This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
In these (Fig. 38), the power is applied between the fulcrum and the weight; hence the power-arm PF, is always shorter than the weight-arm, WF. The moving force acts at a mechanical disadvantage, but swiftness and range of movement are gained ; this is the form of lever most commonly used in the body. For example, when the forearm is bent up towards the arm the fulcrum is the elbow joint (Fig. 29) ; the power is applied at the insertion of the biceps muscle into the radius; the weight is that of the forearm and hand and whatever may be held in the latter, and acts at the centre of gravity of the whole, somewhere on the far side of the point of application of the power. Usually (as in this case), the power-arm is very short, so as to gain speed and extent of movement, the muscles being strong enough to work at a considerable mechanical disadvantage. The limbs are thus also made much more shapely than would be the case were the power applied near or beyond the weight.
Fig. 38. A lever of the third order. F, fulcrum; p, power; TP, weight.
What is the mechanical gain in such levers? What is the loss? Give an example of employment of a lever of the second order in the body, pointing out fulcrum, point of action of the weight, and point of application of the power.
Describe a lever of the third order. What is lost and what gained by it? Is it often used in the body? Give an example.
Fixed pulleys are used in the body; they give rise to no loss or gain of power, but serve to change the direction in which certain muscles pull. One of the muscles of the eye-ball, for example, has its origin at the back of the eye-socket, from there it passes to the front and ends, before it reaches the eye-ball, in a long tendon. This tendon passes on to the margin of the frontal bone, which arches over the front of the eye-socket, and there passes through a ring and turns back to the eye-ball. The direction in which the muscle moves the eye is thus quite different from what it would be if the tendon went directly to the eyeball.