STRIATED MUSCLE. 87 



myoblasts will not be included. In becoming muscle fibers the myoblasts 

 increase to a diameter of from 10 to 100 p.. The fibrils multiply by longi- 

 tudinal splitting so as to form groups of fibrils, or muscle columns, which 

 in cross section are called Cohnheim's areas. Fig. 103 shows four adult 

 muscle fibers cut across, in all of which Cohnheim's areas are distinct. 

 Often such areas are not distinguishable, however, and when present they 

 may appear as though due to shrinkage. Between the areas is the sarco- 

 plasm which may show " interstitial granules" of fat or lecithin. The 

 nuclei of striated muscle fibers, not seen in the figure, are usually flattened 

 and close to the sarcolemma. The fibers just described belong to the 

 pale or white type. In the dark or red form the protoplasm is more 

 abundant and granular, the diameter is less, the fibrils fewer, and the 

 nuclei may be central or imbedded among the fibrils. Clearly this type 



pillaries. 



FIG. 102. CROSS SECTION OF MYOBLASTS 

 AND MESENCHYMAL CELLS FROM AN 

 18 MM. PIG. 



mes., Mesenchymal cell ; f., fibril; n., nu- 

 cleus; s., sarcolemma, of a myoblast. 



Connective tissue. 



FIG. 103. CROSS SECTION OF FOUR MUSCLE FIBERS 

 OF THE HUMAN VOCAL MUSCLE. X 590. 



is intermediate between the myoblast and the pale form. The dark fibers 

 contract more slowly than the light ones, but are less easily fatigued. 

 They are found in the ocular muscles and in those of mastication and of 

 respiration. In some single muscles both types with intermediate forms 

 may be observed. Ordinarily striated muscle is of the pale type. 



The mesenchyma surrounding the myoblasts becomes connective 

 tissue. It envelops each fiber as shown in Fig. 103, and in progressively 

 wider bands it surrounds small bundles of fibers, large groups of these 

 bundles, and the entire muscle as shown in Fig. 104. The connective 

 tissue layer which covers the whole muscle is the external perimysium; 

 its prolongations into the muscle form the internal perimysium. It 

 contains fine longitudinal elastic elements and sometimes fat, chiefly in 

 the outer layer. Elastic substance is particularly abundant in the dia- 



