100 



MUSCULAR TISSUE 



FIG. 109. CARDIAC MUSCLE 



FIBERS. 



A, portion of a fiber from a 

 macerated preparation of a 

 cat's heart, drawn according 

 to its appearance in the opti- 

 cal longitudinal plane. Two 

 nuclei are shown, connected 

 by a continuous axial strand 

 of coarsely granular sarco- 

 plasm. The sarcolemma ap- 

 pears festooned between suc- 

 cessive ground membranes. 

 There is no evidence of a cell 

 membrane separating the cen- 

 tral granular from the periph- 

 eral non- or finely-granular 

 sarcoplasm. X 1000. B, 

 median longitudinal section 

 of a fiber from the ventricle 

 of an adult white mouse. 

 Note the continuity of the 

 axial sarcoplasm. Hematox- 

 ylin and eosin. X 1000. 



corresponding to portions of a single fibril. 

 These units may be granular or compact. 

 The units are bisected or bounded on one side 

 by the Z membrane. Association of the units 

 in transverse lines gives rise to the band 

 forms; they may be drawn into spirals by 

 longitudinal traction of the fibers involved; 

 unequal- transverse and oblique tractions 

 probably produce the step forms; the saw- 

 tooth form arises by process of longitudinal 

 splitting of fibrils, enlargement of fibers, and 

 the various tensions characteristic of hyper- 

 trophying fibers. The exclusive type of 

 hypertrophied heart muscle is the more or 

 less complex saw-tooth type. The practically 

 exclusive type of atrophied heart muscle is 

 the 'comb type/ a type produced from a band 

 type by a modification involving longitudinal 

 tension (Fig. 114, d). In brief, the unit of 

 structure is a modified focus on a myofibril, 

 in essence involving an accumulation of gran- 

 ules about the Z membrane. Such foci asso- 

 ciated in various ways produce the various 

 types of bands and steps, the latter in part 

 due also to external mechanical factors, the 

 extreme condition of such effect being saw- 

 tooth forms. 



A significant point concerns the similarity 

 between the phylogenetic and ontogenetic de- 

 velopment of intercalated disks : that is, below 

 birds, as in all fetal hearts, only simple bands 

 appear; in birds as in young hearts, step forms 

 are present; only in mammals and in old 

 hearts do the more complex types appear. 

 What then is the meaning of these disks ? Any 

 interpretation must be more or less tentative 

 at present. It is easier to say what they prob- 

 ably are not, than what they probably are. 

 They were originally interpreted as cell boun- 

 daries, or intercellular cement lines (Schweig- 



