294 



MUSCULAR TISSUE. 



interval of the sarcomere), and have in optical section the appearance of rows of 

 dark-looking dots. When these dots are carefully regarded they seem each to be 

 surrounded by a bright halo (figs. 331, 339, R), which is apparently due to the manner 

 in which they reflect the light which is transmitted through the muscle, much in the 

 same Avay as an oil-globule or an albuminous granule, when viewed in water under a 



Fig. 338. PART OF A CONTRACTED FIBRE OF INSECT MUSCLE (LEG) 



TREATED WITH VINEGAR. (E. A. S.) 



The moniliform appearance of the sarcostyles and the accumu- 

 lations of the sarcoplasm opposite their constrictions are well 

 seen. The ordinary cross-striation is not visible. 



high power of the microscope, appears when it is 

 exactly focussed, to be surrounded by a bright area. 1 

 Since each dot is encircled by a bright halo, and the 

 dots are arranged in regular rows, the haloes become 

 blended into a stripe, which is much brighter than the rest of the muscle-substance. 

 This is in fact the chief cause of the very bright appearance of the clear bands 

 of the fresh muscle. These clear bands, however, about correspond in level with 



Fig. 339. WAVE OF CONTRACTION PASSING OVER A 



MUSCULAR FIBRE OF DYTISCUS. VERY HIGHLY 

 MAGNIFIED. (E. A. S. ) 



R, R, portions of the fibre at rest ; c, contracted 

 part : i, I, intermediate condition. 



the hyaline substance of the sarcomeres as seen 

 in stained alcohol-preparations, so that their 

 relatively bright appearance in the fresh 

 muscle, as compared with the dim band or 

 row of sarcous elements, is not entirely pro- 

 duced by the light-effect above mentioned, 

 although it is greatly enhanced thereby. 



Changes in contraction. When these 

 muscles contract the sarcous elements, as in 

 the wing-muscles, become bulged out and 

 shortened, while the fluid of the clear inter- 

 vals becomes relatively diminished in amount. 

 The ends of the sarcomeres are thereby con- 

 tracted opposite the membranes of Krause, and 

 the sarcostyles become moniliform (fig. 338). 

 This alteration in shape of the sarcostyles 

 necessarily affects the sarcoplasm which lies in 

 their interstices, which must become squeezed 

 out of the parts which are opposite the 

 bulgings of the sarcostyles and into the parts 

 which are opposite their constrictions. Ill 

 other words, the sarcoplasm must accumulate 

 in greater quantity opposite the clear bands 

 and the membranes of Krause, and must 



diminish in amount opposite the sarcous elements. This is, in fact, what can be seen 

 to take place. In fig. 338 a contracted portion of muscle in which the sarcoplasm 

 has been rendered evident by acid is represented, ari it is seen that the sarcoplasm 



1 It is in fact this action of the dots upon the light which tends to obscure the continuity of the 

 longitudinal lines of the sarcoplasm in the fresh condition, so that in this condition it may be difficult or 

 impossible to make out that continuity (see fig. 331). 



.i 



