336 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



in abeyance, with the understanding, however, that the evidence so far 

 submitted favors the myogenic theory. 



The Nature of the "Internal" Stimulus.— Even if the cause of 

 the heart beat should finally be localized either in the nervous tissue or 

 in the muscle tissue, the nature of the exciting agent must still remain 

 doubtful. It is customary to evade this question by saying that the 

 cardiac muscle possesses the power of automaticity, the implication 

 contained in this statement being that this tissue embraces certain 

 excitatory agents which are capable of acting independently of outside 

 influences. Strictly speaking, however, this cannot be true, because 

 all reactions of living substance are dependent upon material brought 

 to it from the outside. Without stimuli of this kind life is impossible. 



In seeking to discover the nature of the ''inner" stimulus, it is 

 fair to assume that the cardiac contractions result in consequence of 

 an interaction between the chemical constituents of the blood and those 

 of the substance of the heart. If this problem is restricted in this 

 way, further advance in this direction necessitates the determination 

 of those substances which act as exciting agents either individually 

 or when combined with others. In what measure we have succeeded 

 in isolating these agents will be brought out in the succeeding 

 paragraphs. 



It is a well-known fact that the hearts of the cold-blooded animals 

 continue to beat for some time after their excision, while the hearts of 

 the warm-blooded animals cease their activity very soon after the 

 interruption of the circulation. Both types of organs, however, may 

 be kept in an active condition outside the body by supplying them 

 with defibrinated blood or some other nutritive fluid. This difference 

 in their behavior may best be explained upon the basis of metabolism. 

 As the mammalian heart possesses a more vivid metabolism, it requires 

 a more constant supply of nutritive material, and especially, because 

 its storative power is altogether too slight in comparison with the work 

 demanded of it. It is essential, therefore, that it be in possession of 

 an extensive coronary system which enables even its most remote cellu- 

 lar constituents to obtain fresh substances in a very brief time. The 

 heart of the lower animals, on the other hand, does not require a sys- 

 tem of local blood-vessels, because its metabolic processes are less in- 

 tense and are amply safeguarded by direct interchanges with the blood 

 as it traverses its cavities. The cells of the lower hearts also seem to 

 be able to store a considerable portion of their nutritive material, so 

 that it may be made use of whenever the blood supply is cut off. 



It has been found by Merunowicz that an aqueous extract of the ash of the 

 blood exerts a stimulating action upon cardiac muscle. In continuation of these 

 experiments Ringer^ has proven in 1882 that certain inorganic salts, namely the 

 chlorids of sodium, calcium and potassium, affect this tissue in a very specific 

 manner, because they are especially adapted for maintaining the beat. In the case of 

 the heart of the frog, these salts act most efficiently in the following concentration: 



1 Jour, of Physiol., iv, 1883, 222. 



