162 



PHYSIOLOGY FOR DENTAL STUDENTS. 



however, as is shown by experiment, and is explained by the fact 

 that the blood, when it is forced from the ventricle during the 

 cardiac contraction, exerts its force on the apex as well as on the 

 blood in the arteries. This serves to fix the apex in the vertical 

 position and to bring the base of the ventricles downwards 

 during their contraction. In some individuals there is a 

 visible pulsation at about the level of the fifth rib on the left side. 

 This is called the apex beat, and is caused by the rotation of the 

 apex in the transverse diameter and by the sudden change of the 

 ventricle from a soft flabby condition into a firm one. 



Fv 



Fig. 17. Diagram of Valves of the Heart. The valves are supposed to be 

 viewed from above, the auricles having been partially removed. A, aorta 

 with semilunar valve ; B, pulmonary artery and valve ; C, tricuspid, and D, 

 mitral valve ; E, right, and F, left coronary artery ; G, wall of right, and H, 

 of left auricle ; I, wall of right, and J, of left ventricle. ( From Stewart's 

 Physiology. ) 



The walls of the auricles are relatively thin, as they are not 

 required to do heavy work. The ventricular muscles, on the 

 other hand, are well developed, that of the left ventricle being 

 very strong and adapted to the heavy work it must perform. 



The valves guarding the opening between the auricles and 

 ventricles are composed of thin membranes of fibrous tissue, cov- 

 ered with endothelial cells similar to the lining of the heart and 

 the blood vessels (Fig. 17). In acute rheumatism and tonsil- 

 litis, the endothelial covering of the interior of the heart and of 

 the valves is often inflamed, and permanent changes may take 



