614 



or step-like . . . Occasionally they extend wave-like through several 

 adjacent or parallel fibres'' (p. 567). This description, together with 

 his drawings and microphotographs, shows clearly that he was speaking 

 of the structure by Jordan the "intercalated disc''. These "cement- 

 lines'' described by practically all writers upon "segmentation'' and 

 "fragmentation'' correspond exactly with the intercalated discs de- 

 scribed by Heidenhain, Margeau, Palczewska, Werner, Zimmerman and 

 others, referred to in Jordan and Steele's article. 



The interpretation of the intercalated discs as intercellular struc- 

 tures is regarded as untenable by Jordan and Steele. The specific 

 points in the evidence against such an interpretation are thus sum- 

 marized by them: (a) the superficial location of the discs; (b) their 

 relationship to the dark bauds — i. e. they displace these bands ("con- 

 traction bands'' of Rollet) and shade laterally iuto them; (c) their 

 position frequently over a nucleus; (d) their relation to the myofibril- 

 lae ; (e) their random arrangement with respect to the nuclei; (f) 

 structurally they seem to be of the same nature as the so-called 

 anisotrophic substance: (g) their absence before the appearance of 

 cross striations; and (h) their location roughly in the axes of the 

 heart muscle mesh. (Figs. 1 to 7 show the appearance of the discs, 

 their location in the fibres, and the various forms they assume.) 



While the nature and origin of the intercalated discs has not 

 been fully worked out in detail, Jordan and Steele conclude, after a 

 review of the evidence, that "the presence of discs would seem to 

 be correlated with the function of rhythmic contraction characteristic 

 of cardiac muscle, and (a) may represent a fixed phase of a contraction 

 wave (local or general), or (b) more probably is the result (of the 

 nature of an irreversible strain condition) of the total amount of func- 

 tion". Elsewhere, they speak of the irreversible strain condition as 

 an irreversible contraction. The intercalated discs may, therefore, in 

 the normal hearts of man and animals, be caused by the temporary 

 inability of small areas in the muscle (probably areas under unusual 

 temporary stress) to relax. This is borne out somewhat by the ob- 

 servation of Jordan and Steele that the number of discs found in 

 normal hearts is in general greater in hearts having very rapid beats ; 

 and by the variable number of discs in different hearts of the same 

 species, and in different regions of the musculature of the same heart. 



These facts indicate that the intercalated discs can not be inter- 

 preted as "cement-lines"" or "tendon-like structures", which would 



