338 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



While the sodium, calcium, and potassium may not be the only agents con- 

 cerned in this excitation, it must be evident that they play a most important part 

 in the formation of a molecular concentration of the blood which favors the 

 activity of cardiac tissue. It must also be apparent that these salts are specific 

 in their action. The sodium, for example, stimulates contraction, while the 

 calcium maintains the tonus and the potassium favors relaxation. Obviously, 

 therefore, a proper activity of the heart can only be secured by means of a solution 

 which contains these salts in perfectly definite proportions. Howell, in fact, 

 believes that the states of contraction and relaxation of cardiac muscle are depend- 

 ent upon an alternate and opposing interaction of these substances with the 

 contractile elements of this tissue. In this connection, it is of interest to note 



Fig. 176. — A Frog's Heart Poisoned by Excess of Calcium Salts, Recovers 

 Its Spontaneous Rhythm on Adding a trace of KCl to the Perfusion Fluid. 

 {Ringer.) 



that Biedermann^ and Loeb have succeeded in eliciting rhythmic contractions 

 in striated muscle by subjecting it to the influence of isotonic solutions of sodium 

 and lithium. Solutions of calcium, on the other hand, have been proved to 

 possess an inhibitor action. 



As far as the nature of the ''inner stimulus" of the heart is con- 

 cerned, it may be held that the substances just enumerated, actually 

 constitute the exciting agent (Howell), or that they merely furnish a 

 medium in which the true stimulus is then capable of unfolding its action 

 (Engelmann). If the latter view is adhered to, the stimulating agent, 

 whether it be chemical, electrical, or enzymotic in its nature, has not 

 been discovered as yet. 



CHAPTER XXIX 



THE PHYSIOLOGICAL PROPERTIES OF CARDIAC 



MUSCLE 



Refractory Period. Extrasystole. — The heart of the lower animals 

 may be made to register its contractions upon the paper of a kymo- 

 graph by connecting its apex with the free end of a writing lever. A 

 thread and small hook are used to make this connection. Another 

 procedure is to place a delicate rod upon the ventricle and to permit it 

 to act against the long arm of a writing lever. The lower end of this 

 rod should be equipped with a cup-shaped platelet serving to retain 

 the former more firmly upon the surface of the heart. A third method 



1 Wiener Sitzungsber., Ixxxii, 1880. 



