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Page:Popular Science Monthly Volume 24.djvu/393

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THE SOURCE OF MUSCULAR ENERGY.

under the influence of the same exciting cause, is what exerts the power of the contracting muscle. The intensity of this shortening or contracting power has been approximately measured—e. g., by ascertaining experimentally the weight necessary to prevent a muscle from contracting under excitation.[1] The muscles are supplied with blood by the fine ramifications of the arteries, and the blood is conducted away again by the ramifications of the veins, the arterial blood losing oxygen and taking up carbonic acid during its passage, as is the case in the other tissues also.

Regarding the composition of the muscular tissue, it may be simply noted that the tissue itself is composed mainly of albuminoid material (cell-contents) and of the substance of the connective tissue, which is, like the albuminoids, composed mainly of carbon, hydrogen, oxygen, and nitrogen, and in much the same proportions. Besides this, the blood and lymph permeate the muscular tissue throughout, and certain non-nitrogenous substances, mainly glycogen, a substance resembling starch or dextrine in composition and properties, are stored up in the muscular tissue, and always found to be present. Certain other simple compounds containing nitrogen are also present, and are considered to be decomposition products of the more complex albuminoids. When the muscular contraction takes place, mechanical force may be exerted which is produced at the expense of the force stored up as potential chemical energy in the materials which serve as the fuel material. This potential energy is set free or rendered active by the chemical processes which there take place, and appears as work, as sensible heat, or as electrical disturbances.

Before we inquire as to the nature of these chemical processes, it will be of advantage to glance briefly at the results of important investigations which have been made on this subject, as these form the only safe data by which we may judge of the tenability of any theory. It would be out of place here to attempt a full reference to the mass of investigations and experiments which have been published, and which bear on the topic under discussion.[2] We shall therefore simply notice the principal facts which have been established as the results of those investigations, and which are most pertinent to the matter in hand.

The experimental researches on this subject may be classified under four heads: 1. The examination of the muscular tissue itself before

  1. This value has been found in man at about 6,000 to 8,000 grammes per square centimetre of cross-section of muscle (85 to 114 pounds per square inch) for the maximum for voluntary contraction. It is of course evident that the intensity of the force exerted varies with the kind and degree of excitation, so that too much dependence must not be placed on any particular values thus obtained. They simply give an approximate value for ordinary muscular activity.
  2. Quite full references may be found in the excellent and quite recent text-books of F. Hoppe-Seyler, "Physiologische Chemie," and of A. Gamgee, "Physiological Chemistry of the Animal Body."