1873.] 



123 [Dwight. 



the view, and by the multiplicity and rapidity of the changes in pro- 

 gress. It requires much practice and the closest attention, to fix the 

 eye on a part of a fibre at rest, and to keep it there so as to note the 

 successive changes. If the contraction be very rapid it is probably 

 impossible to do so, but when it is becoming weaker it can be done 

 very fairly; but I must admit that I have not been able to satisfy my- 

 self positively on certain points to be presently noticed. This much, 

 however, is certain, namely, that with experience it is easy to follow 

 an individual black band from the state of rest to that of full con- 

 traction, which shows that the homogeneous stage (Zwischensta- 

 dium), supposed by Merkel to occur in partial contraction, has no 

 existence, and that his theory of an exchange of places between two 

 substances in closed cases is impossible. The general impression 

 given by a wave of contraction is that a part of the fibre dilates, 

 that the black bands become more prominent and, while approaching 

 each other, run with the wave along the fibre. This last appearance 

 is partly true and partly deceptive, for to be drawn together the 

 bands must really move; but the effect is exaggerated by new parts 

 in front of the wave, entering into contraction as those behind it are 

 relaxed. The substance between the black bands is evidently the 

 contractile element. Two or three bands are seen to be drawn 

 quickly nearer together, and the gray substance between them disap- 

 pears, so that there is an alternation of black and white stripes. This 

 first step of contraction occurs so rapidly that the observer is 

 scarcely aware of it before it is completed, but as the wave runs 

 along the fibre it is more easily observed. The black bands can be 

 seen to approach one another, and sometimes even the granules com- 

 posing them to get nearer together; in full contraction they are 

 sometimes apparently homogeneous, sometimes granular, but it is 

 important to notice that their edges are rarely sharply defined 

 against the white; they appear irregular and granular. The borders 

 of the fibre are so frequently seen to become scalloped, that I cannot 

 but think it the universal law, though often, owing to the proximity 

 of other fibres, the swellings cannot be seen. At the point of great- 

 est contraction the fibre is much broader than elsewhere, and there 

 is no doubt that the black bands are lengthened; nevertheless they 

 appear longer than they really are, for the bulgings at the ends of the 

 fight bands come so near together that they almost touch one another, 

 and their edges being rendered indistinct by optical effects appear, in 

 part, as prolongations of the black bands. 



