94 MICROSCOPIC CHANGES. [BOOK i. 



Muscular fibres may be examined even under high powers of the 

 microscope while they are yet living and contractile; the contraction 

 itself may be seen, but the rate at which the wave travels is too 

 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 passu 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 com- 

 parison 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 



