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



previously distinct myocardial elements, need not be prejudiced 

 by a forced association with the hypothesis that the discs are 

 essentially irreversible contraction bands. But it may again 

 be emphasized that the discs do not generally occur in the sur- 

 faces of fusion (hence not fundamentally intercellular in char* 

 acter) but laterally to such areas of fusion. As regards the fetal 

 myocardial elements and the Purkinje cells (fibers) this is in- 

 disputable fact. The hypothesis here supported is simply in- 

 terpretative of this, in common with other, facts. Nevertheless 

 w^e believe that the hypothesis can interpret more logically and 

 consistently than any previously proposed the microscopic data 

 concerning the intercalated discs. Moreover, sight must not be 

 lost in evaluating the hypothesis, of the strong support it re- 

 ceives from conditions in the Limulus heart where the simpler 

 discs appear, though sparsely, in a considerably coarser and 

 clearer form. In the Limulus heart the discs seem to admit of 

 no possible interpretation except in terms of modified contraction 

 phenomena . 



VI. LITERATURE CITED 



(1) CuRRAN, E. J. 1909 A constant bursa in relation with the bundle of His; 



with studies of the auricular connections of the bundle. Anat. Rec, 

 vol. 3, pp. 618-640. 



(2) De Witt, Lydia M. 1909 Observations on the sinoventricular connecting 



system of the mammalian heart. Anat. Rec, vol. 3, pp. 475^98. 



(3) Dietrich, A. 1910 Die Elemente des Herzmuskels. Fischer, Jena, pp. 



1-46. 



(4) Heidenhain, M. 1911 Plasma und Zelle, Fischer, Jena. 



(5) Jordan, H. E. 1911 The structure of the heart muscle of the humming 



bird, with special reference to the intercalated discs. Anat. Rec, 

 vol. 5, pp. 517-529: 



(6) 1912 The intercalated discs of hypertrophied heart muscle. x\nat. 

 Rec, vol. 6, pp. 357-362. 



(7) 1912 The intercalated discs of atrophied heart muscle. Proc Soc 

 Exp. Biol, and Med., vol. 10, pp. 1-3. 



(8) 1914 The microscopic structure of mammalian cardiac muscle, with 

 special reference to so-called muscle cells. .\nat. Rec, vol. 8, pp. 

 423-430. 



(9) 1916 A comparative microscopic study of cardiac and skeletal muscle 

 of Lipiulus. Anat. Rec, vol. 10, pp. 210-213. (Proc. .\m. Assoc. 

 Anat. 1915). 



