168 H. E. JORDAN AND K. B. STEELE 



members (e.g., dog, man, sheep). But at other points there is 

 absolutely no correspondence (e.g., sheep, cat, rabbit). The 

 relationship, therefore, if it exists at all, cannot be a simple one. 

 Factors, besides rate of beat, must affect the relative abundance 

 of the discs. Such factors may be the force of the beat, or the 

 instant or total amount of work done. That the rate, simply, does 

 not determine the number of the discs appears furthermore from 

 the fact that the discs are more abundant in the adult than in 

 the young (e.g., guinea-pig and man), whereas the rate of heart- 

 beat relative to age varies in the reverse ratio. 



Moreover, the number of the discs varies in different portions 

 of the same heart and in different individuals of the same species. 

 This may mean, however, that they vary according to the phase 

 or state of function, or perhaps according to the total amount of 

 function (i.e., age of the individual). The observations regarding 

 the relative abundance of the discs above stated may thus have 

 no absolute (final) significance. Sufficient observations under 

 uniform conditions have not yet been made for an accurate seri- 

 ation of heart muscles from the standpoint of the abundance of 

 discs. The physiologic significance (normal and abnormal) of 

 these structures will appear in full only after a careful compara- 

 tive study of the same individual under varying internal and 

 external states, both normal and morbid; of animals of the same 

 species at different ages; and of animals from the various groups 

 under relatively uniform conditions of age, health and function. 

 When all the factors helping to determine the presence and abun- 

 dance of intercalated discs are thus known and accounted for, it 

 may become possible to arrange animals in identical series from 

 the standpoint both of the rate of the heart-beat, and from the 

 number of the intercalated discs. It seems clear that a relation- 

 ship of some degree exists between the presence and abundance of 

 these discs and function (rhythmic contraction), but in detail the 

 relationship remains obscure. 



In a histologic study of the lung of the white mouse one of us 

 (Jordan) recently discovered that the tunica media of the proxi- 

 mal end of the pulmonary arteries consists of striped (cardiac) 

 muscle for a considerable distance. This seemed to offer, there- 



