STRUCTURE OF MUSCLE 



67 



Voluntary muscular fibres are developed from embryonic cells of 

 the mesoderm, which become elongated, and the nuclei of which become 

 multiplied, so as to produce long multi-nucleated fusiform or cylindrical 

 fibres. These become cross-striated at first along one side, the change 

 gradually extending around the fibre and also towards the centre ; 

 but the middle of the fibre, to which the nuclei are at first confined, 

 remains for some time unaltered (fig. 78). Eventually the change in 

 structure extends to this also, and the nuclei pass gradually to occupy 

 their ordinary position under the sarcolemma, which by this time has 

 become formed. 



FIG. 78. DEVELOPING 

 MUSCULAR FIBKE, 



FROM FtETUS OF 2 

 MONTHS. 



p, central protoplasm with 

 several nuclei, n, scat- 

 tered in it ; *, commen- 

 cing sarcolemma, with 

 striated muscular sub- 

 stance developing im- 

 mediately beneath it. 



FIG. 79. MUSCULAR FIBRE-CELLS FROM THE MUSCT- 

 LAU COAT OF THE SMALL INTESTINE, HIGHLY MAG- 



KIFIED. 



A. A complete cell, showing the nucleus with mtra-nuc'ear 

 network, and the longitudinal fibrillation of the cell-sub- 

 stance, with finely vacuolated protoplasm between the fibrils. 

 B. A cell broken in the process of isolation ; the delicate 

 enveloping membrane projects at the broken end a little 

 beyond the substance of the cell. 



Involuntary or plain muscular tissue is composed of long, some- 

 what flattened, fusiform cells (fig. 79), which vary much in length, but 

 are usually not more than -^-^ inch long. Each cell has an oval or 



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