90 



NORMAL HISTOLOGY. 



of the nucleus. The rest of the cell-body is composed of contractile 

 substance, a modification of the cytoplasm of which the cell was 

 first composed, which presents a fine longitudinal and a somewhat 

 coarser transverse striation. The proper intercellular substance is 

 a homogeneous cement, which lies between the ends of the cells. 

 These are arranged end to end so as to form fibres, the lines of 



FIG. 75. 



Section of human heart. The direction of the section is such that the muscular cells are cut 

 exactly perpendicular to their long axes, a, intermuscular areolar tissue. From this, more 

 delicate fibrous tissue penetrates between the muscle-fibres forming the muscular bundles, 

 which are imperfectly separated from each other by the broader septa of fibrous tissue. 

 b, muscle-cell cut beyond the nucleus ; c, cell cut so as to include the nucleus ; d, cell cut 

 just below a branch. The index line d points to that part of the cell which passes into 

 the branch. The granular character of the contractile substance when seen in cross- 

 section has been omitted from the figure. At the lower edge of the figure the section 

 has been torn, but a small amount of the subpericardial areolar tissue is represented. 



junction between the cells, which are occupied by the cement-sub- 

 stance, being usually invisible. The cells give off branches which 

 unite with each other in such a way as to convert the heart-muscle 

 into a reticulum of muscular fibres. The meshes of this reticulum 

 are occupied by areolar tissue, in which the vascular and nervous 

 supply of the tissue is situated. Where this tissue is abundant it 

 may also contain a few fat-cells. The cardiac muscle-cells are 

 destitute of a cell-membrane, in which respect they differ from the 

 voluntary striated muscle-fibres. 



