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.

[Illustration:  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 bursae, similarly lined and containing fluid, are also found in special places between two surfaces where much motion is required.  There are two of these bursae 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.