PROPERTIES OF THE HEART MUSCLE. 567 



It is impossible to say positively whether or not the inorganic salts 

 are directly connected with the cause of the beat, — that is, with 

 the origination of the inner stimulus. According to one point of 

 view, they are necessary only to the irritability and contractility 

 of the heart tissue. The inner stimulus is produced otherwise by 

 some unknown reaction, but it is not able to cause a contraction 

 of the heart muscle in the absence of the proper inorganic salts. 

 According to another view, the reaction of these ions with the 

 living substance constitutes or leads to the development of the 

 inner stimulus. 



Physiological Properties of Cardiac Muscle. — Cardiac muscle 

 exhibits certain properties which distinguish it sharply from skeletal 

 muscular tissue and which have a direct bearing upon the rhyth- 

 micity of the contractions and the sequence shown by the different 

 chambers. The most characteristic of these properties are the 

 following: 



1. The contractions of heart muscle are always maximal. In 

 skeletal muscle and in plain muscle the extent of contraction is 

 related to the strength of the stimulus, and we recognize the exis- 

 tence of a series of submaximal contractions of varying heights. 

 This is not true of heart muscle. As was first shown by Bow- 

 ditch, a piece of ventricular mjjscle when stimulated responds, if 

 it responds at all, with a maximal contraction. The apex of a 

 frog's heart does not beat spontaneously, but contracts upon 

 electrical stimulation. If such an apex is connected with a lever 

 to register its contractions, and the electrical stimulus apphed to 

 it is gradually increased, the first contraction to appear is maxi- 

 mal, and it is not further increased by augmenting the stimulus. 

 This property is sometimes described by saying (Ranvier) that 

 the contraction of the heart muscle is all or none. This fact 

 must not, however, be interpreted to mean that the force of 

 contraction of heart muscle is invariable under all conditions. 

 Such is not the case. The heart muscle under favorable nutritive 

 conditions may give a much larger and more forcible contraction 

 than is possible under conditions of poor nutrition; but the point 

 is, that, whatever may be the condition of the muscle at any 

 given moment, its contraction in response to artificial stimulation 

 is maximal for that condition — that is, does not vary with the 

 strength of the stimulus. As was said above, this property is not 

 exhibited by the crustacean (lobster) heart, but has been shown 

 to be true for the mammahan heart muscle.* It will be remem- 



* For experiments on mammalian heart and literature, see Woodworth, 

 "American Journal of Physiology," 8, 213, 1903. 



