whole question broadly, is it not unrea- 

 sonable to explain the sound resulting from such a complex act 



