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of the contractile substance appears regular, and is easily
recognized. Each fibre consists of a delicate case of thin, elastic, homogeneous membrane, forming a sheath called
a sarcolemma, within which the essential contractile substance is enclosed. The soft contractile substance completely fills and distends the elastic sarcolemma, so that when the latter is broken its contents bulge out or escape. After death, particularly if preserved in weak acid (HCI), the striation becomes marked, and the dead and now rigid contractile substance easily be broken up into transverse plates or discs.
Besides the transverse striation, a longitudinal marking can be seen in the muscle fibre, which indicates the subdivision of the contractile substance into thin threads called primitive fibrillæ. Each primitive fibril shows a transverse marking, corresponding with the transverse striation, which divides the fibrils into short blocks called sarcous, or muscle elements. These markings, and the transverse striations of the muscle fibre in general, depend on different parts of the contractile substance having different powers of refraction, and giving the
appearance of dark and light hands. Two fibres of striated muscle, In the muscle fibre are found long
substance (m) has been rup- granular masses like protoplasm ; these tured and separated from the sarcolemma (a) and (s); are the nuclei of the contractile sub(B) shows a thin strip of
substance stance. They must not be confounded markings are clearer:
with the nuclei of the sarcolemma, which nuclei, (A) space under sarcolemma. (Ranvier.)
are much more numerous along the edge
torn contractile in which the
of the fibre, or with the other short nuclei seen in such numbers between the fibres, which indicate the position of the capillary vessels.
It is stated that each striated muscle fibre has a nerve fibre passing directly into it, but the exact details of the mode of union in mammalia are not yet satisfactorily made out.
PROPERTIES OF MUSCLE IN THE PASSIVE STATE. Consistence.-The contractile substance of muscle is so soft as to deserve rather the name fluid than solid ; it will not drop as a liquid, but its separate parts will flow together again like half-melted jelly. In this respect it resembles the protoplasm of some elementary organisms, the buds from which are so soft that they can unite around foreign bodies and yet have sufficient consistence to distinguish them from fluid.
Chemical Composition.—The chemical constitution of the contractile substance of muscle in the living state is not accurately known. The death of the tissue is accompanied by certain changes of a chemical nature which give rise to a kind of coagulation, resulting in the formation of two substances, viz., muscle serum and muscle clot or myosin. This coagulation can be postponed almost indefinitely in the contractile substance of the muscles of cold blooded animals, by keeping the muscle after its removal at about 5° C. In this way a pale yellow, opalescent, alkaline juice may be pressed out of the muscle, and separated on a cold filter. This substance turns to a jelly at freezing point, and if brought to the ordinary temperature of the room it passes through the stages of coagulation seen in the contractile substance of dead muscle, and gives the same fluid serum and clot of myosin. Since a frog's muscle can be frozen and thawed without the tissue being killed, it is supposed that the thick juice is really the contractile substance, which has been called muscle plasma.
The coagulation of muscle plasma reminds us in many ways of the clotting of the blood plasma, but the muscle clot, or myosin, is gelatinous and not in threads like fibrin.
It is a globulin, and is soluble in 10 per cent. solution of salt. It is
readily changed into syntonin or acid albumin, and forms the preponderant albuminous substance of muscle.
The serum of dead muscle has an acid reaction, and contains three distinct albuminous bodies coagulating at different temperatures, one of which is serum-albumin, and another a derived albumin, potassium-albumin. The serum of muscle also contains : (1) Kreatin, kreatinin, xanthin, etc. (2) Hæmoglobin. (3) Grape sugar, muscle sugar, of inosit, and glycogen. (4) Sarcolactic acid. (5) Carbonic acid. (6) Potassium salts; and (7) 75 per cent. of water. Traces of pepsin and other ferments have also been found.
1. Shows graphically the amount of extension caused by equal weight increments applied
to a steel spring 2. Shows graphically the amount of extension caused by equal weight increments applied
to an india-rubber band. 3. The same applied to a frog's muscle. Showing the decreasing increments of extension :
the gradual continuing stretching, and the failure to return to the abscissa when the weight is removed.
Chemical Change.—In the state of rest a certain amount of chemical change constantly goes on, by which oxygen is taken from the hæmoglobin of the blood in the capillaries, and carbonic acid is given up to the blood. These changes seem necessary for the nutrition, and therefore the preservation of the life and active powers of the tissue, because if a muscle after removal be placed in an atmosphere free from oxygen, it more quickly loses its chief vital character, viz., its irritability.
Elasticity. --Striated muscle is easily stretched, and, if the extension be not carried too far, recovers very completely its original length. That is to say, the elasticity of muscle is small or weak, but very perfect. When a muscle is stretched to a given extent by a weight-say of i gramme-if another gramme be then added, it will not stretch the muscle so much as the first did ; and so on if repeated gramme weights be added one after the other, each succeeding gramme will cause less extension of the muscle than the previous one; so that the more a muscle is stretched the more force is required to stretch it to the given extent, or, in other words, the elastic force of muscle increases with its extension.
If a tracing be drawn, showing the extending effect of a series of equal weights attached to a fresh muscle, it will be found that a great difference exists between it and a similar record drawn by inorganic bodies or an elastic band of rubber.
When a weight is applied to a muscle, it does not immediately stretch to the full extent the weight is capable of effecting, but a certain time, which varies with circumstances, is required for its complete extension. The rate of extension is at first rapid, then slower, until it ceases. As a muscle loses its powers of contraction from fatigue, it becomes more easily extended. Dead muscle has a greater but less perfect elasticity than living, i.e., it requires greater force to stretch it, but does not return so perfectly to its former shape. The importance of the elastic property of muscle in the movements of the body is noteworthy. The muscles are always in some degree on the stretch (as can be seen in a fractured patella, the fragments of which remain far apart and cause the
surgeon much anxiety), and brace the bones together like a series of springs, the various skeletal muscles being so arranged as to stretch others by their contraction. When one muscle--for example, the biceps-contracts, it finds an elastic antagonist already tense, and has to shorten this antagonist as it contracts itself. The triceps in this case acts as a weak spring, opposing the biceps, and it gently returns to its natural length when the contraction of the biceps ceases. The muscles are kept tense and ready for action by their mere
elasticity, and have to act against a gentle spring-like resistance, so that the motions occur evenly, and there is no jarring or jerking, as might take place if the attachments of the inactive muscles were allowed to become slack.
Electric Phenomena.—In a living muscle electric currents may be detected, having a definite direction, and certain relations to the vitality of the tissue. As they seem to be invariably present in a passive muscle, they have been called natural muscle currents.
They are generally studied in the muscles of cold-blooded animals after removal from the body. The muscle is spoken of as if it were a cylinder, with longitudinal and transverse sur
Non-polarizable Electrodes. The glass tubes (a a) contain sulphate of zinc solution
(2. s.), into which well amalgamated zinc rods dip. The lower extremity is plugged with china clay (ch. c.), which protrudes at (c') the point. The tubes can be moved in the holders (h h), so as to be brought accurately into contact with the muscle. (Foster.)
faces corresponding to its natural surface and its cut extremities. In such a block of frog's muscle the measurement of the electric currents requires considerable care, because they are so difficult to detect that a most sensitive galvanometer must be used ; and such an instrument can easily be disturbed by currents due to bringing metal electrodes into contact with the moist saline tissues. Specially constructed electrodes must be used to avoid these currents of polarization taking place in the terminals touching the muscle. These are called non-polarizable electrodes, and may be made on the following plan: Some innocuous