35. The general physical and chemical characters of muscle and its more conspicuous vital properties have been already dealt with (Lesson VII. § 4), so that it remains only to speak of those characters which are revealed by microscopic investigation.

As we have already had occasion to remark, all tissues undergo considerable alteration in passing from the living to the dead state, but, in the case of muscle, the changes

FIG. 105.

A. Part of a muscular fibre (of a frog) seen in a natural condition. d, dim bands; b, bright bands, with the granular line seen in many of them; n, nuclei and the granular protoplasm belonging to them, very dimly seen. B. Portion of prepared mammalian muscular fibre teased out, showing longitudinal portions of variable (1. 2. 3. 4.) thickness; 4 represents the finest portion (fibrilla) which could be obtained; d, dark bands; b, bright bands, in the midst of each of which is seen the granular line g.

which the tissue undergoes in dying, are of such a marked character that the structure of the dead tissue gives a false notion of that of the living tissue.

A living striated muscular fibre of a frog or a mammal is a pale transparent rod composed of a soft, flexible, elastic substance, the lateral contours of which, when the fibre is viewed out of the body, appear sharply defined, like those of a glass rod of the same size; but when

the fibre is observed in the living body, bathed in the lymph which surrounds it, the outlines are not so sharply defined. In neither case can any distinct line of demarcation between a superficial layer and a deeper substance be recognised. The fibre appears transversely striped, as if the clear glassy substance were, at regular intervals (Fig. 105, A. d), converted into ground glass, thus appearing dimmer. Each of these "dim bands " is about 2μ wide, and the clear space or "bright band" which separates every two dim bands is of about the same size, or under ordinary circumstances somewhat narrower. With a high power a very thin dark granular line equidistant from each dim band is discernible in each bright band, dividing the bright band into two. As these appearances remain when the object glass is focussed through the whole thickness of the fibre, it follows that the dim bands, the granular lines, and the clear spaces on each side of each granular line, represent the edges of segments of different optical characters, which regularly alternate through the whole length of the fibre. Let the excessively thin segments, of which the thin granular lines represent the edges, be called g, the thicker, pellucid segments of which the bright bands on each side of a granular line represent the edges, B; and the thickest slightly opaque segments of which the ground glass like dim bands are the edges, D. Then the structure of the fibre may be represented by D. B. g. B. D. B. g. B. indefinitely repeated, and one inch of length of fibre will contain about 30,000 such segments, or alternations of structure.

In a perfectly unaltered living fibre the striated substance presents hardly any sign of longitudinal striation; but near to the surface of the fibre in mammalian muscle, though at various points in the depth of the fibre in the muscles of the frog, faint indications are to be observed of the existence of cavities each filled by a nucleus, surrounded by a small amount of protoplasm (Fig. 105, A. n). These are the so-called muscle corpuscles.

As the muscular fibre dies it undergoes a rapid alteration :-a, parallel longitudinal striæ, often less than 2μ apart, appear in greater or less numbers until sometimes the striated substance appears broken up into a mass of fine delicate fibres; b, the dim bands become much more

opaque, and hence the transverse striation appears better marked, until the dim bands may appear like sharply defined discs; c, the nuclei acquire sharp irregular contours and become much more conspicuous, and c especially under certain circumstances and after particular treatment, a thin superficial layer becomes sharply separated from the deeper substance of the fibre as a membrane of glassy transparency, the sarcolemma, which ensheathes the striated and fibrillated substance.

FIG. 106.-CAPILLARIES OF STRIATED MUSCLE.

A. Seen longitudinally. The width of the meshes corresponds to that of an ultimate fibre. a, small artery; b, small vein.

B. Transverse section of striated muscle. a, the cut ends of the ultimate fibres; b, capillaries filled with injection material; c, parts where the capillaries are absent or not filled.

The bright bands and the granular lines, on the other hand, undergo little alteration.

Under very high powers each granular line looks like a number of minute granules coherent into an extremely attenuated plate, the margins of which are attached to the sarcolemma.

If the sarcolemma of a dead fibre be torn with needles, the striated substance breaks up in different ways according to the treatment to which the fibre has been

previously subjected. It may break up into discs, each of which contains a dim band. Or it may break up into fibrils, each of which presents the same segmentation as the whole fibre. These artificial fibrils vary much in thickness according to mode of preparation and the skill of the operator; they may sometimes be obtained of exceeding fineness (Fig. 105, B.). Transverse sections of muscular fibre, which have been frozen while perfectly fresh, present minute close-set circular dots, which appear to represent the transverse sections of naturally existing longitudinal fibrils. If the muscle substance is

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S

FIG, 107.-A MUSCULAR FIBRE (OF FROG) ENDING IN TENDON. The striated muscular substance, m, has shrunk from the sarcolemma, s, the fibrils of the tendon, t, being attached to the latter.

really in this case unaltered the only possible interpretation of the fact is that the fibre is really made up of fibrils, and that these are invisible in the living muscle on account of their having the same refractive power as the interfibrillar substance. But whether the finest artificial fibrils into which dead muscle may be broken up are identical with these apparently natural fibrils, it is not at present certainly determined. In some cases the artificial fibrils seem smaller than the natural ones, as if the latter, like the fibre itself, were capable of longitudinal cleavage.

These are the most important structural appearances

presented by ordinary striated muscle. But it may further be noticed that the dim bands exert a powerful depolarising influence on polarised light. Hence when a piece of muscle is placed in the field of a polarising microscope and the prisms are crossed so that the field is dark, these bands appear bright. The granular lines have a similar but very much less marked effect.

36. As in the case of the preceding tissues so in that of muscle, the place of the adult tissue is occupied in the embryo by a mass of closely applied, undifferentiated nucleated cells. As development proceeds, some of these cells are converted into the tissues of the perimysium, but others increasing largely in size gradually elongate and take on the form of more or less spindle-shaped rods or fibres. Meanwhile the nucleus of each cell repeatedly divides, and thus each rod becomes provided with many nuclei, so that each fibre is really a multi-nucleate cell. Along with these changes the protoplasmic substance of the original cell becomes, for the most part, converted into the characteristically striated muscle substance, only a little remaining unaltered around each nucleus as a muscle corpuscle.

37. The many-nucleated cell metamorphosed into a muscular fibre is nourished by the fluid exuded from the adjacent capillaries, and it may be said to respire, insomuch as its substance undergoes slow oxidation at the expense of the oxygen contained in that fluid, and gives off carbonic acid. It is, in fact, like the other elements of the tissues, an organism of a peculiar kind, having its life in itself, but dependent for the permanent maintenance of that life upon the condition of being associated with other such elementary organisms, through the intermediation of which its temperature and its supply of nourishment are maintained.

The special property of a living muscular fibre, that which gives it its physiological importance, is its peculiar contractility. The body of a colourless blood corpuscle, as we have seen, is eminently contractile, insomuch as it undergoes incessant changes of form. But these changes take place at all points of its surface, and have no definite relation to the diameter of the corpuscle, while the contractility of the muscular fibre is manifested by a

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