62 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



less refractive more fluid material. The polarizing microscope reveals the 

 fact that the middle piece which corresponds in position to the dark trans- 

 verse band is doubly refractive, isotropic, while the surrounding material, 

 the light band, is singly refractive, anisotropic. 



In transverse section, figure 70, the area of the muscle substance is 

 mapped out into small polygonal areas by a network of clear lines called Cohn- 

 heim's areas. The lines represent the substance between the sarcostyles. 

 This substance probably represents the less differentiated contractile sub- 

 stance, called sarcoplasm. In figure 81 

 the interfibrillar sarcoplasm is indicated 

 by the longitudinal and transverse lines. 

 Heart Muscle. The muscle sub- 

 stance of the heart is composed of 



FIG. 77. FIG. 78. 



FIG. 77. A Section of Cardiac Muscle, Diagrammatic. (From E. A. Schafer, after 

 Heidenhain.) 



FIG. 78. Intercellular Continuity of Muscle Fibrils in Cardiac Muscle. (From E. A. 

 Schafer after Przewosky.) 



mononucleated masses of protoplasm, cardiac muscle cells, in which the 

 substance of the cell presents the transversely striated appearance char- 

 acteristic of the voluntary muscle just described. But the heart muscle is 

 physiologically much more like an involuntary muscle. The cells are rather 

 small, two to four times as long as thick, and the nucleus is usually situated 

 near the middle of the cell, figure 79. There is no sarcolemma; on the other 

 hand, the cells present branched and irregular outlines, but adjacent cells 

 interlock in close-fitting contact. 



Certain observers have described fibrils as extending across the so-called 

 cell boundary and noted that not all such boundaries enclose nuclei. These 



