284 Papers from the Department of Marine Biology. 



location on either side of these membranes. This explanation answers 

 to the description of the formation of a contraction band. In that a 

 disk appears permanent, it seems appropriately described as an ''irre- 

 versible contraction band." In view of what was known of the com- 

 parative morphology of intercalated disks — coupled with our knowledge 

 of the minute structure of Limulus heart-muscle — the simple comb type 

 was to be expected in Limulus. This type of disk is actually present. 

 But disks were expected in greater number. Possibly hearts of older 

 limuli would show more abundant disks. 



The precise information furnished by the Limulus muscle concerning 

 the relation of the ground membrane to the myofibrillse is of prime 

 importance in respect to the interpretation of occasional serrated disks 

 of apparently normal cardiac muscle and the still more complex disks 

 of hypertrophied heart-muscle. This general tj^pe of disk is of very 

 irregular serrated structure. The matter which was difficult of inter- 

 pretation hitherto was the nature of the cross-connections, in the form 

 of delicate membranes, between adjacent elements of the disk proper. 

 In view of the data now available, namely, the close connection of the 

 myofibrillse to the telophragmata and the divisibility of the so-called 

 myfibril units into still finer fibrillse, the matter becomes clear. The 

 essential condition in muscular hj^pertrophy is an increase in size of the 

 fibers due fundamentally to a longitudinal splitting of the myofibrillse. 

 Keeping in mind the connection of the myofibrils with the membrane, 

 and the unequal tensions (relative or absolute) under which the adjacent 

 muscle-fibrils work in the hypertrophied heart, the series of changes 

 from the simpler comb type of disks to the complex serrated type of 

 hypertrophied muscle become intelligible (fig. 16). 



Heart-muscle, then, is clearly a syncytial structure in vertebrates and 

 Limulus, and intercalated disks are presumably invariably present 

 (the matter has not yet received attention below selachii) , representing 

 contraction bands which have become incapable of reversion, and thus 

 undergo structural and chemical changes. 



We are now in a position also to bring into harmony the discrepant 

 descriptions of the relation of the simpler intercalated disks to the 

 telophragmata. Heidenhain (12) describes the disks as invariably 

 bounded on both sides by a ground membrane. They have also been 

 variously described by other observers as bordered only on one side by 

 a telophragma; and as having no definite relationship to these mem- 

 branes. My own observations on mammaUan cardiac muscle led me 

 to conclude that the disks are generally bisected by a telophragma; 

 occasionally they appear bounded on one side by this membrane; and 

 generally in favorable instances the disks can be seen to shade laterally 

 into a Z-membrane. If an intercalated disk of the comb typ^ is cor- 

 rectly interpreted in terms of a contraction band, as I have maintained, 

 then it becomes a simple matter to explain the usual relationship of 



