44 



Scientific Proceedings (51). 



"comb-type" under the influence of longitudinal and transverse 

 gross enlargement, and multiplication of myofibrillar i. e., irregular 

 zig-zag form; (c) mechanical ruptures in normal hearts are not 

 localized along the discs, while in pathological fragmentation and 

 segmentation the lines of fracture are largely limited to these 

 levels. Of further significance is the fact that the presence of step- 

 forms coincides with the condition of more profuse branching, 

 hence absent in forms below birds. Step-forms would seem to be 

 due to tension at various angles to the main longitudinal axis of 

 the fibers. 



The predominant type of disc in atrophied heart muscle is the 

 comb-type. Fractures again occur almost exclusively at these 

 levels. The discs in pathological heart muscle appear to be regions 

 of weakness, probably in part at least due to chemical change. 

 The main steps in the formation of the discs are conceived as 

 follows: (1) irreversible contraction band (apparently homo- 

 geneous); (2) comb-discs, the length of the "teeth" varying 

 according to the total amount, or degree, of traction on the 

 modified regions of the included fibrils; and (3) zig-zag discs, due 

 to a combination of the factors of longitudinal and transverse 

 tension, and increase in number of fibrils by longitudinal splitting, 

 e. g., in hypertrophied cardiac fibers. Under the condition of 

 general compaction which prevails in atrophied cardiac fibers, 

 we should expect to find exactly the type of disc actually present, 

 namely, a comb-type. Comb-discs according to the above inter- 

 pretation of their origin are obviously regions of relative weakness, 

 of relatively greater degree in pathological hearts where there is 

 operating very probably an additional chemical factor. Hence 

 in atrophied hearts fragmentation when present is limited to these 

 levels. The discs once formed are permanent structures under- 

 going various modifications according to physiological conditions. 

 Direct observation reveals a close similarity between the contrac- 

 tion band and the simplest type of disc. Hence the evidence for 

 the origin of the various types of discs from irreversible contraction 

 bands is practically complete. 



