INTERCALATED DISCS OF THE HEART OF BEEF 297 



appear practically identical; if contraction bands are conceived 

 to be rendered incapable of reversion to the relaxed condition, 

 and as such to have become permanent structures modified 

 under unequal functional tensions incidental to the branched 

 and syncytial condition of heart musculature and the spiral 

 t\\dsting of certain bundles of fibers during development, and the 

 lateral fusion of such adjacent and mutually twisted fibers dur- 

 ing growth, the derivation of the various definitive types of 

 intercalated discs becomes clear. 



With the above general features of the origin, structure and 

 relation of the discs in mind we may now more profitably proceed 

 to the further description of the various types of discs. But 

 before doing so a critical estimate should be made of the value 

 of sectioned material for the study of the character of the discs. 

 Sections of 10 microns' thickness include many complete fibers, 

 the diameter of the fibers being on the average from 10 to 15 

 microns. Accordingly little likelihood remains of misinterpreta- 

 tion on account of a peculiarity of the plane of section, or by 

 reason of partial views. In order to reduce the theoretical dis- 

 advantages of sections to a minimum, comparative studies were 

 made with teased material. In teased hemalum-stained frag- 

 ments of the ventricle, mounted in glycerin, the discs appeared 

 exactly as in the sections. Occasional discs may be seen in 

 which a bisecting telophragma is conspicuous. These band- 

 forms of discs are peripheral in position; a certain number have 

 the form of short spirals; some are located superjacent to nuclei. 

 By raising or lowering the level of focus an apparently short 

 disc can occasionally be followed as a complete band (crescent) 

 across the fiber, and in some cases even as a more or less com- 

 plete ring or spiral to the opposite surface. Step-forms also are 

 abundant in the teased material ; they are therefore not generally 

 only the optical expression in sections of transverse cuts of a 

 series of band discs, as might have been suspected; but they 

 must either be due to dislocation of original band discs or they 

 are originally formed as terraces. This material shows clearly 

 also the frequent phenomenon of a division on the part of a disc 

 of a contracted from a relaxed area.- The teased tissue is quite 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 2 



