GROWTH. 



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view, originally promulgated by Eemak, that the fibres are produced as above 

 described by the elongation of single cells, with differentiation of their contents and 

 multiplication of their nuclei. 



Growth. The muscular fibres, after having acquired their characteristic form 

 and structure, continue to increase in size till the time of birth, and thenceforward 

 up to adult age. In a full-grown foetus most of them measure twice, and some of 



Fig. 345. DEVELOPING MUSCULAR FIBRES. HIGHLY MAGNIFIED. 



A, elongated cell with two nuclei and a striation beginning 

 down one side of the cell (from fostal sheep, Wilson Fox). 



B, from foetus of 2 months ; p, granular protoplasm ; 

 ,<7, glycogenous substance ; n, nucleus ; s, commencing sarco- 

 Jemma, with striated muscular substance developing imme- 

 diately beneath it. 



C, from foetus of 3 months, displayed so as to show the 

 contractile substance collected at one side of the fibre, and 

 partially enclosing the unaltered substance of the fibre, g ; 

 f, fibrils. B and C from Ranvier. 



them three or four times their size at the middle of 

 foetal life ; and in the adult they are about five 

 times as large as at birth. This increase in bulk 

 of the individual fibres would, in a measure, account 

 for the enlargement of the entire muscles. 



It is uncertain how far there may be a multi- 

 plication or new formation of muscular fibres 

 during the growth of a muscle ; but it is probable 

 that during growth at least, if not in adult life, 

 & new formation of fibres within the muscles 

 does occur. The new fibres appear to be in 

 part formed by the longitudinal splitting of 

 the existing fibres, a process which was described 

 by Weissmann, and which seems to be of very 

 general occurrence at a certain period of growth. 

 The splitting is preceded by the multiplication of 

 the muscle-nuclei, which form a longitudinal series 

 in the part which is about to become separated. 



The bundle of fine muscular fibres is found to be enclosed within a thick connective 

 tissue sheath formed of several layers, continuous with the layers of Henle's sheath of 

 the nerve-fibre, and since these layers are more developed at the point of entrance of 

 the nerve into the bundle than elsewhere, the whole structure has a spindle-shape, 

 and on this account has received the name of muscle-spindle (Kiihne). The fibres 

 of the muscle-spindle appear gradually to enlarge and eventually to form a bundle 

 of ordinary muscle-fibres. The nerve and nerve-ending must, of course, participate 

 in the cleavage, since each fibre is ultimately found to be furnished with a nerve- 

 ending. But another important mode of new formation is by the transformation 

 of cells (sarcoplasts) which lie between the muscle fibres, and which are presumably 

 undifferentiated cells derived from the original muscle-plate. These cells enlarge 

 End become elongated, and striated muscular substance becomes formed within them 

 usually at one side, and often forming oval masses, which as they grow become longer 

 and more spindle-shaped, the middle part being bulged out and the ends tapering off 

 to fine terminations. At this stage of development the name of muscle-spindle has 

 also been applied to these fibres. The middle bulged part receives a nerve fibre, which 

 is provided with a terminal ramification, as with other muscular fibres, and a multi- 

 plication of nuclei, and formation of sarcolemma having already taken place, the 

 development of the fibre may now be looked upon as completed. Such fibres in 



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