82 



THE MUSCLE TISSUES 



were the mesenchyme cells from which they were derived. Long 

 threads, the myofibrillee, appear enmeshed in the sarcoplasm. The 

 coarse fibrils that appear in the embryonic stage ap])arently undergo 

 longitudinal splitting and give rise to more numerous and much finer 

 fibrils of the fully differentiated cells. Also the lateral connections 

 are no longer apparent and the cells appear as independent units 

 structurally, though not functioning as such. The fully developed 

 cells are fusiform and have an elongated oval nucleus occupying a 

 central position. (Fig. 43.) The size of the cells varies in different 

 species and in different regions in the same species. 



Fig. 43. — Diagram of isolated smooth muscle cells. (Churcliill.) 



In the mature organization the cells are connected with each other 

 by a cement substance possibly derived from the former connecting 

 protoplasmic strands. Although none of this cementing material 

 is evident in ordinary routine preparations a reticulum of fine fibers 

 may be demonstrated by silver techniques. Increase in cells may be 

 effected through development of embryonic cells left among the 

 mature cells or b>' flivision of the mature cells which apparently 

 retain their ability to divide by mitosis. In newly formed tissue 

 the fibrils appear to continue from cell to cell, l>ut this cannot be 

 seen in preparations of older tissues. In general the myofibrilhe 

 of this type of muscle are difficult to distinguish. The sui)erficial 

 myofibrillae appear coarser than those in the interior of the cell. 

 The non-fibrillar sarcoplasm is best seen as a lighter area at either 

 end of the nucleus where the myofibrils diverge in ])assing. By 

 soaking pieces of tissue composed of smooth muscle in weak acid or 



