Page:The American Cyclopædia (1879) Volume XII.djvu/76

 68 MUSCLE bands, apparently homogeneous or finely gran- ular, about yoVfr of an incn in di ameter ? with an elongated nucleus in the central part of each one. The fibres are arranged in paral- FIG. 4. Transverse Section of a Voluntary Muscle, showing the bundles of Muscular Fibres and intervening layers of Areolar Tissue, and the external Fibrous Expansion. lei layers, their pointed extremities interlock- ing with each other, so as to form membra- nous expansions surrounding the cavities of the internal organs. Thus the oesophagus, the stomach, the intes- tines, the bladder and urinary passages, the uterus and Fallopian tubes, the excretory ducts of the glandular organs, and the ar- teries and veins, all have their muscular coat, composed of un- striped fibres, and lia- ble to contraction and relaxation indepen- dently of the will. An exception to the rule that involuntary mus- cular organs are com- posed of unstriped fibres is found in the heart and in the great veins immediately con- tiguous to it. Here the muscular fibres belong to the striped variety, but they are smaller than those of volunta- ry muscles, their stri- ations are less distinct, and they also present the peculiarity of branching and inosculating with each other, which is not seen in other striated muscular fibres. In all probability the difference in structure between the two kinds of fibres, strictly speaking, has reference to their mode of contraction, rather than to its voluntary or involuntary character. The con- traction of the striped muscular fibres is prompt, vigorous, and rapidly followed by relaxation, as in the voluntary muscles and the heart ; that of the unstriped fibres is generally sluggish, grad- ual, and continued, as in the peristaltic action FIG. 5. Unstriped Muscular Fibres, highly magnified, from the walls of the Kenal Vein. of the alimentary canal. Striped fibres have been found in all vertebrates and in articulates ; as we descend the animal scale the movements become more and more automatic, until com- plex muscular action gives place to simple cili- ary vibration. The contractility of muscle de- pends on an inherent property, independent of, though capable of modification by, nervous in- fluence. The stimuli which induce contraction are volition, emotion, impressions conveyed to the nervous centres and involuntarily reflected thence, and various physical and chemical agents applied to any portion of the course of a motor nerve or to the muscular fibres. A muscle in action becomes shorter and thicker, changing its relative proportions without any actual change in bulk. After death muscles become fixed and rigid, a condition constituting the rigor mor- tis. In the active contractions which charac- terize muscles on the application of stimulus, force is exerted against some opposing power ; this is attended with exhaustion or fatigue, and requires intervals of rest. Sustained contrac- tion consists of an infinite number of partial momentary contractions acting in succession. There are altogether in the human body 527 distinct muscles, of which 261 are in pairs, and 5 single on the median line ; of these there are in the head and face 83, the orbicularis oris being single; in the neck 49, the arytenoid of the larynx being single ; in the thorax 78, the triangularis sterni and the diaphragm being single ; in the abdomen 33, the sphincter ani being single ; in the back 78 ; in the upper extremities 98, and in the lower 108. Yet, with all this complex apparatus, everything is in perfect order and harmony. Matteucci and Du Bois-Reymond have investigated the elec- tric currents of muscles. The combination of the muscular movements is in most cases so far independent of the will, that we are apt to lose sight of their perfection ; but let paralysis affect one side of the body or contraction draw up a muscle, and the fact becomes at once evi- dent, as may be seen every day in palsy of one side of the face, or strabismus with the turn- ing in or out of the eye. The simple process of walking, performed it may be unconsciously, with its nice adjustments executed by the au- tomatic guidance of the senses rather than by any act of the will, is what the most ingenious mechanician can never effect in an automaton, from the impossibility of harmonizing the many acts which constitute walking. The energy and rapidity of muscular contraction is more remarkable in the lower animals than in man. The muscular power of insects is seen in the rapid flight of the dragon fly, the leap of the flea and the cricket, the fixed attitudes of some larvae, and the strength of beetles. It is very great in the flight of birds, though their whole structure is organized for aerial motion ; the power of the wings is three times as great as that of the legs in ordinary birds, and their absolute power in proportion to the weight of the body is as 10,000 to 1 ; in small birds the