CARDIAC MUSCLE 



129 



Nucleus. Sarcoplasm. Fibrils. Lateral branch. 







plasm, in both skeletal and cardiac muscle, is separated by the sarcolemma from the 

 myofibrils and sarcoplasm (Fig. 112, A). In regard to smooth muscle, however, 

 Baldwin merely notes that it should be reviewed in the light of these facts. The 

 existence of a membrane around the nu- 

 cleus and granular protoplasm at its peles 

 would place smooth muscle in the same 

 category, and make the fibrils extracellular. 

 With muscle, therefore, as with connec- 

 tive tissue, the distinction between intra- 

 cellular and extracellular appears to be 

 arbitrary and conventional. It is interest- 

 ing to note that the extrusion of the nuclei 

 from the precartilage matrix to its surface, 

 as described by Mall, may be comparable 

 with the passage of the nuclei from the cen- 

 ter to the surface of skeletal muscle fibers. 

 Baldwin's papers are found in the Zeitschr. 

 f. allg. Physiol., 1912, vol. 14, pp. 130-160, 

 and, as regards cardiac muscle, in the 

 Anat. Anz., 1912, vol. 42, pp. 177-181. 



A feature of cardiac muscle which 

 is unlike anything observed in smooth 

 or skeletal fibers is the presence of in- 

 tercalated discs. These are transverse 

 lines across the fibers, which were 

 formerly interpreted as cell bounda- 

 ries, and some authorities still regard 

 them as such. In the guinea-pig , 



Jordan and Steele find that they first appear during the week before birth. 

 Thus they are late in development, and they are relatively less abundant 



and simpler in the young than in adults (Amer. 

 Journ. Anat, 1912, vol. 13, pp. 151-17.3). If 

 the cardiac syncytium ultimately became re- 

 solved into cells, it would resemble certain other 

 syncytia in this respect; and cardiac muscle can 

 be broken up into cell-like blocks, apparently 

 along these discs. However, the discs occur at 

 variable distances from one another, and very 

 frequently they mark off non-nucleated por- 

 tions of the syncytium. As many as four of 

 them may extend partly across a single nucleus, 

 as shown by Jordan and Steele, indicating that 

 they are peripheral modifications of the 

 myofibrils, and cannot be regarded as cell walls. Heidenhain (Anat. 

 Anz., 1901, vol. 20, pp. 33-78) pictures them as always connected on one 



9 



Conn, tissue. Capillaries. 



FIG. 121. LONGITUDINAL SECTION OF A PAPIL- 

 LARY MUSCLB FROM THE HUMAN HEART. X 240 

 The transverse lines (x) are partly light (where the 

 fiber has broken) and partly dark (intercalated 

 iscds.) 



FIG. 122. INTERCALATED Disc (d) 

 FROM HUMAN CARDIAC MUSCLE, 

 STAINED WITH THIAZIN RED AND 

 TOLUIDIN BLUE. Z, Krause's 

 membrane. (Heidenhain.) 



