102 



MUSCULAR TISSUE 



suffered, they represent lines of weakness. These are the locations of frac- 

 ture in fragmented and segmented pathological hearts a significant point 

 in relation to 'heart failure/ 



Heart muscle is syncytial in structure, and the myofibrillas 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 ce- 

 ment lines, combined with the fact that the atria are 

 apparently completely separated from the ventricles by 

 intervening connective 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 elements, that is, to beat automatically and inde- 

 pendently of the nervous system. The neurogenic 

 theory, 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 ap- 

 pearance of nerve fibers. However, there remained the 

 possibility that nerve fibers were actually present but 

 (indemonstrable by the method employed; also that 

 while nerves might be imnecessary for maintaining 

 rhythmic contraction during embryonic life, they 

 nevertheless became necessary in fetal and adult life. 



FIG. 112. SEVEN 

 N UCLEI, THE 

 PRODUCT OF 

 AMITOTIC DIVI- 

 SION, LYING IN 

 AN UNDIFFEREN- 

 TIATED MASS OF 

 SARCO PLASM, 

 FROM THE HEART 



OF LlMULUS. 



The telophrag- 

 mata are seen span- 

 ning the space be- 

 tween the nuclear 

 membrane and the 

 next adjacent myo- 

 fibril. X 700. 



The discovery of the atrioventricular bundle of 

 His (1893) at first added apparently the strongest 

 evidence to the support of the application of the myo- 

 genic theory of heart beat in the mammalian heart. 

 This is a muscular bundle which effects an intimate 

 connection between the atria and the ventricles. (Tt 

 will be further described with the Heart. 



An important matter is the observation that the 

 final ramifications of the bundle of His are identical with the so-called 

 Pur jink e 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 aiv the fibers of the myocardium proper. They would 



