Nothing is easier than to mount a specimen of insect-muscle. The large water-beetle (Dytiscus marginalis) is an excellent subject. It is so easily handled. Having cut off the animal’s head, a leg is pulled out from the thorax. It is split open with a penknife, and a little of the muscle is dug out from within its hard case, placed on a clean slide, and covered with a cover-slip. If the preparation has been made quickly and cleanly, the muscle remains alive for five or ten minutes. Not only can it be studied, with the microscope, unaltered by reagents, but under the most favourable circumstances the progress along its fibres of waves of contraction can be watched. The structure of the fibres is more easily made out if a little salt-solution or white of egg is added to the preparation.
Fig. 16.—A, A Minute Portion of an Insect’s Muscle-Fibre, highly magnified.
B, White Fibre of Mammalian Muscle.
A, The nuclei are in the core of the fibre. B, The nuclei lie immediately beneath the sarcolemma. The disc on the left of this fibre, and the fibril on its right, show the two ways in which striated muscle-fibres tend to cleave. The dark line, or row of dots, is known as Dobie’s line, or Krause’s membrane. The figures are severely diagrammatic.
Striped muscle is crossed by bands, dim, bright, and dark. The sequence is as follows: Starting with the very thin dark line, which often appears as a row of dots, the next band is bright; then comes a dim band about twice as broad as the bright one; then another bright band. This sequence is repeated with extreme regularity from end to end of the fibre. Usually the bands cross the whole breadth of the fibre, although occasionally it is divided by longitudinal lines into parts in which the stratification is shifted a little backwards or forwards. A segment of a fibre comprises the substance between two dark lines—i.e., two bright bands with a dim one between them. If the muscle has been hardened in one of the fluids commonly used for the purpose of preparing tissues for the microscope, with its two ends fixed, say, by binding them to a piece of a match, so that it could not shrink, a thin clear line appears crossing the middle of the dim band. This seems to show that the fibre is not made up of single dim discs between two bright discs, but of couples, comprising half a dim disc and a bright disc. The thin dark lines indicate that the fibre is divided into compartments by transverse septa, which are probably reticulated. The appearance of a transverse line of dots, in place of a continuous line, is due to the existence of very fine longitudinal markings (it is unsafe to give them a name which connotes structure). Where the longitudinal lines cross the transverse lines, the optical effect is the appearance of a dot.
If pieces of muscle are placed in a solution of osmic acid, they become hard and brittle, and their markings are accentuated. Muscle from the claw of a crab or a lobster is very suitable for this purpose, owing to its exceptional freedom from connective tissue. After this hardening the fibres are easily separated with the aid of needles into fibrils immeasurably slender. An isolated fibril shows with extreme distinctness the alternation of dark, bright, dim, bright, dark markings already described. The appearance of a cross-section of a fibre also proves that it is a bundle of fibrils. The cut ends of the fibrils appear as dots surrounded by homogeneous substance. In this respect there is an important difference between red muscle and white. In the red fibres the fibrils are fewer and thicker than they are in white, and the embedding substance is more abundant. It is generally assumed that the homogeneous substance, sarcoplasm, is the nutrient protoplasm of the fibre, the fibrils the contractile elements. The more complete the differentiation of the fibre into fibrils, the more rapid is its action; the more abundant the sarcoplasm, the greater its capacity for continued work.
If a living muscle-fibre is observed while a wave of contraction is passing down it, the ends of the fibre being free, so that its shortening is not prevented, it is noticed that the widening of the fibre is accompanied by the thinning, even to obliteration, of the bright bands. The dim discs extend laterally, without any noticeable diminution of their thickness. It looks as if the bright discs, or something contained in the bright discs, were absorbed into the dim discs. The fibre is, as we have already pointed out, striated longitudinally. The striation is more clearly visible in the dim discs than it is in the bright ones. That the dim disc has an architectural structure absent from the bright disc is placed beyond doubt when a muscle-fibre is illuminated with polarized light. The dim disc is then found to be doubly refracting; the bright disc is not. When the prism in the tube of the microscope is placed with its axis at right angles to the axis of the prism which intervenes between the source of light and the stage of the microscope, a succession of bright bands is seen corresponding to the dim bands seen with unpolarized light. The rest of the fibre is invisible, because it has not the property of twisting the undulations of light which the lower prism has set all in the same plane. Various hypotheses as to the cause of contraction, or, to speak more correctly, as to what happens during contraction, have been based upon the thinning of the bright discs. It is assumed that the dim discs have a definiteness of structure which the bright discs do not possess. They are thought of as being traversed by pores, or as consisting of short rods. Microscopists who take the latter view believe that during contraction the more fluid substance, sarcoplasm, which occupies the bright bands is drawn into the dim bands between the short rods, or sarcostyles, which are consequently separated more widely.
No tissue could be more unsuitable than muscle for microscopic examination; for none other offers the same optical difficulties. This will be evident to anyone who considers the description already given of the markings which it exhibits. Whatever may be the true interpretation of these markings, it is clear that they point to an almost infinite multiplication of minute elements adjusted with absolute accuracy side by side and end to end. A cylinder filled with these transparent objects has to be viewed by transmitted light. The elements, whatever may be their nature, refract light in different degrees. It is impossible to eliminate the effects of internal reflection, refraction, and interference of waves of light. The most alluring hypothesis must be accepted with a considerable amount of reserve. Any fact which seems to militate against it must be taken into consideration. The view set forth above, in general terms, is very attractive to everyone who wishes to bring muscle within the category of machines. Suppose we accept the hypothesis that the dim band is a plate made of sarcostyles surrounded by sarcoplasm then the impulse which reaches a fibre causes an alteration in the surface relations of the rods to the substance in which they are embedded. Molecules of fluid from the bright bands are drawn in amongst them; the rods are pushed farther apart; the fibre broadens with a corresponding diminution in length. This brings muscular contraction into the category of the phenomena which play the most important rôle in bringing about the varied activities of the animal mechanism. Contraction is due to osmosis.
The separation of muscle into fibrils after hardening does not seem to bear out either the rod or the pore hypothesis of the structure of the dim disc. It must be remembered, however, that before the fibrils are teased apart the substance of the fibre has been coagulated. The fluid in the bright disc may thus have become as much a part of the fibril as the rod in the dim disc. The longitudinal striation of plain muscle and the appearance of continuous fibrillation in heart-muscle is more difficult to reconcile with the hypothesis that striped muscle is composed of interrupted rods.