Musculab Tissue 



35 



the centrally located nucleus. Smooth muscles fibers are covered by con- 

 nective tissue in much the same way as are the striped muscles, except that 

 two or more cells in longitudinal contiguity may be enveloped in the same 

 sheath. 



The muscle of the vertebrate heart belongs to the type, designated as 

 cardiac. The fibrils of cardiac muscle are composed of alternated sections 

 of isotropic and anisotropic substances, but the cells have usually a single 

 centrally located nucleus, like the smooth muscle cell. According to some 

 observers, the cardiac muscle does not consist of individual cells, but is a 

 syncytium, that is, a tissue in which there is protoplasmic continuity 





 : i. 



Fig. 35. Muscle fibers of heart, showing syncytial structure. Highly magnified. 

 (SchSfer, Microscopic Anatomy, after Przewoski.) a, septum; b, fibrils bridging 

 septum ; c, nucleus ; d, short segment without nucleus. 



from cell to cell, so that the limits of the individual cells can not be exactly 

 assigned. This anastomosis (union of cells) is not confined to the lon- 

 gitudinal direction, but occurs between lateral surfaces also of contiguously 

 lying cells, so that heart muscle, according to this view is " a network of 

 broad protoplasmic bands, in and near the centers of which nuclei are 

 situated at irregular intervals" (Stohr) [Fig. 25]. According to other 

 observers, there is merely longitudinal continuity of fibrils, not of proto- 



