Intercalated Discs of Atrophied Heart Muscle. 43 



parative anatomical and pathological (experimental — including the 

 phenomena of fragmentation and segmentation) data points to 

 the same conclusion, namely, that the so-called "discs" are 

 aggregations of irreversible contraction foci on the myofibrillar 

 in the form of bands variously modified by a variety of normal 

 mechanical and pathological, both chemical and mechanical, 

 factors. 



The first step, and central fact, in the chain of observations 

 leading to the above conclusion respecting the origin and nature 

 of the discs is the close similarity, amounting practically to an 

 identity, between the normal contraction band in contracted 

 muscle fibers and the simplest type of disc. This correspondence 

 of appearance is very striking in the humming-bird's heart. In 

 the relaxed fiber the Z-line is very conspicuous but delicate. 1 

 The Q-disc is wide and pale. In the contracted fibers the appear- 

 ance is one of an alternation of robust dark (Q) and light (/) bands 

 of approximately equal thickness. These are the same conditions 

 that obtain in striped muscle generally in relaxed and contracted 

 states respectively. In contraction the Z-line appears to become 

 thickened by accumulation about it of dark (<2) substance. Of 

 course the changes take place in the myofibrillar. We need simply 

 postulate local inability (due to strain, whether an instant or 

 cumulative effect) on the part of a contraction band to reverse 

 (or relax) to pass to the first step in the formation of a disc. The 

 similarity between such "bands" and "discs," as also their periph- 

 eral and frequently supernuclear position, renders the assump- 

 tion almost a certainty. Further support to this interpretation 

 is derived from the following ontogenetic facts: (a) the discs 

 begin to make their appearance coincidently with the cross stria- 

 tions; (b) they are at first exclusively of the simple homogeneous 

 band type; (c) they increase in number (and complexity) at least 

 to the time of full growth of the heart; (d) they are permanent 

 structures throughout life. Further confirmative data are these: 

 (a) step-forms are rare in birds, and absent in lower forms, the 

 prevailing disc being of a type corresponding to the earlier onto- 

 genetic forms in mammals; (b) in hypertrophied hearts the discs 

 are exactly of a form expected from a modification of a simple 



1 Prepared according to Zimmermann's technique. 



