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.
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 50 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: (i) Kreatin, kreatinin, xanthin, etc. (2) Haemoglobin. (3) Grape sugar, muscle sugar, of inosit, and glycogen. (4) Sar-colactic acid. (5) Carbonic acid. (6) Potassium salts; and (7) 75 Per cent, of water. Traces of pepsin and other ferments have also been found.
Fig. 180. 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 10 return to the abscissa when the weight is removed.
In the state of rest a certain amount of chemical change constantly goes on, by which oxygen is taken from the haemoglobin 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.