MUSCLE

The muscles of animals, anatomically as well as physiologically considered, are divisible into two separate and distinct systems (1) those which form the chief bulk of the walls of the hollow viscera and of the blood-vessels; (2) those which are attached to the bones. Muscles of the first kind are not subject to the will, and are therefore characterised as involuntary; they are composed of long nucleated cells which are not striped; hence such muscle is characterised as smooth or non-striated. Muscles of the second kind are under the control of the will, and are therefore called voluntary; they are

composed of fibres which are transversely striped; hence such muscle is also characterised as striated. Between these two typical kindsthe smooth involuntary and the striated voluntary-a third kind must be reckoned, viz. cardiac muscle, which is intermediate in its anatomical and physiological characters. Voluntary striated muscle is further distinguishable in many animals into two varieties,

viz. pale and red. The pale variety is the more highly organised; it exhibits more marked striation and less abundant nuclei than

the red variety.

Involuntary muscle is chiefly supplied by non-medullated nerve-fibres, but also receives a smaller proportion of medullated fibres. The actual termination of nerve-fibre in involuntary muscle is not known with certainty; abundant networks of fibres are to be seen, and in connection with them ganglionic cells. Voluntary muscle is chiefly supplied by medullated nerve-fibres, which form definite structures at their junction with the muscular fibres, the most characteristic of these being motor end-plates.

Histological characters of resting, of contracting, and of rigid muscle. The ultimate structure of a striped muscle-fibre has been for long a vexed question. Upon one important point all observers are agreed, viz. that the sarcolemma, or sheath of the fibre, contains contractile solid particles (sarcous elements) and a fluid non-contractile portion. That the fibre is not transversely

divided by a series of partitions (Krause's membrane) is considered to be proved by Kühne's observation of a nematode worm moving freely in a living muscle-fibre without meeting with any apparent resistance until coagulation had occurred. By microscopic observation of a tetanised muscle-fibre, or of a musclefibre fixed in a state of partial contraction by osmic acid, it has been found that the shortening and thickening of the fibre are attended with an obvious approximation of its transverse striæ, and with a by no means obvious reversal of the optical relations of the striæ, the dark broad band of the resting muscle becoming the light narrow band of the contracting muscle. Viewed by polarised light, there is no such transposition; the dark band of

ances.

The accompanying very diagrammatic figure is intended only as a guide to the description given in the text, and not in any degree to imitate the actual appearThe odd numbers 1, 3, 5, point to contractile substance, dark in A, light in C; light in B and D (i.e. doubly refracting or anisotropous). The even numbers, 2, 4, 6, point to non-contractile fluid, light in A, dark in C; dark in B and D i.e. singly refracting, or isotropous).

an uncontracted fibre and the light band of a contracted fibre are both doubly refracting (anisotropous) and appear as light bands in a dark field. We must, however, make the express reservation that these observations refer in both cases to dead muscle, and that it is nowise certain that the partially contracted parts of muscles fixed by osmic acid give the true picture of a natural contraction. And, as a matter of fact, it is only dead muscle which exhibits under polarised light the regular and well-marked alternation of doubly-refracting light band and singly-refracting dark band in living muscle almost the whole of the substance is doubly refracting; singly refractile substance in the form of lines and dots is relatively scanty. By the aid of the phototome described on p. 311, it has been found possible to observe the

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optical changes during the successive phases of a contraction, in the thin hyoglossus muscle of the frog; the contractile bands have been seen to become increased, then diminished, then increased again, in the course of a single contraction (Korányi and Vas).

Chemical composition of muscle: its alteration by activity and in rigor.-Muscle-or, to call it by its popular names, flesh or meat-is mainly composed of water and of proteid; the elementary composition of dry flesh is thus closely similar to that of dry proteid or of dry blood (vide p. 10). Besides its proteid constituents, muscle contains a comparatively small quantity of other nitrogenous bodies, of inorganic salts, and of carbohydrates; a variable amount of fat is usually associated with it, but is not, properly speaking, a constituent of pure muscle. The average composition of muscle is as follows:

Water
Proteid

Other nitrogenous bodies (creatin, xanthin)
Salts (phosph. and K)

Carbohydrates (glycogen, inosit, lactic acid, sugar).

The red colour of muscle is partly due to blood-pigment, partly to a special pigment-myohæmatin (MacMunn); ferments are also present-viz. the myosin ferment and a peptic ferment.

We have to study (1) the chemical differences between living and dead muscle; (2) the chemical differences in dead muscle, which before death was at rest or in forced activity.

The consistence of a living irritable muscle differs from that of dead muscle which has entered into 'rigor mortis'-the former is soft and elastic, the latter is firm and very imperfectly elastic-doughy.' The difference depends upon the fact that living muscle consists of separate solid and liquid parts-the sarcous elements and muscle-plasma-while in dead muscle the liquid part (plasma) has undergone coagulation and has yielded myosin. The rigor or coagulation of muscle is, indeed, closely analogous with the coagulation of blood: it depends upon the conversion of a body present in muscle-plasma (myosinogen) into myosin, just as the coagulation of blood depends upon the conversion of a body present in blood-plasma (fibrinogen) into fibrin, and it is probable that in the case of muscle, as in that of blood, the conversion is effected by the agency of a ferment. The myosin coagulum exhibits the following remarkable difference from the fibrin coagulum: both are soluble in 10 per cent.

solution of sodium chloride, the first-named body the more readily; the dissolved myosin yields a renewed coagulum when its salt solution is diluted; not so the dissolved fibrin (Halliburton). The myosin coagulum having formed is thus easily unmade and remade out of the body; perhaps this is also the case in the body.

Preparation of muscle-plasma.-The coagulation of muscleplasma is delayed by cold. The fresh, irritable muscles, preferably of frogs, are accordingly frozen, minced, pounded, and pressed, all instruments used being cooled; a syrupy alkaline liquid is expressed, which is coagulable at ordinary temperatures or on dilution with cold distilled water. This liquid is muscleplasma, and the body that separates from it when coagulation occurs is myosin. The fluid that remains after removal of the coagulum is muscle-serum, which is acid, and contains in solution. (1) muscle-globulin, coagulable at a temperature of 63°; (2) muscle-albumin, coagulable at 73°.

Preparation of myosin.-If muscle-plasma is allowed to fall drop by drop into distilled water, myosin coagulates at once, and is obtained in the form of little balls. Myosin may also be obtained from rigid muscle in which it has already formed, by pounding with sodium chloride, then adding water to make a 10 per cent. solution of the amount of salt used; myosin is soluble in this, and is precipitated by pouring the solution thus obtained into a considerable quantity of water, or by dialysis. Myosin belongs to the class of globulins, and is as such insoluble in distilled water, soluble in solutions of neutral salts (NaCl, MgSO,, Na,SO1); in solution it coagulates when heated to between 55° and 60°.

Sarcolactic acid.-Living resting muscle has a neutral or alkaline reaction; dead rigid muscle has an acid reaction; muscleplasma is alkaline, muscle-serum is acid. The difference is due to sarcolactic acid produced during rigor, i.e. accompanying though not necessarily caused by the conversion of myosinogen into myosin. We have already seen that the rigor of muscle is accompanied with a production of carbon dioxide (pp. 137-8); we are about to learn that the same two changes also accompany the normal activity of muscle. But whereas in rigor the acidification is the more prominent phenomenon, in normal action the chief event is the production of CO2; these considerations suggest as probable that the lactic acid (C,H,O,) produced in muscular katabolism is a stage towards the production of CO,;

in full action muscle may produce lactic acid, and then decompose it further into CO2 ; in declining action muscle may produce lactic acid, and fail to complete the process.

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Inosit, or muscle-sugar (CH12206, 2H,O), is a minute and variable constituent of muscle. Unlike grape-sugar, it has neither rotating action on polarised light, nor reducing action upon Fehling's solution; it is a crystalline body that can undergo lactic but not alcoholic fermentation, and is now recognised, not to be a true carbohydrate, but to belong to a benzoic series (see p. 588). Rigor mortis as manifested by the human body is of medicolegal interest, as yielding some indication of the time at which death may be presumed to have occurred. From observations on tetanised muscles and on hunted animals we learn that previous muscular activity hastens the advent of rigor; from observations on man-deaths by accident, or in hospitals, or in battle-we learn that the death-stiffening' begins earliest in a person surprised in active exertion at the time of death; on worn-out bedridden patients it begins early, is ill-developed, and soon passes off. It usually begins two or three hours after death, and lasts for two or three days, but soldiers have been found in attitudes indicative of its immediate onset, and it sometimes does not commence before several hours after death. The order of its manifestation on the human subject is from above downwards, in the jaw, neck, arms, and legs. After a period varying from a few hours to a few days, rigor passes off, and in a reverse order, by resolution of the coagulated myosin, and now putrefaction sets in.

Differences between active and resting muscle.-There are only two known chemical differences between living resting and living active muscle, viz. the reaction and the amount of CO2— living resting muscle is neutral or faintly alkaline in reaction, living active muscle is slightly acid, the acidity, like that of rigor, being due to sarcolactic acid. In its chemical effects, activity is thus comparable with partial death, and we may add to the comparison by the observation that whereas death is attended by the complete and final coagulation of myosin, prolonged activity is marked by an incompleteness of relaxation, which may be— though it cannot be proved to be-due to a temporary coagulation of myosin. The similarity of the electrical change, which accompanies the excited and the dying states, and the evolution of heat which accompanies contraction and rigor, may be alluded to as further points of resemblance. Fick offers as the most

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