COMPARISON OF ORDINARY AND WING-MUSCLES. 



293 



of the spongioplasm ; while on the other hand, when the contraction passes off, the hyaloplasm 

 tends to pass out from the pores or meshes of the spongioplasm, and produces in the one case 

 extension of the muscle-fibre, in the other the throwing out of pseudopodia. In short the 

 poriferous substance of the sarcous element corresponds with the spongioplasmic ;< cecoid " of 

 the amoeboid cell, whilst the clear substance which flows in and out of the pores represents 

 the hyaloplasmic " zooid " (see p. 178). 



Fig. 336. DIAGRAM OF A SARCOMERE IN A MODERATELY 



EXTENDED CONDITION A, AND IN A CONTRACTED CONDI- 

 TION B. (E. A. S.) 



K, K, Membranes of Krause ; H, plane of Hensen ; 

 S.E., poriferous sarcous element. 



Comparison of the ordinary muscles 

 of insects and vertebrates with the 

 wing-muscles of insects. The differences 



of structure between the ordinary muscles and the muscular fibres which move 

 the wings of insects, are of no great importance, being differences of degree 

 only. Thus the sarcoplasm is in relatively small amount in the ordinary muscles, 

 and generally has the regular enlargements which appear as dots or transverse 

 networks according to the point from which they are viewed, these enlarge- 

 ments being absent in the wing-fibres ; moreover, the sarcostyles or muscle-columns, 

 probably on account of the smaller amount of sarcoplasm, are less easily isolated, 

 But, like the sarcostyles of the wing-muscles, they are divided into segments or 

 sarcomeres, at regular intervals, corresponding with the middle of the bright strias, 

 by transverse membranes (membranes of Krause), which can be brought distinctly 



Fig. 337. PART OF A MUSCULAR FIBRE OF AN INSECT FIXED WITH 



ALCOHOL AND AFTERWARDS STAINED. (E. A. S. ) 



The rows of sarcous elements which form the dark stripes are 

 stained (S.E.) ; the sarcoplasm has the usual appearance of longi- 

 tudinal lines and dots. The lines produced by the juxtaposed 

 membranes of Krause are just risible (K.). 



UHJHtltimHmUUUA 



-S.E. 



into view on the addition of dilute acid to the fresh or 

 alcohol-fixed muscle (fig. 332, K). Each sarcomere con- 

 tains a sarcous element, with hyaline substance between 

 the sarcous element and the membrane of Krause at 

 either end of the segment : the middle of the sarcous 

 element when the muscular fibre is extended shows a 

 line, as in the wing-element, the band of Hensen. 



As in the wing-sarcostyles, the proportionate amounts of the sarcomere occupied 

 respectively by the sarcous element and the clear intervals vary greatly, especially 

 according to the condition of extension or retraction of the muscle. In 

 retracted or contracted fibres the sarcous elements bulge out and approach the 

 transverse membranes, and the clear intervals diminish proportionately, their fluid 

 being imbibed by the sarcous element, while in extended fibres the sarcous elements 

 become removed from the transverse membranes and narrowed, and the clear 

 intervals become paripassu longer. 



It is said that the ends of the sarcous element may in extended fibres become 

 detached and form accessory disks lying in the clear interval. It is certain, how- 

 ever, that the structures which have often been described as accessory disks in these 

 muscles are merely the sarcoplasmic enlargements which appear as dotted lines. 



Effect of the sarcoplasmic accumulations upon the cross-striation of 

 the ordinary muscles. An important influence upon the optical appearances of 

 the ordinary muscular fibres is exerted by the sarcoplasmic accumulations (transverse 

 networks). These lie, as we have seen, in the region of the bright stripe (clear 



