146 



THE TISSUES. 



cement lines may be especially stained in certain anilin stains. In 

 such preparations it may be seen that they often do not extend 

 through an entire fiber, are frequently irregular, often presenting 

 the appearance of steps, and now and then involve only one or two 

 fibril 'bundles. They are frequently seen to bound portions of a 

 muscle-fiber which are nonnucleated. They are looked upon by 

 M. Heidenhain as representing growth areas. See Fig. 106, in 

 which such intercalated growth areas (cement lines ?) are represented 

 darker than the remaining structures. 



Heart muscle-fibers are surrounded by delicate connective-tissue 

 sheaths, very much as described for nonstriated muscle tissue. 

 These are well shown in tissue fixed in corrosive sublimate and 

 stained after Mallory's differential connective-tissue stain. The 



f& l^- 



s 



b 





Fig. 106. Longitudinal section of heart-muscle of a grown individual, fixed in cor- 

 rosive sublimate and stained in hematein : a, Intercalated disc (so-called cement line); 

 b, nucleus of heart muscle-fiber ; c, red blood-corpuscles ; d, nucleus of blood capillary. 



fibers are grouped into bundles or fasciculi which are surrounded 

 by internal perimysium. 



Development of Heart Muscle-tissue. Heart muscle-tissue de- 

 velops from the mesenchyme, and shows from the beginning a 

 syncytial structure, in that the cells are united by protoplasmic 

 branches (von Ebner, M. Heidenhain, Godlewsky). As development 

 proceeds, the interspaces between the cells become smaller and the 

 protoplasmic bridges larger and more prominent, forming a distinct 

 syncytium, through which the nuclei are scattered. In this syncytial 

 protoplasm are developed the contractile fibrils, which may be traced 

 uninterruptedly for long distances. These fibrils show at first a 

 uniform structure, and later differentiate into isotropic and aniso- 

 tropic discs, Q and j discs appearing first as in voluntary striated 



