102 



MUSCULAR TISSUE 



ing granules about the Z membrane. In the older hearts, where they are 

 mechanically modified, and in diseased hearts, as in hypertrophies, where 

 probably a chemical modification is also suffered, they represent lines of 

 weakness. These are the locations of fracture in fragmented and segmented 

 pathological hearts a significant point in relation to 'heart failure.' 



Heart muscle is syncytial in structure, and the myofibrillae pass unin- 

 terruptedly through the intercalated disks. These facts are of special im- 

 portance because of their bearing on the opposed theories of the origin and 

 conduction of the stimulus to the heart beat, the myogenic and the neuro- 

 genic. A complete cellular structure with actual cement lines, combined 

 with the fact that the atria are apparently completely 

 separated from the ventricles by intervening con- 

 nective tissue, was once urged as a strong argument 

 against the validity of the myogenic theory of heart 

 beat the theory which proclaimed the adequacy of 

 heart muscle to initiate and conduct the stimulus to 

 contraction without the intervention of nerve ele- 

 ments, that is, to beat automatically and indepen- 

 dently of the nervous system. The neurogenic the- 

 ory, which holds that nerve elements are essential 

 for the conduction of stimuli for contraction, on the 

 other hand, seemed contradicted by the observation 

 that in the chick the heart beat rhythmically before 

 the appearance of nerve fibers. However, there re- 

 mained the possibility that nerve fibers were present 

 but undemonstrable by the method employed; also 

 that while nerves might be unnecessary for maintain- 

 ing rhythmic contraction during embryonic life, they 

 nevertheless became necessary in fetal and adult life. 

 The discovery of the atrioventricular bundle of 

 His (1893) at first added apparently the strongest 

 evidence to the support of the application of the myogenic theory of heart 

 beat in the mammalian. This is a muscular bundle which effects an inti- 

 mate connection between the atria and the ventricles. See heart, p. 199. 

 An important matter is the observation that the final ramifications 

 of the bundle of His are identical with the so-called PurJcinje fibers. 

 These have long been known, especially in the sheep's heart, where 

 they are unusually large and abundant. They are limited to a region 

 directly under the ventricular endocardium. They are coarser, less 

 branched, with fewer intercalated disks, almost exclusively of the band 

 type, than are the fibers of the myocardium proper. They would 



FIG. 112. LONGI- 

 TUDINAL SECTION 

 OF PORTION OF 

 ATRIO - VENTRICU- 

 LAR BUNDLE OF 

 HEART OF BEEF. 

 MANY OF THE 

 CELLS ARE BINU- 



CLEATED. X 160. 



