36 



CONTRACTILITY 



the sarcostyles, which are separated from one another by a 

 granular substance — sarcoplasm. 



The relative amount of sarcostyle and sarcoplasm in a 

 fibre is variable, and confers upon the fibre its form of 

 contraction. Where the sarcoplasm is scanty {white fibre), 

 the contraction is in the form of a rapid twitch; where 

 abundant {red fibre), the contraction is slow and sustained. 

 While in some animals individual muscles are composed 

 exclusively of either red or white fibres, in man both types 

 of fibre are often found in the same muscle. There is some 

 evidence to show that both sarcostyle and sarcoplasm are 

 endowed with contractihty, the movement being rapid 

 in the case of the former, slow in the case of the latter. 



^.'EA 



SJL 



Fio. 4. — Sarcomere (diagrammatic). A, relaxed; B, contracted. 

 - K, membrane of Krause ; H, line of Henle ; S.E., sarcostyle (Schafer) 



When a muscle-fibre is observed under the microscope 

 the act of contraction is. seen to consist of a broadening of 

 the fibre and a thinning of the individual discs. At the 

 same time the dark band becomes hghter and the hght 

 band darker, until a complete reversal is obtained. When, 

 however, the fibre is observed through polarised hght the 

 dark bands are anisotropic, or doubly refracting, appearing 

 hght ; while the hght bands are isotropic, or singly refracting, 

 and appear dark. In contraction there is no reversal of 

 this effect, but an increase of the anisotropic at the expense 

 of the isotropic substance. 



In the following pages we shall deal primarily with 

 striated muscle, indicating how cardiac and imstriated 

 muscle resemble or differ from it. 



