262 



THE BODY AT WORK 



hard case, placed on a clean slide, and covered with a cover- 

 slip. If the preparation has been made quickly and cleanly, the 

 muscle remains alive for five or ten minutes. Not only can it 

 be studied, with the microscope, unaltered by reagents, but 

 under the most favourable circumstances the progress along 

 its fibres of waves of contraction can be watched. The 

 structure of the fibres is more easily made out if a little salt- 

 solution or white of egg is added to the preparation. 



Striped muscle is crossed by bands, dim, bright, and dark. 

 The sequence is as follows : Starting with the very thin dark line, 

 which often appears as a row of dots, the next band is bright ; 

 then comes a dim band about twice as broad as the bright one ; 



Sarcotemma 



'Dim Band. 



cleus,. 

 -BflghlBand. 



CoTvtheirra Jfreas. 



'Fbril 



FIG. 16. A, A MINUTE PORTION OF AN INSECT'S MUSCLE-FIBRE, HIGHLY MAGNIFIED. 

 B, WHITE FIBRE OF MAMMALIAN MUSCLE. 



A, The nuclei are in the core of the fibre. B, The nuclei lie immediately beneath the sarcolemma. 

 The disc on the left of this fibre, and the fibril on its right, show the two ways in which 

 striated muscle-fibres tend to cleave. The dark line, or row of dots, is known as Dobie's 

 line, or Krause's membrane. The figures are severely diagrammatic. 



then another bright band. This sequence is repeated with 

 extreme regularity from end to end of the fibre. Usually the 

 bands cross the whole breadth of the fibre, although occasion- 

 ally it is divided by longitudinal lines into parts in which the 

 stratification is shifted a little backwards or forwards. A 

 segment of a fibre comprises the substance between two dark 

 lines i.e., two bright bands with a dim one between them. 

 If the muscle has been hardened in one of the fluids commonly 

 used for the purpose of preparing tissues for the microscope, 

 with its two ends fixed, say, by binding them to a piece of a 

 match, so that it could not shrink, a thin clear line appears 

 crossing the middle of the dim band. This seems to show that 

 the fibre is not made up of single dim discs between two bright 



