THE HEART BEAT 163 



place which injure the valves and produce what is known as val- 

 vular disease of the heart. The chordae tendineae connect the 

 free margins of the valves with the papillary muscles, which arise 

 from the musculature of the ventricle like little knobs of tissue. 

 This arrangement prevents the valves from being everted into 

 the auricle during the contraction of the ventricle. The valves 

 on the left side consist of two flaps and are called the mitral 

 valves; those 011 the right side have three flaps and hence are 

 on 1 1 cd tricuspid valves. The valves guarding the arterial orifices 

 consist of three cup-shaped membranes and are known as the 

 semilunar valves, because of their crescent-shape when they are 

 closed. Whenever the pressure in the arteries is greater than 

 that in the ventricles, these valves are tightly closed, and prevent 

 any blood entering the ventricle from the arteries. 



The Physiologic Properties of Heart Muscle. 



The Character of Cardiac Contraction. The contraction of 

 our voluntary muscles is not due to a single stimulus sent from 

 the brain through the nerves, but rather to a series of such stim- 

 uli, which produce a more or less continued or tonic contraction 

 of the muscle. If this were not the case, our movements would 

 be very quick and jerky, similar to those made by a person suf- 

 fering with St. Vitus dance. In the case of the heart muscle, 

 however, each beat consists of a single complete muscular con- 

 traction, and it is impossible to produce a tonic or continued con- 

 traction in the heart such as can be produced in voluntary mus- 

 cle by rapid successive stimuli. Another peculiarity of heart 

 muscle is that each time it contracts it does so with all the force 

 that it has at the moment. Skeletal muscle contracts with great- 

 er or less intensity according to the strength of the stimulus it 

 receives. 



Heart muscle, and in a lesser degree some other muscles, such 

 as those of the intestinal tract and spleen, have the power of 

 making automatic rhythmic contractions which follow each other 

 in a definite sequence. This phenomenon in the case of cardiac 

 muscle is not dependent on the influence of the nerves, as can be 

 shown by the fact that the heart removed from the body will con- 



