344 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



a disturbance of the rhythm. It might also be mentioned that a large 

 number of the so-called ''premature" beats are caused by impulses 

 which arise in a hypersensitive auricular tissue. Whether these stimuli 

 originate in this particular area or nearer the pace-maker, can readily 

 be determined by noting the length of time intervening between them 

 and the next systole, because a compensatory pause must arise as soon 

 as the distance between their place of origin and the ventricle becomes 

 sufficiently great to allow them to reach the latter while in systole. 

 The method of auscultation is not well adapted for the detection of 

 these irregularities in rhythm, and especially not if they are of the inter- 

 polated type, but it is possible to recognize them without difficulty with 

 the aid of the electrocardiograph.^ 



The Tonus of Cardiac Muscle. — The functional capacity of the 

 heart depends upon the tonus of its muscular elements. This fact 

 implies that the latter are normally held under a certain physiological 

 tension, i.e,, they are retained in a state intermediate between com- 

 plete relaxation and contraction. The tonus, however, does not re- 

 main the same for a long period of time, but varies with the character 

 of the internal stimuli. This fact may readily be deduced from any 

 continuous record of the beating heart of a frog, because the curve 

 as a whole does not follow along a straight horizontal line, but shows 

 long wave-like oscillations. In this respect, cardiac muscle does not 

 differ from striated or non-striated muscle tissue, because both of these 

 are continuously exposed to tonic impulses and are able to relax fully 

 only if separated from the central nervous system. It need scarcely 

 be emphasized that a muscle when held in a position of partial contrac- 

 tion, can reach the condition of maximal shortening with much greater 

 rapidity. This statement also applies to the arteries and other tubular 

 organs, because their walls are ordinarily kept in a position intermedi- 

 ate between constriction and dilatation. 



The property of tonicity of a tissue is dependent upon the activity 

 of the nervous elements with which it is connected. It is believed that 

 the nervous centers give origin to a series of subminimal impulses which 

 tend to keep the tissue continually in a condition of functional alertness. 

 Concurrently, it may be reasoned that if these impulses are prevented 

 from reaching their destination for any length of time, the tissue 

 loses its tonicity and eventually becomes functionally useless. In 

 the case of the isolated heart, however, the tonus is retained in a meas- 

 ure, because its intrinsic nervous elements are capable of generating 

 those impulses which under normal conditions are derived from its 

 extrinsic centers. 



The nature of the stimuli upon which the tonus depends is still 

 unknown. It is commonly held that the tonicity is due to the same 

 stimuli which produce the contractions. In the former case, however, 

 they remain subminimal, while in the latter case they become supra- 



^ Lewis, Clinical Disorders of the Heart Beat, London, 1913. 



