After a bone has been moved by the contraction of a muscle, it is brought back to its position by the contraction of another muscle on the opposite side, the former muscle meanwhile being relaxed. Muscles thus acting in opposition to each other are called antagonistic. Thus the biceps serves as one of the antagonists to the triceps, and the various flexors and extensors of the limbs are antagonistic to one another.
71. The Tendons. The muscles which move the bones by their contraction taper for the most part, as before mentioned, into tendons. These are commonly very strong cords, like belts or straps, made up of white, fibrous tissue.
Tendons are most numerous about the larger joints, where they permit free action and yet occupy but little space. Large and prominent muscles in these places would be clumsy and inconvenient. If we bend the arm or leg forcibly, and grasp the inside of the elbow or knee joint, we can feel the tendons beneath the skin. The numerous tendons in the palm or on the back of the hand contribute to its marvelous dexterity and flexibility. The thickest and strongest tendon in the body is the tendon of Achilles, which connects the great muscles in the calf of the leg with the heel bone (sec. 49).
When muscles contract forcibly, they pull upon the tendons which transmit the movement to the bones to which they are attached. Tendons may be compared to ropes or cords which, when pulled, are made to act upon distant objects to which one end is fastened. Sometimes the tendon runs down the middle of a muscle, and the fibers run obliquely into it, the tendon resembling the quill in a feather. Again, tendons are spread out in a flat layer on the surface of muscles, in which case they are called aponeuroses. Sometimes a tendon is found in the middle of a muscle as well as at each end of it.
Fig. 34.--The Biceps Muscle dissected to show its Tendons.
72. Synovial Sheaths and Sacs. The rapid movement of the tendons over bony surfaces and prominences would soon produce an undue amount of heat and friction unless some means existed to make the motion as easy as possible. This is supplied by sheaths which form a double lining around the tendons. The opposed surfaces are lined with synovial membrane,[11] the secretion from which oils the sheaths in which the tendons move.
Little closed sacs, called synovial sacs or bursæ, similarly lined and containing fluid, are also found in special places between two surfaces where much motion is required. There are two of these bursæ near the patella, one superficial, just under the skin; the other deep beneath the bone (Fig. 29). Without these, the constant motion of the knee-pan and its tendons in walking would produce undue friction and heat and consequent inflammation. Similar, though smaller, sacs are found over the point of the elbow, over the knuckles, the ankle bones, and various other prominent points. These sacs answer a very important purpose, and are liable to various forms of inflammation.
Experiment 21. Examine carefully the tendons in the parts dissected in Experiment 18. Pull on the muscles and the tendons, and note how they act to move the parts. This may be also admirably shown on the leg of a fowl or turkey from a kitchen or obtained at the market.
Obtain the hoof of a calf or sheep with one end of the tendon of Achilles still attached. Dissect it and test its strength.
73. Mechanism of Movement. The active agents of bodily movements, as we have seen, are the muscles, which by their contraction cause the bones to move one on the other. All these movements, both of motion and of locomotion, occur according to certain fixed laws of mechanics. The bones, to which a great proportion of the muscles in the body are attached, act as distinct levers. The muscles supply the power for moving the bones, and the joints act as fulcrums or points of support. The weight of the limb, the weight to be lifted, or the force to overcome, is the resistance.
74. Levers in the Body. In mechanics three classes of levers are described, according to the relative position of the power, the fulcrum, and the resistance. All the movements of the bones can be referred to one or another of these three classes.
Levers of the first class are those in which the fulcrum is between the power and the weight. The crowbar, when used to lift a weight at one end by the application of power at the other, with a block as a fulcrum, is a familiar example of this class. There are several examples of this in the human body. The head supported on the atlas is one. The joint between the atlas and the skull is the fulcrum, the weight of the head is the resistance. The power is behind, where the muscles from the neck are attached to the back of the skull. The object of this arrangement is to keep the head steady and balanced on the spinal column, and to move it backward and forward.
Fig. 35.--Showing how the Bones of the Arm serve as Levers.
P, power;
W, weight;
F, fulcrum.
Levers of the second class are those in which the weight is between the fulcrum and the power. A familiar example is the crowbar when used for lifting a weight while one end rests on the ground. This class of levers is not common in the body. Standing on tiptoe is, however, an example. Here the toes in contact with the ground are the fulcrum, the power is the action of the muscles of the calf, and between these is the weight of the body transmitted down the bones of the leg to the foot.
Levers of the third class are those in which the power is applied at a point between the fulcrum and weight. A familiar example is where a workman raises a ladder against a wall. This class of levers is common in the body. In bending the forearm on the arm, familiarly known as "trying your muscle," the power is supplied by the biceps muscle attached to the radius, the fulcrum is the elbow joint at one end of the lever, and the resistance is the weight of the forearm at the other end.
Experiment 22. To illustrate how the muscles use the bones as levers. First, practice with a ruler, blackboard pointer, or any other convenient object, illustrating the different kinds of levers until the principles are familiar. Next, illustrate these principles on the person, by making use of convenient muscles. Thus, lift a book on the toes, by the fingers, on the back of the hand, by the mouth, and in other ways.
These experiments, showing how the bones serve as levers, may be multiplied and varied as circumstances may require.
75. The Erect Position. The erect position is peculiar to man. No other animal naturally assumes it or is able to keep it long. It is the result of a somewhat complex arrangement of muscles which balance each other, some pulling backwards and some forwards. Although the whole skeleton is formed with reference to the erect position, yet this attitude is slowly learned in infancy.
In the erect position the center of gravity lies in the joint between the sacrum and the last lumbar vertebra. A line dropped from this point would fall between the feet, just in front of the ankle joints. We rarely stand with the feet close together, because that basis of support is too small for a firm position. Hence, in all efforts requiring vigorous muscular movements the feet are kept more or less apart to enlarge the basis of support.
Now, on account of the large number and flexibility of the joints, the body could not be kept in an upright position without the cooperation of certain groups of muscles. The muscles of the calf of the leg, acting on the thigh bone, above the knee, keep the body from falling forward, while another set in front of the thigh helps hold the leg straight. These thigh muscles also tend to pull the trunk forward, but in turn are balanced by the powerful muscles of the lower back, which help keep the body straight and braced.
The head is kept balanced on the neck partly by the central position of the joint between the atlas and axis, and partly by means of strong muscles. Thus, the combined action of these and