96 NERVE. [CH. vn. 



are simply elongated cells in which the nucleus becomes rod- 

 shaped. In cardiac 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 Purkinje's fibres ; these are large clear quadrangular 

 cells with granular protoplasm containing several nuclei in the 

 centre, and striated at the margin. It appears that the differen- 

 tiation of these cells is arrested at an early stage, 

 though they continue to grow in size. 



Voluntary muscular fibres are developed from 

 cells which become elongated, and the nuclei of 

 which multiply. In most striated muscle fibres the 

 nuclei ultimately take up a position beneath the 

 cell-wall or sarcolemma which is formed on the 

 surface. Striations 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 dif- 

 ferentiation of embryonic cells (sarcoplasts) found 

 between the fully formed fibres. 

 Fig. 115. Deve- In plain muscle, growth occurs in a similar 

 laT^LT'from wa y : this is well illustrated in the enlargement of 

 foetus of two thg u terus during pregnancy ; this is due in part 



months. (Kan- or j > r 



vier.) 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. 



CHAPTER VII. 



NERVE. 



NERVOUS tissue is the material of which the nervous system 

 is composed. The nervous system is composed of two parts, the 

 central nervous system, and the peripheral nervous system. The 

 central nervous system consists of the brain and spinal cord ; the 



