29() H. E. JORDAN AND J. B. BANKS 



myofibrils; tlie 'cells' elongate into fusiform elements, the con- 

 stituent myofibrils meanwhile increasing in number, and subse- 

 (juently by fusion form a close-meshed network of delicate 

 trabeculae (fig. 15) with still more delicate branches, for the 

 most part originating at very acute angles; coarser trabeculae 

 arise by growth and further fusions, their coarser branches com- 

 ing off at more obtuse angles; the myocardial plexus may suffer 

 still further local fusions, and becomes meanwhile subjected to 

 the functional stresses of opposed and oblique tensions in part 

 the consequence of a spiral twisting of its constituent fibers and 

 branches. (2) Intercalated discs develop gradually during fetal 

 life; they are from the beginning closely associated with the telo- 

 phragmata, having the appearance of thickened membranes or 

 portions of telophragmata; at this time the only conspicuous 

 stripes are the telophragmata, which appear very delicate and 

 irregular; these apparently developed out of the spongioplasm of 

 the myoblasts, while the sarcolemma develops from the cell 

 membrane; the discs are at first granular, and only subsequently 

 show the typical comb structure. (3) The discs increase in 

 size and number coincident with the pre- and post-natal devel- 

 opment of the heart, the results respectively of a longitudinal 

 splitting of the fibrils with their intercalated discs and a new 

 formation of discs, and persist under modification throughout 

 life; once formed the discs are apparentl}^ persistent structures 

 subject to growth and extensive mechanical alterations. (4) 

 The discs are peripherally placed, always in association wdth telo- 

 phragmata, and with the sarcolemma. (5) The discs are more 

 commonly located in the regions where the coarser trabeculae 

 branch, and freciuently dixdde areas of different physiologic 

 states. (6) The myofibrils pass \\dthout interruption through 

 the discs; the discs are essentially modified portions of the 

 involved myofibrils, among the structural units of which a 

 relatively more abundant tissue fluid occurs. (7) In the sim- 

 plest condition they are similar to the contraction bands both in 

 structure and in their relation to the myofibrils and the telo- 

 phragmata; in sections stained with iron-hematoxylin the 

 narrower band-forms of discs and complete contraction bands 



