STRIATED MUSCLE 83 



staining and refracting power, and these materials are distributed with 

 absolute. regularity and evenness. One of them, the so-called isotropic 

 substance, which does not readily stain, is probably responsible for the 

 tensile continuity of. the fibril and is also, probably, a less specialized 

 form of cytoplasm than the second, or anisotropic substance, which is 

 doubly refractive and stains in our subject a jet black. 



The anisotropic substance is deposited, in resting or relaxed sucker 

 muscle, in regular areas of the fibril, which areas are of equal length 

 and spaced equally apart. Each of these areas is designated for conven- 

 ience by the capital letter Q. Such a portion of anisotropic substance, 

 together with one half of the isotropic substance on each side of it, is 

 called a sarcous element or sarcomere. This same arrangement holds foi 

 all the other fibrils in the sucker's voluntary muscle, and where a number 

 of fibrils are grouped together in a bundle, the sarcous elements are all 

 in perfect alignment and directly opposite one another. This gives the 

 fiber a banded or cross-striped appearance, from which it gets its name, 

 striated muscle (see Fig. 81). These broad black stripes, as shown in a 

 portion of resting sucker muscle, are almost exactly two thirds the width 

 of the intervening light stripes. 



In most instances this band of the sarcous element is divided at its 

 middle by a lighter band. This is caused by the beginning of the physi- 

 ological act of contraction in which broad black bands of the sarcous 

 elements separate into two parts. Figure 81, A, shows this condition. 



Looking closely at the light stripe which lies between the broad 

 median band of the sarcous elements, we see that it is divided midway 

 by a black line into two equal parts. This line we shall call the inter- 

 mediate septum (Krause's membrane}. Referring to one of the com- 

 ponent fibrils again, it is seen that this intermediate septum is repre- 

 sented in the fibril by a dot or spot which may be called the intermediate 

 granule. A very close inspection under high power will probably show 

 that a transparent membrane connects the various intermediate granules 

 into one plane, and the whole structure forms the intermediate septum, 

 which has been called Krause's membrane from its discoverer. This 

 is very difficult to see in the sucker muscle, and may be mudi better 

 observed in our next specimen, the lobster's muscle. Turning now to a 

 section taken at right angles to the sucker's muscle fiber (Fig. 82), a 

 number of points of interest can be made out to corroborate impressions 

 formed while examining the longitudinal sections. 



The fibers are here seen to be of several sizes, and the continuous 

 outline of each sarcolemma can easily be made out. The connective- 

 tissue nuclei of the sarcolemma are sharply distinguishable from the 

 muscle nuclei which lie in the sarcoplasm of the fiber between the 

 sarcolemma and fibril bundles. 



