56 THE MUSCULAK TISSUES 



CARDIAC MUSCLE. The muscular tissue of the heart is inter- 

 mediate in its phylogenic position between smooth and striated 

 muscle. Its cells consist of a granular cytoplasm which presents 

 distinct longitudinal fibrillations, but only indistinct transverse 

 striation. The latter appearance indicates an incomplete differ- 

 entiation of its ultimate fibrillae into alternate disks of light and 

 dark (isotropic and anisotropic) substance, a differentiation which 

 is much more highly developed in the striated variety of muscle. 



The cardiac muscle cell is short and broad as compared with 

 that of smooth muscle, and is joined to its neighbors at either end. 



FIG. 58. CARDIAC MUSCLE CELLS FROM THE PIG'S HEART, ISOLATED IN EQUAL PARTS OF 



ALCOHOL, GLYCERIN, AND WATER. 

 Unstained. (The nuclei are somewhat darker than they actually appear.) x 410. 



This firm union by abutment produces long bands, cardiac muscle 

 fibres, which are united into bundles, and as such may be fol- 

 lowed, by careful dissection, for long distances through the cardiac 

 wall, their course maintaining a peculiar figure-of-eight direction. 

 The union of cells end to end to form these fibres is accom- 

 plished by means of a cement substance, which is occasionally 

 bridged across by fine protoplasmic fibrils. A cell membrane is 

 wanting. 



The cardiac muscle cell frequently branches, and its processes 

 anastomose with those of adjacent cells to form a coarse network 

 of muscle fibres, which insures harmony in physiological contrac- 



