MUSCLE 259 



cell. Save for its central nucleus and a little granular proto- 

 plasm in which the nucleus is embedded, the cell may show no 

 architectural features. But in most varieties of plain muscle, 

 and especially in that of the alimentary canal, the substance of 

 the fibres is striated longitudinally. This is visible evidence of 

 the orientation of the molecules of protoplasm and metaplasm in 

 the direction of the long axis of the fibre. It shows that the 

 streaming of particles occurs along these lines. It is, as it were, 

 a diagram of the lines of force. 



Heart-muscle has been described already (p. 224). Its 

 striation, which is both transverse and longitudinal, is so 

 delicate as almost to defy microscopical analysis. The trans- 

 verse striae are the darker and more distinct. But close ex- 

 amination shows that the transverse striae do not indicate the 

 direction in which the particles of cell-subs 'ance are oriented. 

 They are oriented longitudinally. The cell is a bundle of rods 

 of substance A, embedded in substance B. The transverse 

 markings are very thin lines which cross the bundles at right 

 angles. 



The third variety of muscle is the kind by which locomotion 

 is effected. It is present in large masses all the red tissue to 

 which the term " meat " is commonly applied. It accounts for 

 about 35 per cent, of the body- weight. This kind of muscle is 

 not composed of single cells, but of compound cells, or cell- 

 complexes, termed " fibres." A fibre may attain a length of 

 upwards of 2 inches, with a breadth of about ^Jo inch. In 

 most cases the fibres are attached by one end to a bone, by the 

 other to a tendon ; and since they are shorter than the muscle 

 as a whole, the tendon commences as a membrane which 

 covers the surface of the muscle, sloping to it from the bone to 

 which by their other ends the fibres are attached. A fibre is 

 developed from a single cell. The cell elongates, its nucleus 

 divides, and the daughter-nuclei divide until several hundred 

 have been formed ; but cell-division does not follow. The 

 result is a cylindrical mass enclosed within a delicate mem- 

 branous sheath, the sarcolemma. In the early stages of its 

 development its nuclei are in the axis of the fibre, but sub- 

 sequently they are displaced outwards. In the most highly 

 specialized muscle, known as the " white " variety, they lie 

 just beneath the sarcolemma (cf. Fig. 16, B). 



172 



