MUSCLE. 



83 



muscles are made up of fasciculi of fibres of this striated muscle- 

 tissue, each being enclosed in a sheath (sarcolemma) (Fig. 21). 

 The arrangement of the elements of each 

 fibre is such that its microscopic appearance FIG. 20. 



is as though there were alternate light and 

 dark bands about it, the dark being the 

 wider. These are the elements of the muscle- 

 tissue, the dark bands representing disks of 

 contractile tissue, while the brighter bands 

 represent a disk of interstitial substance. 



What are sarcous elements? 



The fibres may be split up longitudinally 

 into minuter fibrils which have the micro- 

 scopic appearance of alternate light and 

 dark particles ; these particles are the sar- 

 cous elements (Fig. 21. B, d), and it is by 

 the fusion of these fibrils that the fibres are 

 made up. 



How large are striped muscle-fibres? 



They are about an inch in length and 

 ^j-g- inch in diameter. They join the con- 

 nective-tissue cells of a tendon or aponeu- 

 rosis or another muscle-fibre by adhesion of 

 the sarcolemma at the ends, and thus unite 

 the muscle-bundles in a firm mass ; and this 

 union is further strengthened by the cohe- 

 sion of the fibres. 



Non-striated Elementary 

 Fibres, from the human 

 colon : a, treated with 

 acetic acid, showing the 

 corpuscles; fc, fragment 

 of a detached fibre not 

 touched with acid. 



What is peculiar in the heart-muscle? 



It is an exception to the rule that involuntary muscles are un-^ 

 striped. The striation of the heart-muscle fibres is not so marked" 

 as in ordinary muscle, and the form of the fibres is different, for 

 they are branched and more slender. Each fibre is nucleated, a 

 large oval nucleus occurring at the centre. The appearance of 

 the heart-fibres indicates that they occupy an intermediate position 

 between typical plain and striped fibres. 



What is myosin? 



Myosin is an albuminoid substance belonging to the class of 



