GENERAL CONSIDERATIONS 



91 



tial difference seems to be one of degree 

 differentiation. In general, skeletal mus- 

 cle is most highly differentiated, cardiac 

 muscle being intermediate between 

 smooth and the voluntary striped type. 



For a proper understanding of the 

 structure of these three types it is neces- 

 sary that we now consider the process of 

 muscle histogenesis. The student should 

 gather the several criteria by which he 

 may distinguish between smooth, cardiac, 

 and skeletal muscle, both in transverse 

 and longitudinal sections. 



HISTOGENESIS AND STRUCTURE 



Smooth Muscle. As stated above, the 

 germ layer involved in muscle histogene- 

 sis is the mesoderm. Smooth muscle is 

 derived chiefly (exception: arrectores pil- 

 orum) from the visceral or splanchnic 

 layer. This is at first an epithelial struc- 

 ture of a single layer of cells, the prim- 

 itive mesothelium. The cells subsequent- 

 ly proliferate and change their shape in 

 general to a fusiform type. Intercellular 

 connections (cytodesmata) are either 

 maintained or established, and the tissue 

 is permanently more or less in a syncytial 

 condition. These so-called intercellular 

 bridges are particularly pronounced and 

 can be readily demonstrated in the tunica 

 media of the blood-vessels of the umbilical 

 cord. It must be emphasized, however, 

 that the outlines of the genetic units in 

 smooth muscle are always distinct, where- 

 as in striped, especially the cardiac type, 

 the outlines of the original myoblasts are 

 lost. 



FIG. 97. SMOOTH MUSCLE 

 CELLS. 



A, an isolated cell from the 

 cat's intestine. The nucleus is 

 surrounded by coarsely granular 

 sarcoplasm, continuous periph- 

 erally with the finely granular 

 interfibrillar sarcoplasm. The 

 innermost myofibrils may er- 

 roneously suggest a cell mem- 

 brane. The fusiform element is 

 invested by a true cell mem- 

 brane, or sarcolemma. X 750. 

 B, oblique transverse section of 

 a cell from the muscularis mu- 

 cosse of the cat's esophagus. 

 The perinuclear sarcoplasm has* 

 contracted away from the nu- 

 cleus leaving a clear space limited 

 by a sharp line, external to which 

 lies the perinuclear granular sar- 

 coplasm. Hematoxylin and 

 eosin. X 750. 



