INTERCALATED DISCS OF HEART MUSCLE 157 



The ' teeth' of the 'comb/ we believe, must be interpreted as 

 local, elongate contractions of the myofibrillae. Occasionally 

 such discs consist of only one or several 'teeth;' again three or 

 four 'combs' may appear in longitudinal series, giving the appear- 

 ance of a contracted striped insect muscle. Frequently successive 

 discs limit a region of darker sarcoplasm; frequently also discs 

 lie in regions of darker sarcoplasm. The significance of these 

 observations is that they indicate a relationship between discs 

 and a condition of contraction. Possibly the discs outline lim- 

 its of special physiological states. The loose granular discs are 

 perhaps simply 'comb'-discs with shorter 'teeth.' 



2. Bat 



In the heart of the bat, in addition to the types of discs de- 

 scribed for monkey, there appear discs in the form of a line of 

 dark-staining, connected, spherical granules extending across 

 as many as eight separate fibers; and directly over underlying 

 nuclei (fig. 7). Such a structure does not even vaguely suggest 

 a cell membrane or cement line. It becomes intelligible only 

 in terms of local modifications (contractions?) at approximately 

 the same level in many adjacent fibers. It would seem to rep- 

 resent the morphologic picture of a physiologic state sharply 

 localized longitudinally, widespread laterally. In fig. 8 are shown 

 two series of narrow discs connected by more or less delicate 

 granular inter-disc membranes ('risers'). These inter-connec- 

 tions, and the localization of the discs in the axial region of the 

 muscular mesh, here again suggest the effect, or phase, of a defi- 

 nite physiologic state. The inter-disc membrane may represent 

 less extreme local contractions in the intervening fibrillae, or a 

 dislocated modified membrane of Heidenhain, or possibly simply 

 ' anisotropic' granules obliquely aligned. Careful focussing reveals 

 the fact that these complex step-like discs lie superficially through- 

 out and encircle the fiber in ring-fashion. This single observation 

 disposes definitely of a cell-border interpretation. 



