158 LOCOMOTION. [CHAP. vn. 



contractile material gradually takes place with them, commencing 

 on the inner surface, and advancing towards the centre, till the 

 whole is solidified. The deposition occurs in granules, which, as 

 they come into view, are seen to be disposed in the utmost order, 

 according to the two directions already specified. These granules, 

 or sarcous elements, being of the same size as in the perfect muscle, 

 the transverse stripes resulting from their apposition are of the same 

 width as in the adult ; but, as they are very few in number, the 

 fibres which they compose are of corresponding tenuity. From the 

 very first moment of their formation, these granules are parts of a 

 mass, and not independent of one another ; for, as soon as solid 

 matter is deposited in the cells, faint indications of a regular ar- 

 rangement in granules are usually to be met with. It is common 

 for the longitudinal lines to become well-defined before the trans- 

 verse ones. When both are become strongly marked, as is always 

 the case at birth, the nuclei of the cells, which were before visible, 

 disappear from view, being shrouded by the dark shadows caused 

 by the multitudinous refractions of the light transmitted through 

 the mass of granules : but they can still be shewn to exist in the 

 perfect fibre, in all animals, and at all periods of life, by immersion 

 in a weak acid ; which, while it swells the fibrous materials of the 

 granules, and obliterates their intervening lines, has no action on 

 the nuclei. 



These nuclei in insects are arranged, in the early con- 

 dition of the fibre, as a single or double series along the 

 axis (fig. 45) ; and, in the adult state, they retain the 

 same position (fig. 42). In vertebrate animals they are 

 scattered more irregularly, but at pretty equal distances 

 throughout the mass in both foetal and adult conditions. 

 In the fully formed fibre, if it be large, they lie at various 

 depths within it ; but, if small, they are at or near the sur- 

 face. They are oval and flat, and of so little substance, 

 that though many times larger than the sarcous elements, 

 and lying amongst them, they do interfere with their 

 m,r en from mutual apposition and union. These corpuscles are fre- 

 the Libeiiu- quently the cause of irregular longitudinal dark streaks, 

 eariy m stage seen in the fibre by transmitted light. They usually con- 

 - tain some central granules or nucleoli. It is doubtful 

 row 6 of whether the identical corpuscles, originally present, remain 

 agnmed through life, or whether successive crops advance and decay 

 during the progress of growth and nutrition : but it is 

 certain that, as development proceeds, fresh corpuscles are de- 





