MUSCLE 



641 



material of the dim and light stripes. Schafer has explained it as due 

 to the squeezing of sarcoplasm from between the sarcous elements 

 into the position of the light stripe, when they bulge laterally in 

 contraction. This accumulation of sarcoplasm in the stripes that 

 were previously light makes them look darker in comparison with 

 the true dim stripe. 



Appearance of the Fibres in Polarized Light. A ray of ordinary 

 light consists of vibrations of the ether in all planes at right angles 

 to the direction of the ray. In a ray of plane polarized light all the 

 particles vibrate in one plane. A ray of light which has been 

 polarized by a Nicol's prism cannot pass through another Nicol's 

 prism with its principal plane at right angles to that of the first. 

 If the second or analyzing prism be rotated so that the principal 

 planes are no longer at right angles, some of the light will pass 

 through. The same effect is produced if, without altering the 

 original ' crossed ' position of the nicols, a substance capable of 

 rotating the polarized ray is introduced between the prisms. A 

 rough illustration will perhaps 

 tend to make this point clearer. 

 Suppose that a string fixed at 

 one end is set vibrating in 

 various directions by a twist- 

 ing movement. If the string 

 has to pass through a narrow 

 vertical slit e.g., between two 

 fingers held vertically all vi- 

 brations except those in the 

 vertical plane will be extin- 



FIG. 227. LIVING MUSCULAR FIBRE 

 (FROM GEOTRUPES STERCORARIUS). 



i, in ordinary ; 2, in polarized light. 

 (Van Gehuchten.) In living muscle (at 

 least in fibres which are not extended) in 

 contrast to dead muscle after treatment 

 with reagents, the doubly refracting or 

 anisotropous substance is present in the 

 greater part of the fibre ; and with crossed 

 nicols the position of the singly refracting 

 or isotropous material is indicated only 

 by narrow transverse black lines or rows 

 of dark dots. 



guished ; but vertical vibra- 

 tions will be able to pass be- 

 yond the slit. The movement 

 may be said to be plane polar- 

 ized, and the effect of the slit 

 corresponds to that of the first 

 nicol. Now make the string 

 pass also through a horizontal 

 slit ; the vertical vibrations 

 will then be extinguished too ; 

 in other words, none of the 

 movements will pass beyond 

 the ' crossed ' slits. This cor- 

 responds to the dark field of the crossed nicols. But if the vertical 

 vibrations which have passed the first slit could be in any way 

 changed into horizontal vibrations, they would no longer be ex- 

 tinguished by the second. This would correspond to rotation of 

 the plane of polarization through 90. A ray of light polarized 

 by the first nicol will, if its plane of polarization be rotated through 

 90, pass entirely (except for loss by ordinary reflection and absorp- 

 tion) through the second. If the angle of rotation is less than 90, 

 a portion will pass through. 



The substance of the sarcous element which forms the dark stripe 

 is doubly refracting, and therefore rotates the plane of polarization, 

 but the clear substance of the light stripe is singly refracting. When 

 an uncontracted fibre is viewed with crossed nicols, the dim stripe 

 accordingly appears bright in the otherwise dark field. In the con- 

 tracted fibre the doubly refractive material remains in the stripe 



41 



