330 H. E. JORDAN AND J. B. BANKS 



PLATE 1 



EXPLANATION OF FIGURES 



1 Portion of a longitudinal section of a muscular trabecula from the myo- 

 cardium of the right ventricle. The trabecula is in the relaxed condition. Two 

 intercalated discs are shown, at the levels of successive telophragmata, having 

 apparently displaced these membranes. They are of the simple band form, the 

 constituent elements being modified rod-like portions of the included myo- 

 fibrils. The fiber is somewhat narrower at the point of the location of the discs, 

 as if stretched in this region. The discs are comparable, structurally, to con- 

 traction bands which have become stretched out by reason of the tension exerted 

 by the adjacent contractile portion of the trabecula. 



2 Portion of a longitudinal section of a branching trabecula of right ven- 

 tricle. A terraced form of intercalated disc divides an upper relaxed from a 

 lower (left) contracted region. The contracted stains more deeply than the 

 non-contracted region. In the latter the telophragmata, Q-discs and J-discs 

 are conspicuous; in the contracted region only relatively narrow contraction 

 bands occur, alternating with lighter broader discs. At the left is shown the 

 sarcolemma, thrown into arcades or festoons. These span the spaces between 

 successive telophragmata. Where intercalated discs occur, the point of at- 

 tachment generally corresponds with the midportion of the disc. In the case 

 of the upper two elements of this step-like disc, the telophragmata at the right 

 also pass to the mid-line of the discs. In so far, these elements correspond to 

 slightly modified contraction bands. In teased preparations of stained myo- 

 cardium, mounted in glycerin, the telophragma can occasionally be seen actually 

 bisecting a disc. The lower four elements are of identical structure, and placed 

 at successive sarcomeric segments in such a manner that their upper limits are 

 on a level with the telophragmata at the right, and their lower borders corre- 

 spond to the level of the delicate contraction bands at the left. The second and 

 third element are connected by a slightly deeper-stainipg membrane, probablj- 

 a sarcolemma remnant. Similar complex discs appear in teased preparations, 

 hence not due to any peculiarity of section. 



3 a and b Portion of a longitudinal section of a myocardial trabecula of the 

 right ventricle at a higher (a) and a lower (6) level of focus. This disc is of the 

 simple band-type, peripherally located, apparently displacing a telophragma. 

 In lowering the level of focus the disc is seen to have become dislocated or dis- 

 torted on the opposite surface. 



4 A broad two-step disc from a trabecula of the right ventricle. The steps 

 are connected' by a 'riser' in the form of a deeper-staining membrane. The disc 

 may represent two successive bands, but is more probably a band-disc secondarily 

 dislocated. 



5 Complex terraced disc bounding a wedge-shaped, lighter-stained, relaxed 

 portion of the fiber at the left. In passing from a lower to a higher level of focus 

 the group of discs marked 1 and 2 come successively into view. These discs are 

 peripherally located. Those marked {I') shade laterally into the telophragmata. 

 The intermediate terraced portion {2) consists of 'steps' connected by deeply- 

 staining membraneous 'risers.' A complete interpretation probably requires 



