MUSCLE. 353 



fibres may be traced for a considerable distance along the 

 fibre, and may be observed to contain as many as twenty 

 nuclei. 



From these several facts it would thus appear, that striped 

 and unstriped muscular fibre, do not represent distinct 

 types of structure, but that each is to be regarded as a dif- 

 ferent stage in the development of the same. The unstriped 

 muscular fibrilla passes through but one stage of growth, 

 and then its development becomes permanently arrested : the 

 fibrilla? of the heart, &c. attain a higher degree of development, 

 the nucleated fibres becoming marked with transverse striae ; 

 after which their growth permanently ceases ; lastly, the 

 striped fibrillaa of the voluntary muscles reach the third and 

 last period in the development of the muscular tissue, having 

 their nuclei obliterated, and becoming exceedingly slender. 



It would appear also that new fibrilla3 are constantly being 

 developed, even in the adult muscular fibre. 



But certain appearances may be observed which render it 

 extremely probable that not merely new fibrilla3 are con- 

 stantly being developed, but also that new fibres are con- 

 tinually being formed. 



Thus, it is a common thing to meet with unstriped and 

 even striped fibrilla?, which are slightly adherent to the ex- 

 ternal surface of the sarcolemma; again, small muscular fibres 

 attached to the larger fibres, and consisting of but very few 

 fibrilbe, may constantly be observed. (Plate XLII.jfy. 4.) 



In the uterus we have a very remarkable example of a 

 periodic development and subsequent absorption of unstriped 

 muscular fibrilla. 



The last point to which reference need be made is, to the 

 doubt expressed as to " whether the identical corpuscles 

 originally present " (in the fibre) " remain through life, or 

 whether successive crops advance and decay during the pro- 

 gress of growth and nutrition." * This matter is no longer 

 doubtful : the particulars observed in relation to the develop- 

 ment of muscular fibre enable us to give a solution of the 



* Loc. cit pp. 182, 183. 



