MUSCULAR ACTIVITY 117 



If the same treatment causes lactic acid to be produced in smooth 

 muscle, we are obliged to conclude that the presence of this acid 

 may cause relaxation in this tissue instead of contraction. This, 

 again, is in harmony with the general idea that in smooth muscle 

 any change in length may require the liberation of energy. 



Physiology of Cardiac Muscle. In some features of its activity 

 heart muscle resembles skeletal muscle; in others it is more like 

 smooth muscle. The contraction of heart muscle is rapid and 

 vigorous, in this respect corresponding to skeletal muscle. The 

 liberation of energy in heart muscle is associated with contraction 

 rather than with change in length as such. Here again the re- 

 semblance is with skeletal muscle rather than with smooth. 



Cardiac muscle is like smooth muscle in that it has the power of 

 executing spontaneous contractions. The heart receives, also, 

 the double innervation referred to above as characteristic of 

 smooth muscle. 



In a number of important regards the heart differs from either 

 of the other types of muscle. For convenience discussion of these 

 special features is deferred to the section in which the heart is 

 studied as an organ of the circulation (Chap. XX). 



