534 PHYSIOLOGY. 



structure of muscle. As to their structure, the wing-fibers are in 

 complete agreement with ordinary muscles. 



Wing-fibers occur in large bundles of muscle-columns or sarco- 

 styles imbedded in considerable amount of granular sarcoplasm, 

 while the whole of the structure is inclosed within a sarcolemma. 

 The nuclei are scattered here and there. The quantity of sarco- 

 plasm in wing-muscle is relatively far greater than in the ordinary 

 muscle. 



When wing-muscle has been carefully teased into muscle-col- 

 umns, or sarcostyles, it is found that they contract while the sarco- 

 plasm is quiescent. The muscle-columns can then be very carefully 

 studied, when they show, like other muscles, the alternate bright and 

 dark cross-striping. Each bright stria is bisected by a line which is 

 the optical section of a transverse membrane: the membrane of 

 Krause. These membranes divide the fibers into a series of seg- 

 ments, called sarcomeres. 



In a muscle hardened by spirits each sarcomere is seen to con- 

 tain: (1) in its middle, a strongly refracting, disclike sarcous ele- 

 ment; (2) at either end (next the membrane of Krause) a clear 

 interval occupied by hyaline substance. With strong lenses the 

 sarcous elements can be made out to be composed of a sarcous sub- 

 stance which stains with logwood; it is pierced by short, tubular 

 canals which extend from the clear interval as far as the middle of 

 the disc. It is these canals which give to the sarcous element its 

 longitudinal striping. 



If, for any reason, the sarcostyle becomes extended, the sarcous 

 elements tend to separate into two parts with an interval between 

 them; vice versa, if the muscle be contracted or retracted the sarcous 

 elements tend to encroach upon the clear intervals. At the same 

 time the sarcous elements become swollen, so that the sarcomeres 

 are bulged out at their middle and contracted at their ends. 



Changes in Contraction. When these muscles contract, the sar- 

 cous elements become bulged out and shortened, while the fluid of 

 the clear interval becomes relatively diminished in amount. The 

 ends of the sarcomeres are thereby contracted opposite the membrane 

 of Krause, so that the sarcostyles become moniliform. This altera- 

 tion in the shape of the sarcostyle necessarily affects the sarcoplasm 

 which lies in their interstices. It must become squeezed out of the 

 parts which are opposite the bulgings of the sarcostyles and into 

 those parts which are opposite their constrictions. In other words, 

 the sarcoplasm must accumulate in greater quantity opposite the 





