A Practical Physiology: A Text-Book for Higher Schools. Albert F. Blaisdell
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the sarcolemma. These fibers, in turn, are further bound together to form larger bundles called fasciculi, and these, too, are enclosed in a sheath of connective tissue. The muscle itself is made up of a number of these fasciculi bound together by a denser layer of connective tissue.
Experiment 17. To show the gross structure of muscle. Take a small portion of a large muscle, as a strip of lean corned beef. Have it boiled until its fibers can be easily separated. Pick the bundles and fasciculi apart until the fibers are so fine as to be almost invisible to the naked eye. Continue the experiment with the help of a hand magnifying glass or a microscope.
67. The Involuntary Muscles. These muscles consist of ribbon-shaped bands which surround hollow fleshy tubes or cavities. We might compare them to India rubber rings on rolls of paper. As they are never attached to bony levers, they have no need of tendons.
Fig. 31.--A, Muscular Fiber, showing Stripes, and Nuclei, b and c. (Highly magnified.)
The microscope shows these muscles to consist not of fibers, but of long spindle-shaped cells, united to form sheets or bands. They have no sarcolemma, stripes, or cross markings like those of the voluntary muscles. Hence their name of non-striated, or unstriped, and smooth muscles.
The involuntary muscles respond to irritation much less rapidly than do the voluntary. The wave of contraction passes over them more slowly and more irregularly, one part contracting while another is relaxing. This may readily be seen in the muscular action of the intestines, called vermicular motion. It is the irregular and excessive contraction of the muscular walls of the bowels that produces the cramp-like pains of colic.
The smooth muscles are found in the tissues of the heart, lungs, blood-vessels, stomach, and intestines. In the stomach their contraction produces the motion by which the food is churned about; in the arteries and veins they help supply the force by which the blood is driven along, and in the intestines that by which the partly digested food is mainly kept in motion.
Thus all the great vital functions are carried on, regardless of the will of the individual, or of any outward circumstances. If it required an effort of the will to control the action of the internal organs we could not think of anything else. It would take all our time to attend to living. Hence the care of such delicate and important machinery has wisely been put beyond our control.
Thus, too, these muscles act instinctively without training; but the voluntary need long and careful education. A babe can use the muscles of swallowing on the first day of its life as well as it ever can. But as it grows up, long and patient education of its voluntary muscles is needed to achieve walking, writing, use of musical instruments, and many other acts of daily life.
Fig. 32.--A Spindle Cell of Involuntary Muscle. (Highly magnified.)
Experiment 18. To show the general appearance of the muscles. Obtain the lower part of a sheep's or calf's leg, with the most of the lean meat and the hoof left on. One or more of the muscles with their bundles of fibers, fascia, and tendons; are readily made out with a little careful dissection. The dissection should be made a few days before it is wanted and the parts allowed to harden somewhat in dilute alcohol.
68. Properties of Muscular Tissue. The peculiar property of living muscular tissue is irritability, or the capacity of responding to a stimulus. When a muscle is irritated it responds by contracting. By this act the muscle does not diminish its bulk to any extent; it simply changes its form. The ends of the muscle are drawn nearer each other and the middle is thicker.
Muscles do not shorten themselves all at once, but the contraction passes quickly over them in the form of a wave. They are usually stimulated by nervous action. The delicate nerve fibrils which end in the fibers communicate with the brain, the center of the will power. Hence, when the brain commands, a nervous impulse, sent along the nerve fibers, becomes the exciting stimulus which acts upon the muscles and makes them shorter, harder, and more rigid.[10]
Muscles, however, will respond to other than this usual stimulus. Thus an electrical current may have a similar effect. Heat, also, may produce muscular contraction. Mechanical means, such as a sharp blow or pinching, may irritate a muscle and cause it to contract.
We must remember that this property of contraction is inherent and belongs to the muscle itself. This power of contraction is often independent of the brain. Thus, on pricking the heart of a fish an hour after removal from its body, obvious contraction will occur. In this case it is not the nerve force from the brain that supplies the energy for contraction. The power of contraction is inherent in the muscle substance, and the stimulus by irritating the nerve ganglia of the heart simply affords the opportunity for its exercise.
Contraction is not, however, the natural state of a muscle. In time it is tired, and begins to relax. Even the heart, the hardest-working muscle, has short periods of rest between its beats. Muscles are highly elastic as well as contractile. By this property muscle yields to a stretching force, and returns to its original length if the stretching has not been excessive.
Fig. 33.--Principal Muscles of the Body. (Anterior view.)
69. The Object of Contraction. The object of contraction is obvious. Like rubber bands, if one end of a muscle be fixed and the other attached to some object which is free to move, the contraction of the muscle will bring the movable body nearer to the fixed point. A weight fastened to the free end of a muscle may be lifted when the muscle contracts. Thus by their contraction muscles are able to do their work. They even contract more vigorously when resistance is opposed to them than when it is not. With increased weight there is an increased amount of work to be done. The greater resistance calls forth a greater action of the muscle. This is true up to a certain point, but when the limit has been passed, the muscle quickly fails to respond. Again, muscles work best with a certain degree of rapidity provided the irritations do not follow each other too rapidly. If, however, the contractions are too rapid, the muscles become exhausted and fatigue results. When the feeling of fatigue passes away with rest, the muscle recovers its power. While we are resting, the blood is pouring in fresh supplies of building material.
Experiment 19. To show how muscles relax and contract. Lay your left forearm on a table; grasp with the right hand the mass of flesh on the front of the upper arm. Now gradually raise the forearm, keeping the elbow on the table. Note that the muscle thickens as the hand rises. This illustrates the contraction of the biceps, and is popularly called "trying your muscle" Reverse the act. Keep the elbow in position, bring the forearm slowly to the table, and the biceps appears to become softer and smaller,--it relaxes.
Experiment 20. Repeat the same experiment with other muscles. With the right hand grasp firmly the extended left forearm. Extend and flex the fingers vigorously. Note the effect on the muscles and tendons of the forearm. Grasp with the right hand the calf of the extended right leg, and vigorously flex the leg, bringing it near to the body. Note the contractions and relaxations of the muscles.
70. Arrangement of Muscles. Muscles are not connected directly with bones. The mass of flesh tapers off towards the ends, where the fibers pass into white, glistening cords known as tendons. The place at which a muscle is attached to a bone, generally by means of a tendon, is called its origin; the end connected with the movable bone is its insertion.
There are about 400 muscles in the human body, all necessary for its various movements. They vary greatly in shape and size, according to their position and use. Some are from one to two feet long, others only a fraction of an inch. Some are long and spindle-shaped, others thin and broad, while still others form rings. Thus some of the muscles of the arm and thigh are long and tapering, while the abdominal muscles are thin and broad because they help form walls for cavities. Again, the muscular fibers which surround and by their contraction close certain orifices, as those of the eyelids and lips, often radiate like the spokes of a wheel.
Muscles are named according to their shape, position, division of origin or insertion, and their function. Thus we have the recti (straight), and the deltoid (Δ,