280 ^ THE MECHANICS OF THE HEART 



The cause of this sudden loss of regularity of contraction is not 

 fully understood. Kronecker^ beheves that it is due to the destruction 

 of the coordinating cardiac center, while McWilliam^ states that it is 

 dependent upon an interference with conduction. The work of Gar- 

 rey^ has greatly strengthened the block-hypothesis of Porter^ which 

 proposes that the fibrillation is due to an interruption of the contrac- 

 tion wave. In consequence of this blocking, this wave is prevented 

 from running its usual course until the normal coordinated action of 

 the cardiac musculature gives way to the confused ^'circus" motions 

 of fibrillation. A similar confusion of contraction may be produced 

 in the tongue by reestablishing the circulation after it has been inter- 

 rupted for some time. As this organ embraces muscle fibers which are 

 arranged in different directions, it has been thought that this peculiar 

 motion is caused by a loss of functional continuity between the adjoin- 

 ing areas of tissue. It is possible that a similar dissociation takes 

 place in the fibrillating heart. 



A fibrillating heart, or ventricle, is, of course, quite unable to expel 

 the blood and to sustain the circulation. Death results very sud- 

 denly. A fibrillating auricle, on the other hand, is not necessarily 

 incompatible with life, because the ventricles are still in a condition of 

 responding. To be sure, the contractions of the latter become irregu- 

 lar, because they are now played upon by numerous impulses derived 

 from the fibrillating auricles. This condition is characterized by an 

 irregular arterial pulse and an absence of the auricular summit from 

 the venous pulse, as recorded from the external jugular vein. The 

 electrocardiogram taken at this time does not show the P-wave which 

 represents the electrical variation produced by the normally acting 

 auricles. 



CHAPTER XXVI 

 THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 



The different cardiac cycles follow one another in rapid succession, 

 every additional one adding another unit of work to that already 

 accomplished. Like any other mass of Uving substance, cardiac 

 muscle generates mechanical, thermal, and electrical energy. The 

 first of these is at present of greatest interest to us, because it furnishes 

 the basis for the dynamics of the circulation. While the heart is en- 

 gaged in this process of kinetically innervating the blood, it exhibits 



1 Compt. rend., Soc. de Biol., 1891. 



2 Jour, of Physiol., viii, 1887. 



3 Am. Jour, of Physiol., xxi, 1908, 283. 

 * Ibid., vi, 1902, 25. 



