14 H. E. JORDAN 
is still longer than in fiber B, but the sarcomere as a whole is 
shorter. In fiber D appear successively later phases in the for- 
mation of the contraction band (levels 2’, 3, 4, 5, and 6). The 
sarcomere has become reduced to less than half the length of 
that of the relaxed fiber. The contraction band consists of the 
fused opposite halves of successive Q discs, and the two involved 
accessory discs, the whole bisected by a telophragma. That the 
horizontal seriation as here given is correct is proved by the 
fact that the same phases follow in the same order in passing 
from top to bottom of fiber D, which is still relaxed above but 
fully contracted below. 
Three additional important data are contributed by this 
illustration. The sarcolemma of the contracted fiber (D) is 
not normally festooned. ‘The second datum relates to the modi- 
fications produced in the structure of a fiber by stretching. <A 
fiber may be stretched at any phase of contraction. This simple 
fact is of prime importance; it may not safely be ignored in the 
interpretation of any particular fiber when judging of its fune- 
tional condition on morphologic grounds. In fiber D, by reason 
of the condition of full contraction at the lower end, the middle 
portion, which is only at midphase of contraction, has become 
greatly stretched. The effect of this stretching influences 
chiefly the length of Q. Comparison of the length of the sarco- 
meres of this portion of the fiber with the length of the sarcomeres 
in an unstretched fiber in the relaxed condition, in disregard of 
the special condition obtaining in the case of the former fiber, 
would lead to the erroneous conclusion, widely held at the 
present time, that the presence of an H disc is an indication of 
stretching superimposed upon a sarcomere in repose. This 
matter will also be further discussed below. The portion of 
fiber D, here under consideration, discloses a third important 
fact, namely, that the apparent absence of a mesophragma may 
be an illusion, for the reason that its structure is too delicate to 
come within the limits of microscopic vision. The longitudinal 
tension imposed upon the middle portion of this fiber by the 
stretching relaxes the horizontal tension under which the meso- 
phragma exists in the resting and especially in the contracted 
