334 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



3. By cutting and removing the nerve cord which passes along the tubular 

 heart of the horseshoe crab, Carlson^ has succeeded in showing that the cause of 

 the contraction of this organ lies in the ganglion cells of the median cord, and that 

 the conduction is effected by the nervous and not by the muscular elements. In 

 this particular case, therefore, it would appear that the cessation of the heart beat 

 is brought about by an interference with the automatic discharges of the ganghon 

 cells (Weber) and not by an inhibition of the activity of the cardiac musculature 

 (Engelmann). These results, however, do not permit of generalizations, because 

 they cannot justly be applied to the vertebrate heart without certain modifications. 

 The reason for this is that the heart of vertebrates may possess certain physiological 

 properties which are very different from those displayed by the heart of the 

 crustaceans. 



4. It has been found by Kronecker and Schmey^ that the regular and forceful 

 contractions of the ventricle may be changed into mere fibrillary undulations 

 (delirium cordis) at any time by puncturing the interventricular septum at a 

 point near the junction of its upper and middle thirds. While this phenomenon 

 has been interpreted as proving that the coordinated action of the ventricle is de- 

 pendent upon a center situated in the aforesaid region, this hypothesis can scarcely 

 be defended in the light of our present knowledge regarding the conduction paths 

 of the heart. Moreover, it has been shown subsequently by McWilliams^ that 

 the cardiac musculature may also be made to fibrillate in other ways, for example, 

 by mechanical, thermal, and electrical stimulation of the surface of the heart in 

 the vicinity of the apex. 



5. The contractions of the mammalian heart may also be incited by perfusing 

 the coronary circuit with non-nutritive fluids. It seems that in this particular 

 case the distention of the coronary blood-vessels suffices to stimulate the nervous 

 receptors in a mechanical way. 



The Myogenic Theory of the Heart Beat. — This theory has been 

 more fully developed in recent years by the work of Gaskell and 

 Engelmann. It is held that the wave of excitation arises in the muscle 

 tissue and that the nervous elements serve the sole purpose of cor- 

 relating the action of the different parts of the heart, and secondly, of 

 bringing the activity of this organ into functional relation with other 

 structures. Furthermore, as the beat originates in the venous vesti- 

 bule, the tissue composing this particular area, is said to possess 

 certain functional peculiarities which render it especially suitable for 

 the generation of those impulses which later on give rise to the con- 

 traction. The arguments favoring the myogenic theory may be cited 

 as follows: — *-- 



1. Bernstein's Experiment. — If the apical portion of the heart of a frog or 

 turtle is separated by a ligature which is tightly drawn around the ventricle, it 

 ceases to contract almost immediately. When isolated in this way, it may be made 

 to beat again by applying electrical or mechanical stimuli to its surface or by raising 

 the pressure within its cavity. The latter end may be attained at times by tem- 

 porarily compressing the aortse. 



2. Strips of tissue may be cut from the apex which may be made to beat 

 rhythmically by placing them in an isotonic solution of sodium chlorid or in 

 Ringer's fluid. These strips frequently continue their activity for several hours. 

 These experiments become especially significant, if it is remembered that the apex 

 of these hearts contains no ganglion cells. 



. 1 Am. Jour, of Physiol., xiii, 1905, 217. 



^ Sitzungsber. der Akad. der Wissensch., Berlin, 1884. 

 3 Jour, of Physiol., viii, 1887, 296. 



