CARDIAC MUSCLE. 



81 



adult, cardiac muscle being a network of broad protoplasmic bands, in 

 and near the centers of which nuclei are situated at irregular intervals 

 Lateral union. ( Fi g- 94)- The intercellular spaces 



are reduced to clefts occupied by a 

 small amount of connective tissue, 

 which is either a part of the original 



Nucleus of Nucleus of Intercalated 

 a muscle a connective disc. 



fiber. tissue cell. 



FIG. 94. FROM A LONGITUDINAL SECTION OF 

 A PAPILLARY MUSCLE OP THE HUMAN 

 HEART. X 360. 



FIG. 95. PART OF THE MUSCULAR SYNCYTIUM 

 FROM THE HEART OF A DUCK EMBRYO' OF 

 3 DAYS. (M. Heidenhain, from McMurrich's 

 " Embryology.") 



mesenchyma or a later ingrowth accompanying the blood vessels. 



The protoplasm of cardiac muscle contains longitudinal fibrils, 

 in development they are few in number and situated 

 near the periphery of the bands of protoplasm. They 

 extend for considerable distances through the syncytium 

 regardless of cell areas (Fig. 95). Their origin is a 

 subject for speculation. It has been suggested (i) that 

 they are bundles of ultra-microscopic molecular fibrils; 

 (2) that they develop by the coalescence of granules 

 in the hyaloplasm between the reticular network of 

 protoplasm; and (3) that they are parts of this network, 

 supposed to be retractile, which is irregularly arranged 

 in ordinary cells but which in muscle cells has acquired 

 rectilinear meshes. At first homogeneous, they soon 

 become marked by alternating light and dark bands. 

 They increase in number by longitudinal splitting. 

 6 



Early 



FIG. 96. MUSCLE 

 FIBER OF A FROG 

 X 240. 



f ., Fibrillae ; k., nucleus. 



The protoplasm 



