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



fied type, a sarcomere in width, perhaps fused adjacent halves of 

 successive contraction bands. 



Figure 21 illustrates a rare type of disc. It is of the simple 

 band type, bounded on both borders by a telophragma, and shad- 

 ing on the right into the lateral extension of these membranes. 

 Viewing the sarcomeric section as a unit, the modified disc por- 

 tion appears contracted. Such a disc may seem to support 

 Heidenhain's interpretation in terms of a differentiating sarco- 

 mere; but it can equally well be interpreted as the result of the 

 only partial relaxation of adjacent halves of successive contrac- 

 tion bands, the disc representing the irreversible fused product 

 on the left. 



Figures 22 and 23 illustrate peculiar atypical forms of discs, 

 several sarcomeres in width. In figure 23 the central element is a 

 simple band-disc with which are associated outlying modified 

 portions of fibrils. The entire disc-area here stains more deeply 

 than adjacent portions of the trabecula, indicating a different 

 physiologic condition in the disc region. Such discs seem im- 

 possible of interpretation with any degree of plausibihty on 

 a basis of growth phenomena, tendinous fibrils, or cement 

 substances. 



The complex disc in figure 24 likemse defies interpretation in 

 terms of any of these hypotheses. It is one of the most compli- 

 cated types, and perhaps involves secondary distortions coinci- 

 dent with a spiral twisting and concomitant dislocations. Fig- 

 m*e 25 possibly represents a transection of a similar fiber, in 

 which portions of the originally peripheral band-discs have been 

 transferred centrally through spiral twistings of the fiber. Simi- 

 lar discs are shown in figures 26 and 27, where the irregular con- 

 dition is associated with a secondary longitudinal splitting of the 

 primary trabeculae into smaller bundles. Such splitting of 

 trabeculae, followed by a partially independent and opposed 

 functional activity, may be a factor in the formation of these 

 types of discs. The connecting deeply-staining membrane may 

 be the distorted telophragma of the original disc, or perhaps an 

 inturned portion of the sarcolemma. 



