MUSCLE 263 



discs, but of couples, comprising half a dim disc and a bright 

 disc. The thin dark lines indicate that the fibre is divided into 

 compartments by transverse septa, which are probably re- 

 ticulated. The appearance of a transverse line of dots, in place 

 of a continuous line, is due to the existence of very fine longi- 

 tudinal markings (it is unsafe to give them a name which 

 connotes structure). Where the longitudinal lines cross the 

 transverse lines, the optical effect is the appearance of a dot. 



If pieces of muscle are placed in a solution of osmic acid, they 

 become hard and brittle, and their markings are accentuated. 

 Muscle from the claw of a crab or a lobster is very suitable for 

 this purpose, owing to its exceptional freedom from connective 

 tissue. After this hardening the fibres are easily separated 

 with the aid of needles into fibrils immeasurably slender. An 

 isolated fibril shows with extreme distinctness the alternation 

 of dark, bright, dim, bright, dark markings already described. 

 The appearance of a cross-section of a fibre also proves that it 

 is a bundle of fibrils. The cut ends of the fibrils appear as dots 

 surrounded by homogeneous substance. In this respect there 

 is an important difference between red muscle and white. In 

 the red fibres the fibrils are fewer and thicker than they are in 

 white, and the embedding substance is more abundant. It is 

 generally assumed that the homogeneous substance, sarco- 

 plasm, is the nutrient protoplasm of the fibre, the fibrils the 

 contractile elements. The more complete the differentiation of 

 the fibre into fibrils, the more rapid is its action ; the more 

 abundant the sarcoplasm, the greater its capacity for continued 

 work. 



If a living muscle-fibre is observed while a wave of contraction 

 is passing down it, the ends of the fibre being free, so that its 

 shortening is not prevented, it is noticed that the widening of 

 the fibre is accompanied by the thinning, even to obliteration, 

 of the bright bands. The dim discs extend laterally, without 

 any noticeable diminution of their thickness. It looks as if 

 the bright discs, or something contained in the bright discs, 

 were absorbed into the dim discs. The fibre is, as we have 

 already pointed out, striated longitudinally. The striation is 

 more clearly visible in the dim discs than it is in the bright 

 ones. That the dim disc has an architectural structure absent 

 from the bright disc is placed beyond doubt when a muscle- 



