36 PHYSIOLOGY. 



verse strice, quite as frequently as in the longitudinal direction, thus 

 forming a series of discs as seen in fig. 9. If a general disintegration 

 took place along all the lines in both directions, there would result a 

 series of particles, which have been c^Wed^ primitive particles, or 

 sarcous elements, the union of which constitutes the mass of the fibre. 



According to the opinion of Messrs. Todd and Bowman, (in 

 which they are supported by MuUer, Lauth, and Schwann, the 

 cross-stripes of the fibres are formed by the apposition side by side 

 of the dark points seen on the separated fibrillse. 



The elements of the muscular fibre are held together by a tubular 

 sheath adapted to its surface and adhering to it ; this is called the 

 sarcolemma, or myolemma. It is a transparent, very delicate, but 

 tough and elastic membrane, which isolates the fibre from all other 

 tissues. It has nothing to do with the production of the transverse 

 strioe. Neither is it perforated by either nerves or blood-vessels. 



Muscular fibres are rather polygonal than cylindrical, owing to 

 the pressure to which they are subjected by juxtaposition. Their 

 average diameter in man is about 1 -400th of an inch, being rather 

 larger in the male than in the female. 



The fibrillse, when examined, are found to present an alternation 

 of dark and light spots, corresponding with the transverse strife of 

 the fibre, and the lighter intervals between them. Each dark spot 

 is surrounded by a pellucid border ; the whole constitutes a complete, 

 though minute cell, and the entire fibrilla may be considered as 

 'made up of a linear aggregation of such cells. The dark spot within 

 the bright border is the cavity of the cell, filled with a refracting sub- 

 stance. . When the fibrilla is relaxed, the longitudinal diameter of 

 these cells is greatest ; when contracted, the transverse. Thus the 

 act of muscular contraction sefems to consist in a change of form in 

 the cells of the ultimate fibrillte, consequent upon an attraction 

 between the walls of their two extremities or their nuclei. This 

 corresponds with the contraction of certain vegetable tissues, the 

 component cells of which change their form when irritated, and thus 

 produce a movement. 



When muscular fibre contracts therefore, it is not thrown into 

 zigzag lines, since fibres in this state, cannot be supposed to exercise 

 any force of traction, but it is shortened in length by the approxima- 

 tion of its elements, at the same time that the transverse diameter is 

 increased. 



Muscles grow by an increase, not of the number, but of the bulk 

 of their elementary fibres ; the number of fibres probably remains 

 the same through life, as it was in the foetus. Notwithstanding the 

 energy of its growth, and the constant interstitial changes, it is 

 doubtful whether muscular tissue is ever regenerated where loss of 

 substance has taken place. It is generally replaced by areolar tissue, 

 which gradually becomes condensed, but never contractile. 



The great property of muscular tissue is that of contra/Aility, by 



