THE MUSCULAR SYSTEM. 



397 



wholly parallel lines. Each fibre is found to consist of broad dim bands 

 of highly refractive substance representing the contractile portion of the 

 muscle fibre the contractile discs alternating with narrow bright bands 

 of a less refractive substance the interstitial discs. After hardening, 

 each contractile disc becomes longitudinally striated, the thin oblong 

 rods thus formed being the sarcous elements of Bowman. The sarcous 

 elements are not the optical units, since each consists of minute doubly- 

 refracting elements the disdiaclasts of Brucke. When seen in trans- 

 verse section the contractile discs appear to be subdivided by clear lines 

 into polygonal areas Cohnheim's fields (Fig. 278), each corresponding to 

 one sarcous element prism. The clear lines are due to a transparent in- 



FlG. 276. 



FIG. 277. 



FIG. 276. Part of a striped muscle-fibre of a water beetle prepared with absolute alcohol. A, 

 sarcolemma; B, Krause's membrance. The sarcolemma shows regular bulgings. Above and below 

 Krause's membrane are seen the transparent " lateral discs." The chief mass of a muscular com- 

 partment is occupied by the contractile disc composed of sarcous elements. The substance of the 

 individual sarcous elements has collected more at the extremity than in the centre: hence this latter 

 is more transparent. The optical effect of this is that the contractile disc appears to possess a 

 " median disc " (Disc of Hensen). Several nuclei of muscle corpuscles, C and D, are shown, and in 

 them a minute network, x 300. (Klein and Noble Smith.) 



FIG. 277. A. Portion of a medium-sized human muscular fibre, x 800. B. Separated bundles 

 of fibrils equally magnified; a, a, larger, and 6, 6, smaller collections; c, still smaller; d, d, the 

 smallest which could be detached, possibly representing a single series of sarcous elements. 

 (Sharpey.) 



terstitial fluid substance pressed out of the sarcous elements when they 

 coagulate. The sarcolemma is a transparent structureless elastic sheath 

 of great resistance which surrounds each fibre (Fig. 275). There is still 

 some doubt regarding the nature of the fibrils. Each of them appears 

 to be composed of a single row of minute dark quadrangular particles, 

 called sarcous elements, which are separated from each other by a bright 



