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



at coordination of functionally incooordinated myofibrils of 

 fusing trabeculae (/cells') which involve unusual strains, but 

 they do not themselves effect the coordination; they are effects 

 of functional coordination not primarily causes of such coordi- 

 nation, as urged by Dietrich (3). 



V. SUMMARY 



1. Intercalated discs are described in sections from the atria, 

 ventricles, moderator band, and Purkinje fibers of the adult 

 heart of the beef. No striking numerical or structural differ- 

 ences obtain between thd discs of the right and left ventricle, 

 nor between those of the ventricles and atria. The types of 

 discs include the simple band-forms, more or less complex terraced 

 forms, and serrated forms. These occur in frequency in the order 

 named, the serrated type being relatively sparse. Discs are 

 somewhat more abundant in the papillary muscles than in the 

 ventricular wall, and are more predominantly of the band-form. 

 A similar statement applies also to the moderator band. Con- 

 sidered in toto many of the 'band-forms ' of disc are more or 

 less complete rings or spirals. In the Purkinje fibers the discs 

 are relatively less abundant than in the ventricular myocardium 

 proper, and they are predominantly of the band-form, with occa- 

 sional short step-forms. The several technics employed in- 

 clude maceration, treatment with silver nitrate solutions, and 

 fixation by the Zimmermann nitric-acid-alcohol mixture \vith 

 hemalum and iron-hematoxylin staining respectively. The 

 stained tissues were studied in sections, and in teased condition 

 mounted in glycerin. The investigation included further the 

 study of hearts of fetuses of the second, fourth and seventh 

 months, and of young calves' hearts. 



2. Discs are present already towards the end of the second 

 fetal month (ventricle) as delicate peripheral bands, apparently 

 as local thickenings of the telophragmata. Subsequently to the 

 second fetal month the discs become progressively more abun- 

 dant and more robust, and after birth they become altered into 

 more complex terraced and irregular forms. 



