cular fibres without entering them; d, the motor nerves which also at first lie in the sheath and in the partitions between the compartments, but which eventually enter into the muscular fibres.

The perimysium forms a complete envelope around the muscle, which, when it is sufficiently strong to be dissected off, is known as a fascia; at each end it usually terminates in dense connective tissue (tendon), which becomes continuous with the bone or cartilage to which the tendon is attached. The partitions given off from the inner surface of the perimysium form at first coarse compartments, inclosing large bundles, each consisting of a very great number of fibres. These large bundles are again divided by somewhat finer connective tissue partitions into smaller bundles, and these again into still smaller ones, and so on, the smallest bundles of all being composed of a number of individual muscular fibres. In this way the partitions become thinner and more delicate, until those which separate the chambers in which the individual muscular fibres are contained are reduced to little more than as much connective tissue as will hold the small nerves, arteries and veins and capillary networks together. As the perimysium consists of connective tissue, it may be destroyed by prolonged boiling in water. In fact, in "meat boiled to rags" we have muscles which have been thus treated; the perimysial case is broken up, and the muscular fibres, but little attacked by boiling water, are readily separated from one another.

If a piece of muscle of a rabbit which has been thus boiled for many hours, is placed in a watch-glass with a little water, the muscular fibres may be easily teased out with needles and isolated. Such a fibre will be found to have a thickness of somewhere about 60μ (they vary, however, a great deal), with a length of 30 or 40 millimetres, i.e. about 1 inch. It is a cylindroidal or polygonal solid rod, which either tapers or is bevelled off at each end. By these it adheres to those on each side of it; or, if it lies at the end of a series, to the tendon.

The structure and properties of striated muscular tissue in the histological sense means the structure and properties of these fibres.

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

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; ", 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

m

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