INTERCALATED DISCS OF HEART MUSCLE 153 



III. DESCRIPTIVE 

 A. ADULT MATERIAL 



In every case the myocardium consists of a close-meshed net- 

 work of coarser and finer branching striped muscle trabeculae. 



1. Monkey 



When a large expanse of tissue is under view, the discs are 

 seen to lie more or less closely aggregated in definite regions. 

 There is evidently roughly an alternation of disc-containing and 

 disc-free areas (figs. 1 and 2). On close examination the disc- 

 containing areas for the most part correspond to the axial regions 

 of the mesh. In physical terms these regions correspond to 

 areas of greatest stress during contraction (figs. 1 and 2). The 

 discs appear commonly in three distinct forms: (1) they may be 

 compact and wide, 4 i.e., as wide as a single fiber; (2) they may 

 be narrow, i.e., not much wider than a single fibril; (3) they may 

 consist of rows of spherical granules, connected severally by a 

 delicate deep-staining membrane. These three main types of 

 discs are all represented in fig. 1. The 'compact' discs under 

 higher magnification are seen to be composed of rows of longer 

 or shorter rod-like granules. The rows of spherical granules 

 usually span the intervals between adjacent discs of more com- 

 pact forms (forms 1 and 2). The myofibrillae are clearly evi- 

 dent, singly and in bundles, giving the fiber a distinct longitu- 

 dinal striation. The striations (fibrillae) passing through the 

 smaller discs of form 2 are much coarser than the adjacent fibrils 

 (fig. 1). These fibrils are evidently modified, being probably in 

 a state of greater contraction; and the narrower discs would seem 

 to be foci of still greater contraction in the fibrils. If this in- 

 terpretation is valid — as many facts about to be detailed very 

 strongly indicate — then the wider discs are reasonably conceived 

 as the result of a fusion or association of narrower discs; and the 



4 'Wide' and 'narrow' refer to the transverse axis of longitudinal sections, 

 'coarse' or 'robust' and 'delicate' to the longitudinal axis, and 'deep' and 'super- 

 ficial' to the diameter of the uncut fiber. 



