202 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



appears that the Myolemma, or external transparent tube, is the part 

 first formed: this being distinctly visible, long before any traces of 

 fibrillse can be observed in it. This tube takes its origin, like the 

 straight ducts of Plants, in cells laid end to end; the cavities of which 

 coalesce, by the disappearance of the partitions, at a subsequent period. 

 The nuclei of these original cells may be distinctly seen, for some time 

 after the appearance of the transverse striae, which indicate the forma- 

 tion of the fibrillse in their interior ; and they project considerably from 

 the sides of the fibres. In the fully-formed muscle of animal life, how- 

 ever, they are not perceptible, except when the fibre is treated with 

 weak acid ; the effect of which is to render the nuclei more opaque, 

 whilst the surrounding structure becomes more transparent (Fig. 66). 



Fig. 66. 



Mass of ultimate fibres from the pectoralis major of the human foetus, at nine months. These fibres 

 haye been immersed in a solution of tartaric acid, and their " numerous corpuscles, turned in various direc- 

 tions, some presenting nucleoli," are shown. 



They are usually numerous in proportion to the size of the fibre. There 

 is every probability that these nuclei continue to act, like the " germinal 

 spots" of the glandular follicles, as centres of nutrition; from which the 

 minute cells that compose the fibrillse are developed as they are required. 

 The diameter of the Muscular Fibre of the foetus is not above one-third 

 of that which it possesses in the adult ; and as the size of the ultimate 

 particles is the same in both cases, their number must be greatly multi- 

 plied during the growth of the structure. But we shall find reason to 

 believe, that the decay of these particles is constantly taking place, 

 with a rapidity proportional to the functional activity of the Muscle ; 

 and their generation, which occurs as continually, when the nutrient 

 operations proceed in their regular course, is probably accomplished by 

 a development from these centres, at the expense of the blood with 

 which the Muscle is copiously supplied. 



339. From the preceding history it appears, that there is "no diffe- 

 rence, at an early stage of development, between the striated and the 

 non-striated forms of muscular fibre. Both are simple tubes, containing 

 a granular matter in which no definite arrangement can be traced, and 

 presenting enlargements occasioned by the presence of the nuclei. But 

 whilst the striated fibre goes on in its development, until the fibrillse, 

 with their alternation of light and dark spaces, are fully produced, the 

 non-striated fibre retains throughout life its original embryonic condi- 

 tion. And it may further be remarked, that the contractile cells, of 

 which many of the non-striated muscular structures are composed, are 

 the permanent types of those which are found in the earliest condition 



