498 CIRCULATION OF BLOOD AND LYMPH. 



beat and its variations under experimental conditions may be 

 explained in terms of the theory, or at least do not contradict it. 

 The same statement, however, may be made regarding the myogenic 

 theory. Both theories may be applied successfully from a logical 

 standpoint to the explanation of known facts. 



2. No single fact is known which can be cited as positive proof 

 that the nerves participate in the production of the normal beat 

 of the vertebrate heart. The experiment by Kronecker and Schmey 

 is sometimes given this significance. These observers have shown 

 that, when a needle is thrust into a certain spot in the dog's 

 ventricle, the regularly contracting heart falls suddenly into fibrillar 

 contractions so far as the ventricles are concerned. The ex- 

 periment is certainly a striking and interesting one. The needle 

 may be thrust many times into certain portions of the muscu- 

 lar mass without affecting the powerful co-ordinated contractions, 

 but in the region specified by Kronecker a single puncture, if 

 it reaches the right spot, causes the ventricle to fall into ir- 

 regular fibrillar twitches from which it does not recover. The 

 spot as described by Kronecker is along the line of the septum at the 

 lower border of its upper third. The experiment frequently fails; 

 and it would seem that there must be a definite and quite circum- 

 scribed structure whose lesion produces the effect described. We 

 have no evidence as yet what this structure is, and are therefore in 

 no condition to make positive inferences with regard to the bearing 

 of the experiment upon the origin of the heart beat. Recently 

 Carlson* has described experiments upon the heart of the horseshoe 

 crab (Limulus) which seem to show conclusively that in this animal 

 the rhythmical contractions are dependent upon the intrinsic nerve 

 cells. These latter are placed superficially, forming a cord that 

 runs the length of the tubular heart. When this cord is removed 

 the heart ceases to beat. There are reasons, however, which at 

 present make it impossible for us to apply the results of this ex- 

 periment to the vertebrate heart. The crustacean heart differs 

 from the vertebrate heart in its fundamental properties; unlike the 

 latter, it has no refractory period (see p. 504), can be tetanized, and 

 gives submaximal contractions. f It is a tissue, therefore, that 

 resembles in its properties ordinary skeletal muscle in the verte- 

 brate, and, like this muscle, it seems to be lacking in automaticity. 

 Carlson's experiments give, however, another instance of automatic 

 rhythmicity in nerve tissue, and to that extent supports the 

 neurogenic theory. 



The Myogenic Theory of the Heart Beat. The myogenic 



* Carlson, "American Journal of Physiology," 12, 67, and 471, 1905. 

 t Hunt, Bookman, and Tierney, " Centralblatt f. Physiologic," 11, 275, 

 1897. 



