CHAP. VII.] 



STRIPED MUSCLE. 



151 



and semitendinosus ; and it is very common for them to fall short of 



the length of the whole organ, in consequence of an oblique dispo- 



sition, as seen in penniform muscles. In the sartorius they often 



exceed two feet in length, while in the sta- /y^.so. 



pedius they are not two lines. They vary in 



diameter from -^ to j yVo f an i ncn ; "being 



largest in Crustacea, fish, and reptiles, where 



their irritability is enduring; and smallest in' 



birds, where it is mpst evanescent. The in- 



dividual fibres, however, vary considerably 



in thickness in the members of the several 



tribes, and even in the same animal and 



muscle. Their average width in man is 



about -^I-Q of an inch. They are not cylin- 



drical,but flattened more or less, by being 



closely packed together. This may be ascer- 



tained in the recent state, or still better by 



a transverse section of a dried muscle. Small 



interspaces are left, however, for the passage 



of the capillary blood-vessels along the angles 



of junction, and sometimes between the con- 



tiguous sides. (Fig. 36.) rower elementar y fibres - 



Internal Structure. The beautiful cross-markings on the volun- 



tary fibre have been known from the early days of microscopical 

 research, and have given occasion to a variety of hypothetical 

 and generally mechanical solutions of the problem of contraction; 

 which, by warping the minds of observers, have had the effect of 

 greatly complicating an already difficult subject, that of the internal 

 anatomy of the fibre, which can only be determined by pure obser- 

 vation. Fontana alone among the older anatomists abstained from 

 vague speculation ; and he arrived nearest to the truth. He found 

 that the fibre was apt to split up into fine fibrillse, each of which 

 was a scries of particles; and he imagined that the transverse lines 

 were caused by the regular apposition side by side of the parti- 

 cles of the contiguous fibrillse. It was customary both before and 

 since his time, as at the present day, to regard the fibre as a bundle 

 of smaller ones, whence the term primitive fasciculus, first given to 

 it by him and adopted byMiiller: but this view of the subject 

 is imperfect. The fibre always presents, upon and within it, longi- 

 tudinal dark lines, along which it will generally split up into fibrilhe; 

 but it is by n fracture alone that such fibrillse are obtained. They 

 do not exist as such in the fibre. And, further, it occasionally 



