438 



PRINCIPLES OF GENERAL PHYSIOLOGY 



It seems possible that photographs of the living fibre by ultra-violet light, in 

 the manner described in a previous chapter (page 9), might give valuable infor- 

 mation with regard to the structure of muscle. 



A fact of interest is that the dark bands of striated muscle are doubly 

 refracting, that is, they appear bright on the dark field between crossed Nicol 

 prisms. Tf a fibre is made to contract while under the microscope, the dark band 

 becomes light and the light band dark ; but the double refraction does not alter, 

 that is, it is the light band which is now doubly refracting. According to 

 Engelmann (187.3, p. 166), the isotropic (singly refracting) part diminishes in 

 volume in contraction, while the anisotropic (doubly refracting) part increases ; 

 that is, fluid passes out of the isotropic into the anisotropic elements (p. 167 of 

 the paper). Fig. 133 shows this and other facts. It will be seen, in the 



appearance under polarised light, that 

 whereas in the resting fibre the two 

 parts are of about equal size, in the 

 contracted part, in the middle of the 

 portion of fibre represented, the clear 

 part considerably exceeds in volume 

 the dark part. Further, Engelmann 

 holds (1875) that contractility is always 

 associated with double refraction ; in 

 the striated muscle fibre it is the aniso- 

 tropic part which is the active con- 

 stituent, and Engelmann has detected 

 double refraction in the contractile 

 parts of Hydra, and of various uni- 

 cellular organisms. The statement is 

 made (p. 460) that contractility, in 

 whatever form it may occur, is con- 

 nected with the presence of doubly 

 refracting, positive, uniaxial particles, 

 whose optical axis coincides with the 



direction of shortening. The isotropic 

 part is supposed capable of excitation, 

 but not of contraction. 



Hiirthle (1909) concludes, from 

 the evidence of photographs of living 

 fibr ' th > chane of volume occurs 



FIG. 134. FIBBK OF LEG MUSCLE OF CHRvso- 



MKLA CORRULEA WITH (FIXED) CONTRACTION in tne doubly retractive, contractile 



WAVE, PHOTOGRAPHED UNDER POLARISING elements. The isotropic parts, on the 



MICROSCOPE. contrary, increase in volume at the 



A, with parallel Nicols. B, with crossed Nicols. expense of the Sarcoplasm. Although 



(After Engelmann. Schafer's " Essentials the photographs of the living fibres 



of Histology," Fig. 166, p. 136.) with waves of contraction are certainly 



very interesting, it is difficult to make 



out from them whether the statement is justified (see Schafer's criticism of 

 Hiirthle's views, 1910, pp. 72-73). Fig. 134 shows photographs of living muscular 

 fibres under polarised light. 



The essential point in the mechanics of muscular contraction is that the 

 properties of the tissue change in contraction, so that, if it be not permitted to 

 shorten, a state of tension is developed. It is a difficult matter to give an adequate 

 mechanical illustration of the process, but perhaps it will assist comprehension if 

 we imagine that we have two spirals, one of hardened steel wire, the other of soft 

 lead wire. We now stretch them to the same extent. The coil of steel wire will 

 be in a state of tension, as felt by the necessity to exert a continuous pull upon 

 it to prevent its returning to its original length, whereas the lead wire will have 



