CARDIAC MUSCLE. 



muscle cell contracts it broadens, and shortens, even to one tenth of its 

 length when at rest (Prenant). The ground membranes approach one 

 another (Fig. 98). It has been said that by a transfer of light substance 

 to the dark the staining reactions are reversed, but this has been denied. 

 The retreat of the protoplasm into capillary spaces between the dark 

 fibrils has been described. The process is known to be most complex, 

 involving physical (electrical) and chemical changes which are but im- 

 perfectly expressed in the histological pictures. With prolonged activity 

 B the muscle nuclei are said to shrink 



and to stain less deeply. 



In ordinary specimens of cardiac 

 muscles the student will observe only 

 the alternating light and dark bands, 

 with possibly the ground membrane 

 Z. On changing the focus the dark 

 bands may appear light and vice 

 versa, but in the proper focus for ad- 

 jacent nuclei and connective tissue, 





FIG. 98. FIBRILS FROM THE WING MUSCLES 

 > r OF A WASP. (Schafer.) 



A, Contracted; B, stretched; C, uncontracted. 

 The dark bands are bisected by the light 

 stripes (H), but they do not show the 

 median membranes (M). 



FIG. 99. INTERCALATED Disc (d) FROM HUMAN 

 CARDIAC MUSCLE, STAINED WITH THIASIN 

 RED AND TOLUIDIN BLUE. THE GR9UND 

 MEMBRANES ARE LETTERED z. (Heiden- 

 hain.) 



the bands appear as has been described. At irregular intervals, in cardiac 

 muscle only, transverse lines of another sort may be found, called inter- 

 calated discs and formerly known as cement lines. 



Intercalated discs are seen in Fig. 94, and as pictured by Prof. Heid- 

 enhain, in Fig. 99. He describes them as deeply staining plates almost 

 invariably not as wide as a muscle segment. The segment in the human 

 heart is 2 //, whereas the intercalated discs Vary from i to 1.7 //. A disc 

 may extend straight across a fiber, or it may be interrupted so as to form 

 a succession of steps, usually from two to four. The discs are always 



