ORGANISATION AND STRUCTURE OF MUSCLE 



51 



that muscle-fibres exhibiting such fixed waves of contraction, 

 when considered on a gypsum ground in the plus and minus 

 condition, exhibit no particular alteration of colour in the con- 

 tracted as compared with the relaxed parts, although normally 

 increase of bulk in a muscle-layer does perceptibly deepen the 

 colour, e.g. when two relaxed fibres partially cover each other.~ 

 Even high waves of contraction exhibit, in comparison with 

 the relaxed portions of the fibre, little or no alteration qua increase 

 or decrease of colour, in the ascending or descending stages. 



This leads us to infer that in contracted muscle-fibres the 

 increase of colour which should go along with 

 the thickening of the fibre is compensated, or 

 over -compensated, by a diminution of the 

 double refraction coincident with the con- 

 traction. 



The method of polarised light enables us 

 further to form a conclusion with regard to 

 another important point in the behaviour of 

 striated muscle-fibres during contraction. If 

 the height of the metabolous and arimetabolous 

 segments is compared in an appropriate pre- 

 paration (Fig. 32) with crossed prisms, during 

 the transition from the relaxed to the contracted 

 portions of the fibre, it may be seen that with 

 increasing contraction the height of the iso- 

 tropous (arimetabolous) segments diminishes 

 more than that of the anisotropous (metabol- 

 ous), so that the volume of the latter increases 

 at the expense of the former, the total volume 

 of the section in question, like that of the entire 

 fibre, remaining constant. Engelmann has established these facts 

 in appropriate objects by micrometric measurements. In order to 

 explain the effect he assumes that fluid passes from the isotropous 

 to the anisotropous substance in contraction ; the anisotropous 

 substance swells, the isotropous shrinks. This water-exchange 

 between the metabolous and arimetabolous segments must natur- 

 ally be imagined as between the elements of the muscle-column, or 

 single fibril, corresponding with these segments. An inverse 

 change in volume occurs upon relaxation, and the surplus of fluid 

 returns to the isotropous (arimetabolous) system. This theory is 



FIG. 32. Muscle-fibres of 

 Telephones during con- 

 traction, a, In ordin- 

 ary > &> i n polarised 

 light. (Engelmann. 

 From Foster's Text-Book 

 of Physiology.) 





