TISSUES OF SPECIAL FUNCTION. 



95 



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

 first composed, which presents a tine longitudinal and a somewhat 

 coarser transverse striation due to the arrangement of the " sarco- 

 plasmic disks " of which the contractile substance or sarcoplasm is 

 composed. The proper intercellular substance was formerly regarded 

 as a homogeneous cement, which lay between the ends of the cells, 

 but there appear to be delicate protoplasmic bridges connecting con- 

 tiguous cells (Figs. 78 and 79). The cells are arranged end to 



Fig. 79. 



Fig. 80. 



Longitudinal section of adult human heart 

 muscle, showing the junction of two 



cells. (MacCallum.) 



Longitudinal section of heart muscle Irom 

 an adult dog, showing protoplasmic 

 bridges between two cells. (MacCallum.) 



end so as to form fibres, the lines of junction between them 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 fibres of 

 voluntary striated muscle. 



When seen in longitudinal section it is difficult to trace a given 

 muscle-fibre for any considerable distance, because the occasional 

 anastomosing branches of the cells cause a blending of the neigh- 

 boring fibres with each other. In cross-section the cells have a 



