XXXI. MUSCLES Muscles and Movement. We are all aware that motion in any of the higher animals is caused by the action of the muscles. These contract and expand, thus giving the required amount of movement. In man and the other vertebrate animals the muscles are fastened to bones, which, acting as levers, give wide range of motion. Study of the muscles in the leg of a frog will help us to a better understanding of the subject. pe tr ta sm Muscles of the left leg semimembrano- Study of the Muscles of a Frog. (Material preserved in 70 per cent alcohol or 4 per cent formol is best for this purpose.) Remove the skin from the leg of a frog by stripping it downward as you would take off a glove. Notice the yellowish-white muscles which form the "meat" of the frog's leg. Notice that they are more or less separated into bundles each of which tapers at the end. Look for dark-colored blood vessels and tiny white nerves which lead into the bundle of muscles. The wide part of the muscle is called the belly. The glistening white part which attaches the muscle to the bone is called the tendon. Notice the large muscle forming the calf of the leg (the gastrocnemius). Try to make out what movements are performed by means of this muscle. (Pull it and note the resulting movements.) Arrangement of Voluntary Muscles in the Human Body. Muscles are usually placed in pairs; one, called the extensor, serves to straighten the joint; the other, the flexor, bends the joint. Try to find examples of flexor and extensor muscles in the leg of the frog. Locate, by means of feeling the muscles when expanded and contracted, the extensors and flexors in your own arm. This paired arrangement of muscles is of obvious importance, a flexor muscle balancing the action of an extensor on the other side of the joint. The end of the muscle that has the wider movement in a contraction is called the insertion; the part that moves least is the origin. Muscles are named biceps or triceps (two-headed or three-headed, depending on the number of tendons found at the origin): the gastroc nemius, already referred to, is a biceps muscle; the large muscle forming the front part of the thigh is a triceps muscle. MICROSCOPIC STRUCTURE OF VOLUNTARY MUSCLE.- With a sharp pair of scissors cut through the belly of a muscle at right angles to the long axis; examination will show that it is composed of a number of bundles of fibers. These fibers are held together by a sheath of connective tissue called the perimysium (Lat. = around the muscle). Each of these bundles may be separated into smaller ones. If we continue this so as to separate into the smallest possible bits that can be seen with the naked eye, and then examine such a tiny portion under the compound microscope, it will present some A bit of voluntary muscle fiber, showing the cross striations as seen under the microscope. what the appearance of the figure. The muscle is seen to be made up of a number of tiny threads which lie side by side, held together by the perimysium. Each thread consists of several muscle fibers, each of which is inclosed in a tiny sheath. A single fiber under the compound microscope appears to have alternate light and dark bands running at right angles to its long axis. This gives the bundle of fibers a striped appearance. Nuclei are seen here and there, showing that the muscle is made up of cells. Such is the structure of muscle fibers which are under the control of the will, the voluntary muscles as they are called. If it were possible to prepare muscle fibers in such a manner as to trace the path of the nerves in these fibers, such nerves would be seen to end in little Motor nerve endings in striated muscle fibers as seen under the microscope. A bundle of involuntary muscle fibers, as seen under the microscope. knobs or plates on the muscle cells; thus, the action of every muscle fiber, perhaps every cell, is under the control of the nervous system. STRUCTURE OF INVOLUNTARY MUSCLE. - Muscles in which the movement is not controlled by the will are called involuntary. The most apparent difference between voluntary and involuntary muscle fibers (such as those found in the wall of the stomach and intestines of man) is that the latter do not show the cross striation characteristic of the former. The involuntary fibers are supplied with blood vessels and nerves. The latter, however, come from what is known as the sympathetic nervous system. Of this we shall learn more later. Fibers from the blood vessels. The act of breathing and the heart-beat STRUCTURE OF HEART MUSCLE.- Heart muscle differs slightly in appearance from both voluntary and involuntary muscle fibers. The cross striation is seen here as in voluntary muscle. The outline of the individual cells can be made out in heart muscle. (See diagram.) RATE OF MUSCULAR CONTRACTION. - A wave of muscular contraction moves rather slowly in an involuntary muscle; a little more rapidly in heart muscle; and most rapidly of all in voluntary muscle fibers. Muscle Tissue and its Uses. As we have seen in the frog, muscles evidently form a large part of Striated muscular fibers, from the heart of man, divided by transverse septa into separate nucleated portions, much magnified the body. In man, nearly half the body weight is muscle. Nearly every muscle in the human body is attached to a bone either at one or at both ends. Movement is performed by means of the muscles, leverage being obtained by means of their attachment to the bones. Movement is, indeed, the chief function of muscles. In the human body there are over five hundred muscles, varying from the size of a pinhead to a band almost two feet in length. Every movement of the body, be it merely a change of expression or change in the pitch of the voice, directly results from contraction or expansion of a muscle. Muscles also give form to the body, and are useful in protecting the delicate organs and large blood vessels within. them. STANDING. Certain muscles of the body attached to bones are used in standing. These are muscles of the legs, trunk, and neck. Certain joints must be kept rigid by the pull of flexor and extensor muscles attached to them; coöperation of muscles with the central nervous system, especially the balancing organ in the ear, must also be had. WALKING AND RUNNING. Walking is more difficult than standing. Two sets of muscles are always used, one set being extended while the other set is contracted. Walking may be considered as a series of falls in a forward direction, equilibrium being maintained by putting forward one foot; the swinging motion of the arms and swaying of the body aids in keeping this equilibrium. Running differs from walking in the fact that, when walking, one foot is always on the ground, while in running, a brief period exists when both feet are off the ground. Diagram of some of the muscles which tend to keep the body erect. Work done by Muscles. The heart is an excellent example of an organ which does muscular work. The constant pulsation of the heart during one day releases energy sufficient to raise one pound 434,112 feet from the earth's surface. During this period the heart is estimated to rest about one third of the time between the pulsations. Thus, it is evident that muscles may liberate a vast amount of energy; in order to do this, food material, which is brought by the blood to the muscles, must be oxidized. As we A bundle of involuntary muscle fibers, as seen under the microscope. knobs or plates on the muscle cells; thus, the action of every muscle fiber. perhaps every cell, is under the control of the nervous system. STRUCTURE OF INVOLUNTARY MUSCLE. Muscles in which the movement is not controlled by the will are called involuntary. The most apparent difference between voluntary and involuntary muscle fibers (such as those found in the wall of the stomach and intestines of man) is that the latter do not show the cross striation characteristic of the former. The involuntary fibers are supplied with blood vessels and nerves. The latter, however, come from what is known as the sympathetic nervous system. Of this we shall learn more later. the sympathetic system organs in the body cavity of man- the the blood vessels. The act of breathing and the heart-beat are examples of an automatic muscular action where nervous control comes largely from the sympathetic system. The churning movements of the stomach and intestines are examples of muscular action entirely beyond control of the will. STRUCTURE OF HEART MUSCLE. - Heart muscle differs slightly in appearance from both voluntary and involuntary muscle fibers. The cross striation is seen here as in voluntary muscle. The outline of the individual cells can be made out in heart muscle. (See diagram.) RATE OF MUSCULAR CONTRACTION.-A wave of muscular contraction moves rather slowly in an involuntary muscle; a little more rapidly in heart muscle; and most rapidly of all in voluntary muscle fibers. Muscle Tissue and its Uses. As we have seen in the frog, muscles evidently form a large part of Fibers from go to most Striated muscular fibers, from the heart of man, divided by trans verse septa into separate nucleated portions, much magnified |