Lever arm

“ II. Lever of second class. III. Lever of third class. F. Fulcrum. P. Power. W. Weight. a. Power-arm. b. Weight-arm. Application to the Body.—In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the [pg 252] body, or things to be lifted, serve as weights. For these levers to increase the motion of the muscles, it is necessary that the muscles be attached to the bones near the joints, and that the parts to be moved be located at some distance from the joints. ”

“ In the human machine the levers of the arm have been fashioned, not for climbing, but for work of another kind—the kind which brings us a livelihood. We must have perfect control over our hands; the longer the lever of the forearm is made, the more difficult does control of the hand become. Hence, in the human machine the forearm is made relatively short and the upper arm long. ”

“ If such a set of holds is lacking altogether, he will be employing press holds with the hands, lever holds with arm or leg, twisted pressure-thrusts from his foot and ankle against one wall to his knee against the other—any device that will relieve him of direct pulls and will leave his body free to help him at any second with some balance adjustment. For, once the body is really jammed up against the rock or in a crack, arms and legs are usually working only against one another or [Pg 166] against the friction of shoulders or thighs, and the mechanism can merely struggle and exhaust itself. ”

“ This gives the muscle greater play than in us, and enables it to lift, more slowly perhaps, but more efficiently, for the arm-bone between the joint and the place where the muscle is attached, is the long arm of a lever which is shorter in man. Below the globular head of the arm-bone is the shaft or cylindrical part of the bone which gives origin to the three-headed muscle called triceps, and is covered by the two-headed one (biceps) already mentioned, besides the deltoid. ”

“ Separate the parts, open the sheath of the long tendon of the biceps muscle, and draw out and hold it on one side with a blunt hook. Rotate the arm outward, (if it will rotate,) to facilitate the division of the tendon of the sub-scapularis; then rotate the arm inward, to aid in the division of the tendons of the supra-spinatus, infra-spinatus, and teres minor muscles, inserted into the great tuberosity. Complete the division of the capsular ligament, push the bone through from below, using the arm as a lever if you can, and saw it off. No arteries of consequence are wounded. ”

“ When an upward pressure is applied to the foot of the lever a, it causes the upper arm c d of the elbow piece b c d to describe an arc of about a quarter of an inch from left to right, thereby communicating a corresponding movement to the pair of levers e e, working on the pivots f f ”

“ A pang of dreadful pain shot through me as I fell; my arm was rebroken. After this I knew no more; all the accumulated excitement of the evening bore down with one fell swoop upon my brain. Ere day broke, I was delirious.
I have a vague and indistinct remembrance of hurried and anxious faces around my bed, of whispered words and sorrowful looks; but my own thoughts careered over the bold hills of the far west as I trod them in my boyhood, free and high of heart, or recurred to the din and crash of the battle-field, with the mad bounding of the war-horse, and the loud clang of the trumpet. ”

“ But along with the extension a broad lever is to be used to force the bone from without inward, the lever being placed at the nates or a little farther up, and some person is to steady the patient's body, so that it may not yield, either by grasping him at the buttocks with his hands, or this may be effected by means of another similar lever, adjusted to one of the grooves, while the patient has something laid below him, and he is secured, and the dislocated thigh is to be turned gently from within outward at the knee. ”

“ No matter what part of the human body we examine, we shall find that its mechanical work is performed by means of bony levers. Having seen how the head is moved as a lever of the first order, we are now to choose a part which will show us the plan on which levers of the second order work, and there are many reasons why we should select the foot. It is a part which we are all familiar with; every day we can see it at rest and in action. The foot, as we have already noted, serves as a lever in walking. ”

“ It is true that the bones of the arch are loosely bound together by passive ties or ligaments, but as these cannot be lengthened or shortened at will, Nature had to fall back on the use of muscular engines for the maintenance of the foot as an arched lever. Some of these are shown in Fig. 8. The foot, then, is a lever of a very remarkable kind; all the time we stand or walk, its rigidity, its power to serve as a lever, has to be maintained by an elaborate battery of muscular engines all kept constantly at work. No wonder our feet and legs become tired when we have to stand a great deal. ”

“ The musculo-spiral nerve, g, winds under the brachial artery at the middle of the arm, but as this nerve passes deep between the short and long heads of the triceps muscle, P, and behind the humerus to gain the outer aspect of the limb, a little care will suffice for avoiding the inclusion of it in the ligature. ”

“ Besides these peculiarities of the hands and feet, which, moreover, are supplied in every joint with tendons and muscles of great motive power, the fore-arm is capable of turning the wrist forwards or backwards, or, as it [Pg 213] is called, rotating, and also of bending. Again, the upper arm is loosely but firmly attached to the shoulders and neck, so as to admit of great range of motion, so what with the bending and rotation of the fore-arm, and the mobility and cushioned state of the fingers, these creatures possess a wonderful apparatus, suited for extreme action and safe holding on. ”

“ But, in actual work, thrusting movements are nearly always made with the elbow bent to a right angle or almost so, then the pressure transmitted through the forearm piece is borne almost entirely by the steels of the armlet.
3. Resistance to Rotation.—A well-adjusted artificial arm cannot rotate on the limb because—
(1) The forearm is elliptical in section and not circular, this is specially so in the lower third.
(2) Flexion of the elbow is only possible if the artificial joint is in the same plane as the axis of the elbow joint—that is, the sagittal plane. ”

“ For a leg hanging like a flail, or shortened by some inches, is not so good for purposes of locomotion as a wooden leg is, while an arm, even though powerless at the elbow, and perhaps much shortened, can be so strengthened and supported by slings and bandages as to give a most useful hand, the complex movements and uses of the fingers of which no mechanism can at all imitate. ”

“ The machine, as represented by Fig. 8, is brought to the desired elevation, about as high as the shoulders, and the hand-holder is attached. One arm is extended horizontally, and the hand grasps the hand-holder, while rapid motion is given by turning the wheel. An alternate twisting motion is communicated to the arm, which causes corresponding pressure and relaxation of all the soft tissues of the limb, combined with slight rubbing or attrition. The action is increased by contracting the muscles, and also by grasping at greater distance from the center. ”

“ For the back stroke therefore, and that the arm may not only bend at the elbow, but also extend and straighten itself with force, other muscles, the longus, and brevis brachiæus externus, and the aconæus, placed on the hinder part of the arms, by their contractile twitch, fetch back the fore-arm into a straight line with the cubit, with no less force than that with which it was bent out. The same thing obtains in all the limbs, and in every moveable part of the body. ”

“ At the psychologic moment the right foot is advanced half a pace, the left leg straightened into a brace, the right arm, with its extended cutlass, stiffened to a bar of steel—and the thing is done. The keen two-foot blade, slipping between the shoulder blade and the first rib, shores its way through heart and lungs, and its point may even penetrate to the abdominal cavity. ”

“ All forms of the lever and all the principal kinds of hinges are to be met with in our own frames. The valvular arrangements of the blood-vessels are unapproached by any artificial apparatus, and the arrangements for preventing friction are so perfect that two surfaces will play on each other for fourscore years or more and never once trouble their owner by catching or rubbing so as to be felt or heard. ”

“ Weight.
Fig. 118—Diagram of the foot lever. F. Fulcrum at ankle joint. W. Body weight expressed as pressure against the earth. While the muscle power acts through the distance ab, the fulcrum support (body) is forced through the distance FE. Loss of Muscular Force.—Using a small spring balance for measuring the power, a light stick for a lever, and a small piece of metal for a weight, and arranging these to represent some lever of the body (as the [pg 254] forearm) , it is easily shown that the gain in motion causes a corresponding loss in muscular power. ”

“ The ipselateral reflex is flexion at shoulder, elbow and wrist. The contra lateral fore limb at the same time is extended at shoulder, elbow and wrist. When the reflex spreads to the hind limbs the hind limb of the same side is extended at hip, knee and ankle, that of the crossed side is sometimes flexed at hip, knee and ankle, but sometimes is instead extended at hip, knee and ankle. The reflex sometimes spreads to the neck, causing the head to be turned toward the fore limb, which is the seat of the stimulation. ”

“ Thus, for instance, if the left arm is being stroked upward with the right hand, the muscles of the right hand are not only exercised, but likewise those of the left arm and shoulder, the latter are especially put into play because the left arm is resisting the pressure of the right hand upon it. ”

“ The evils attendant upon being over-bowed are various: the left (bow) arm, wrist, and elbow, the fingers of the right (loosing) hand and its wrist, are strained and rendered unsteady; the pull becomes uncertain and wavering, and is never twice alike; the whole system is overworked and wearied ”

“ Several of the muscles in the forearm run into tendons in the wrist because if the muscle part were to extend along the wrist, this part of the arm would be large and clumsy instead of graceful and slender. ”

“ And see, under the big deltoid, which fits round the shoulder like an epaulette and pulls the arm up, is the scapular group, things like tidily sorted skeins, thick on the shoulder-blades, diminished to a tendon string at their insertion in the arm; their business is to pull the arm back, in opposition to the big pectoral muscle which pulls it forwards. ”