PHYSIOLOGICAL PROPERTIES OF CARDIAC MUSCLE 345 



minimal. Fano/ on the other hand, beHeves that there are two dif- 

 derent kinds of excitatory agents at work. In support of this conten- 

 tion, Gaskell and Mines ^ have found that weak acids and carbon 

 dioxid diminish the power of contraction as well as the tonus, whereas 

 an increased alkahnity gives rise to just the opposite effect. It seems 

 certain, however, that an optimum degree of tonus can only be obtained 

 if the body fluid possesses a perfectly definite reaction. As the re- 

 action of the blood depends chiefly upon the tension of carbon dioxid, 

 it may be inferred that this gas plays a most important part in the 

 production of tonicity.^ 



It must be clear that the tonicity of cardiac muscle furnishes a 

 means of determining its functional capacity. Under ordinary con- 

 ditions it is sufficient to note the amplitude and force of the contrac- 

 tions of the exposed or isolated heart, or to measure the pressure which 

 the normally beating organ is capable of developing in the blood-vessels. 

 To begin with, the individual cells must of course be capable of entering 

 the state of complete relaxation, as well as that of maximal contrac- 

 tion. Hence, they must possess a wide range of movability. The 

 former quality is as important as the latter, because it determines the 

 capaciousness or power of filling of the entire organ. It must be 

 evident that a loss of the relaxing power of the muscular units must 

 place the heart under a certain disadvantage, because it lessens the 

 capacity of its chambers. Quite similarly, it may be said that an 

 unusual degree of relaxation must act unfavorably, because it tends to 

 invite an undue distention and imperfect emptying of the cardiac 

 chambers. The latter condition indicates a loss of tonus approaching 

 fatigue, and may lead to a general dilatation of the organ when called 

 upon to perform an extra amount of work. It stands to reason that a 

 muscular unit which is not tonically set is not in a favorable position 

 to resist those strains which 'frequently arise in the vascular system 

 in consequence of physical exertions and emotions. A loss of tonus^ 

 therefore, exposes the heart to the danger of becoming hyperdistended 

 and dilated. 



Opposed to the condition of dilatation is the condition of hypertrophy, which 

 presents itself in the form of either a deposition of perfectly new cells or an increase 

 in the volume of those already present. In either case, an organ larger and heavier 

 than normal is the result. Hypertrophy finds its origin in the fact that the cardiac 

 cells are in a tonic condition and react to excessive stimulation by increasing 

 their power of contraction. This change eventually produces a compensatory 

 increase in the size and massiveness of the heart, while the condition of dilatation 

 is a simple distention without a deposition of new material. But it is not always 

 true that these changes affect the organ as a whole, in fact, in many instances only 

 single compartments are involved. Thus, mitral stenosis is usually associated 

 with a hypertrophy of the left auricle and aortic stenosis with a hypertrophy of 

 the left ventricle. 



^ Festschr. fiir C. Ludwig, Leipzig, 1887. 



2 Jour, of Physiol., xlvi, 1913, 23. 



3 Patterson, Piper and Starling, Jour, of Physiol., xlviii, 1914, 465. 



