92 MICROSCOPIC CHANGES. [BOOK i. 



rapid to permit satisfactory observations being made as to the 

 minute changes which accompany the contraction. It frequently 

 happens however that when living muscle has been treated with 

 certain reagents, as for instance with osmic acid vapour, and sub- 

 sequently prepared for examination, fibres are found in which a 

 bulging, a thickening and shortening, over a greater or less part of 

 the length of the fibre, has been fixed by the osmic acid or other 

 reagent. Such a bulging obviously differs from a normal contraction 

 in being confined to a part of the length of the fibre, whereas, as 

 we have said, a normal wave of contraction, being very much longer 

 than any fibre, occupies the whole length of the fibre at once. We 

 may however regard this bulging as a very short, a very abbre- 

 viated wave of contraction, and assume that the changes visible in 

 such a short bulging also take place in a normal contraction. 



Admitting this assumption, we learn from such preparations 

 that in the contracting region of the fibre, while both dim and 

 bright bands become broader across the fibre, and correspondingly- 

 thinner along the length of the fibre, a remarkable change takes 

 place between the dim bands, bright bands, and granular lines. 

 We have seen that in the fibre at rest the intermediate line in 

 the bright band is in most cases inconspicuous ; in the contracting 

 fibre, on the contrary, a dark line in the middle of the bright 

 band in the position of the intermediate line becomes very distinct. 

 As we pass along the fibre from the beginning of the contraction 

 wave, to the summit of the wave, where the thickening is greatest, 

 this line becomes more and more striking, until at the height 

 of the contraction, it becomes a very marked dark line or thin 

 dark band. Pari passa with this change, the distinction between 

 the dim and bright bands becomes less and less marked; these 

 appear to become confused together, until at the height of the 

 contraction, the whole space between each two now conspicuous 

 dark lines is occupied by a substance which can be called neither 

 dim nor bright, but which in contrast to the dark line appears 

 more or less bright and transparent. So that in the contracting 

 part there is, at the height of the contraction, a reversal of the 

 state of things proper to the part at rest. The place occupied 

 by the bright band, in the state of rest, is now largely filled 

 by a conspicuous dark line which previously was represented 

 by the inconspicuous intermediate line, and the place occupied 

 by the conspicuous dim band of the fibre at rest now seems 

 by comparison with the dark line the brighter part of the fibre. 

 The contracting fibre is like the fibre at rest striated, but its 

 striation is different in its nature from the natural striation of 

 the resting fibre ; and it is held by some that in the earlier phases 

 of the contraction, while the old natural striation is being replaced 

 by the new striation, there is a stage in which all striation is lost. 



We may add that the outline of the sarcolemma, which in the 

 fibre at rest is quite even, becomes during the contraction indented 



