CH. VI.] 



DEVELOPMENT OF MUSCLE 



muscle, the likeness to the original cells from which the fibres are 

 formed is not altogether lost, and in certain situations (immediately 

 beneath the lining membrane of the ventricles) there are found peculiar 

 fibres called after their discoverer Piirkinje's fibres ; these are large clear 

 quadrangular cells with granular protoplasm contain- 

 ing several nuclei in the centre, and striated at the 

 margin. It appears that the differentiation of these 

 cells is arrested at an early stage, though they con- 

 tinue to grow in size. 



Voluntary muscular fibres are developed from cells 

 which become elongated, and the nuclei of which mul- 

 tiply. In most striated muscle fibres the nuclei ulti- 

 mately take up a position beneath the cell-wall or 

 sarcolemma which is formed on the surface. Stria- 

 tions appear first along one side, and extend round the 

 fibre (fig. 115), then they extend into the centre. 



During life new fibres appear to be formed in part 

 by a longitudinal splitting of pre-existing fibres ; this 

 is preceded by a multiplication of nuclei ; and in part 

 by the lengthening and differentia tion of embryonic FIG. 115. Develop 

 cells (sarcoplasts) found between the fully formed 

 fibres. 



In plain muscle, growth occurs in a similar way : 

 this is well illustrated in the enlargement of the uterus during 

 pregnancy; this is due in part to the growth of the pre-existing 

 fibres, and in part to the formation of new fibres from small 

 granular cells lying between them. After parturition the fibres 

 shrink to their original size, but many undergo fatty degeneration 

 and are removed by absorption. 



ing muscular fibre 

 from foetus of two 

 months. (Ran- 

 vier). 



