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



as favorable for a study of the discs and striations as are sec- 

 tions; it reveals clearly also the H-discs at certain stages in the 

 process of contraction (fig. 32). 



h. The atrial myocardium 



The several new types of discs to be described for the atrium 

 must not be thought to be characteristic of this portion of the 

 myocardium. Similar discs are found also in about equal pro- 

 fusion in the ventricles. They are considered in this order be- 

 cause in some respects this selected group represents a more 

 complex condition. 



In figure 16 is illustrated a simple and common type of disc. 

 In so far however as these discs are bounded on both borders by 

 a telophragma they represent a secondary modification of the 

 original disc which is in contact with only one telophragma, 

 which generally bisects the disc. Both of these discs moreover 

 separate areas of different physiologic condition. Both discs 

 have the width of a complete sarcomere. The one on the left 

 is a two-step form, and probably arose from a dislocation 

 through one sarcomere of an original band disc. In terms of a 

 contraction band such a disc might conceivably be the result of 

 the fusion of adjacent halves of successive bands, or of a second- 

 ary modification of a single band in such a manner as to cause 

 it to spread to the adjacent telophragma. 



Appearances like that illustrated in figure 18 strongly suggest 

 that the former interpretation of discs of the width of a complete 

 sarcomere as the result of a fusion of adjacent halves of succes- 

 sive contraction bands, is, at least in some cases, the correct one. 

 Here, in figure 18, two discs appear within a single sarcomere, 

 each bounded on opposite sides by a telophragma. If the disc- 

 condition is conceived to spread between the two moieties, such 

 discs as are illustrated in figure 16 would be formed. Whatever 

 the fundamental modifying factor may be that operates to con- 

 vert the myofibrils to the disc-condition — whether of the natm'e 

 of an irreversible contraction following uncommon stresses or 

 not — it acts along the line of a telophragma, causing the myo- 



