THE TISSUES 41 



markings appear across the cell. Lastly, a covering develops, 

 and the fully-formed fibre is produced (Fig. 15, 6). 

 This consists of three parts 



1. The Sarcolemma is a delicate, tough, elastic membrane 

 closely investing the fibre, and attached to it at Dobie's 

 lines. 



2. The Muscle corpuscles consist of little masses of 

 protoplasm with a nucleus, which lie just under the sarco- 

 lemma. 



3. The Sarcous substance is made up of a series of 

 longitudinal fibrils consisting of alternate dim and clear bands 

 the former staining deeply with eosin. In the middle 

 of the clear band is a narrow dim line, Dobie's line. The 

 fibres and fibrils tend to break across in the region of the 

 clear band, showing that they are weakest at that part. The 

 clear band differs from the dim band, not only in not taking 

 up eosin, but also in the fact that it entirely prevents the 

 passage of polarised light except in one position of the 

 analysing prism, while the dim band allows polarised light to 

 pass, whatever be the position of the prisms. 



Two explanations of these facts have been suggested: 

 (1) That the sarcous substance is made up, like other proto- 

 plasm, of a mitoma and plasma ; that the mitoma is arranged 

 in a series of longitudinal fibres, which are broader and 

 stronger in the dim band, and lie closely applied to one 

 another, side by side, while in the clear band they are 

 thinner, and are separated from one another by plasma ; and 

 that at Dobie's line there is a swelling on each fibril ; (2) 

 thai each fibril is a hollow tube consisting of a sponge-work 

 in the dim band with a fluid part in the clear band inter- 

 sected by a septum at Dobie's line. 



2. Chemistry of Muscle. 



Like all other living tissues, muscle is largely composed of 

 water. It contains about 75 per cent. The 25 per cent, 

 of solid constituents is made up of a small quantity, about 3 

 per cent., of ash, and 21 per cent, of organic substances. 

 The ash consists chiefly of potassium and phosphoric acid, 

 with small amount of hydrochloric acid and sodium, mag- 



