CH. VI. 



SARCOPLASM 



85 



the whole muscle is only the sum total of the contraction of all the 

 constituent sarcomeres. 



In an ordinary muscular fibre it is stated that when it contracts, 

 not only does it become thicker and shorter, but the light stripes 

 become dark and the dark stripes light. This again is only an optical 

 illusion, and is produced by the alterations in the shape of the sarco- 

 styles, affecting the sarcoplasm 

 that lies between them. When 

 the sarcous elements swell during 

 contraction, the sarcoplasm accu- 

 mulates opposite the membranes 

 of Krause, and diminishes in 

 amount opposite the sarcous 

 elements; the accumulation of 

 sarcoplasm in the previously 

 light stripes makes them appear 

 darker by contrast than the dark 

 stripes proper. This is very 

 well shown in fig. 110. There 

 is no true reversal of the strip- 

 ings in the sarcostyles them- 

 selves. 



That this is the case can be seen 

 very well when a muscular fibre is 

 examined with polarised light. A 

 polarising microscope contains a Nicol's 

 prism beneath the stage of the micro- 

 scope which polarises the light passing 

 through the object placed on the stage. 

 The eye-piece contains another Nicol's 

 prism, which detects this fact. If the 

 two Nicols are parallel, the light pass- 

 ing through the first passes also through 

 the second ; but if the second is at right 

 angles to the first, the light cannot 

 traverse it and the field appears dark. 

 If an object on the microscope stage is 

 doubly refracting it will appear bright 

 in this dark field ; if it remains dark 

 it is singly refracting. The sarcoplasm 

 is singly refracting or isotropous ; it 

 remains dark in the dark field of the 

 polarising microscope. The muscle columns or sarcostyles are in great measure 

 doubly refracting or anisotropous, and appear bright in the dark field of the 

 polarising microscope. The sarcostyle, however, is not wholly doubly refracting ; 

 the sarcous elements are doubly refracting, and the clear intervals are singly 

 refracting. On contraction there is no reversal of these appearances, though o*f 

 course the relative thickness of the singly refracting intervals varies inversely 

 with that of the doubly refracting sarcous elements. 



Ending of Muscle in Tendon. A tendon-bundle passes to each 

 muscular fibre, and becomes firmly united to the sarcolemma. The 



FIG. 110. Wave of contraction passing over a mus- 

 cular fibre of water-beetle. R, R, portions of 

 the fibre at rest ; c, contracted part ; i, i, inter- 

 mediate condition. (Schafer.) 



