Heart-muscle differs from ordinary striped muscle by having shorter oblong cells, which are branched. There is no sarcodemma and the cells contain but one or two nuclei, which are situated in the centre of the muscle-substance. Blood-vessels, lymphatics and nerves are plenti ful in the heart-muscle.
Groups of these muscle-cells, with connective tissues, tendons and fat, make up the gross muscles of the human body. The voluntary muscles are all attached to bony structures; the involuntary muscles are found in the softer parts. Contraction and expansion are the ex pressions of their functions, each set of muscles being provided with antagonists, and it is char acteristic that in response to pleasure-giving stimuli movements of expansion result, whereas tinder painful stimuli contraction is marked. Modern psychological theories have been founded on this fundamental, principle, emo tional states being interpreted as being founded on visceral muscular activities. The forms of external stimuli that can cause muscular action are usually classed as mechanical, chemical, thermal and electrical. Excess of stimulus brings about a condition of fatigue in muscle. This is accompanied by diminished muscular power, by pain or discomfort, by diminished reflex excitability and by vague symptoms in the body indicative of some perversion of metabolism.
Muscles are classified in various ways, ac cording to structure or according to their func tion, or by their positions and situations in the body. For example, some muscles are attached to bones, which they move after the fashion of levers. Such muscles are said to arise or take origin from definite points of bones and are generally inserted into bones by tendinous pro longation of the muscular substance. The in sertion is the moving point, and the origin the fixed point of the muscle. The tendons of muscles vary in length and breadth. They rep resent inelastic bands of fibrous tissue, the fibres of which insensibly merge into and become continuous with their attached muscular fibres. When the tendinous fibres of muscles become greatly broadened out, so as to form fibrous webs or membranes, which separate or enclose muscles, or which afford extensive surfaces for their attachments, the term aponeuroses is then applied to them. Such aponeurotic expansions are seen in the terminations of the muscles of the abdominal wall, in the scalp and in other situations. The limit or extent of the action of a muscle is determined by the length of its fibres, whilst its degree of force or strength depends on the number of the fibres. Other muscles are not attached to bones as levers, but on the contrary surround and enclose cavities, which they limit or expand as required. Such
hollow muscles are exemplified in the heart and uterus, in the muscular fibres of blood-vessels, in the muscles of the digestive tract, in the iris of the eye, etc.
The nature, mode and effects of muscular action may be briefly considered in connection with the present subject. The muscles which have the most active functions are those most abundantly nourished. Every action on the part of a living being results in the production of a certain amount of waste material, evinced by perceptible differences in the chemical com position of the tissue. And when it is remem bered that the nervous and vascular supply of muscle is also concerned in muscular work and waste, the entire question is seen to assume aspects of a very intricate and complicated nature. Increased exercise of muscles — as seen in gymnastic exercises, or in the exercise of certain trades (for example, the arms of the blacksmith and the lower limbs of the ballet dancer)— demanding increased nutrition, re sults in the increased growth of the muscle and in the formation of new tissue. This re sult, it is evident, can take place only when the nutrition of the tissue keeps pace with or slightly outstrips its waste and wear.
The property of contractility distinctive of muscular tissue, and through which its func tions are manifested, is generally, though not always or invariably, brought into action through the stimulus of the nervous system, or more widely speaking, through stimuli con veyed to the muscular fibres through the nerves. The subject of the various kinds of muscular actions involves both physiological and me chanical considerations. The voluntary muscles thus constitute moving powers for the bones as levers; and in the living body examples of the three kinds of levers which mechanical science distinguishes are found. In the familiar action of the biceps muscle, which flexes or bends the fore upon the upper arm, is seen an instance of a lever of the third kind, in which the power (represented by the insertion of the muscle on the radius or bone of the fore-arm) is placed between the fulcrum (at the elbow-joint) and the weight (in the hand). The lever of the second order may be illustrated by the raising of the body upon the toes, as in the act of making a step forward in walking. Here the weight (represented by the body pressing on the ankle) is placed between the fulcrum (formed by the fixed toes) and the power rep resented by the muscles of the calf. The head moving on the spine illustrates a lever of the first order; the fulcrum being represented by the atlas vertebra, the power by the muscles of the neck and the weight by the heavier por tion of the skull situated in front of the spine. See ANATOMY.