THE TENDONS.--The ends of the muscles are generally attached to the bone by strong, flexible, but inelastic tendons. [Footnote: The tendons may be easily seen in the leg of a turkey as it comes on our table; so we may study Physiology while we pick the bones.] The muscular fibers spring from the sides of the tendon, so that more of them can act upon the bone than if they went directly to it. Besides, the small, insensible tendon can better bear the exposure of passing over a joint, and be more easily lodged in some protecting groove, than the broad, sensitive muscle. This mode of attachment gives to the limbs strength, and elegance of form. Thus, for example, if the large muscles of the arm extended to the hand, they would make it bulky and clumsy. The tendons, however, reach only to the wrist, whence fine cords pass to the fingers (Fig. 16).
Here we notice two other admirable arrangements. 1. If the long tendons at the wrist on contracting should rise, projections would be made and thus the beauty of the slender joint be marred. To prevent this, a stout band or bracelet of ligament holds them down to their place. 2. In order to allow the tendon which moves the last joint of the finger to pass through, the tendon which moves the second joint divides at its attachment to the bone (Fig. 16). This is the most economical mode of packing the muscles, as any other practicable arrangement would increase the bulk of the slender finger.
[Illustration: _Muscles of the Right Eye:_ A, _superior straight,_ B, _superior oblique passing through a pulley,_ D; G, _inferior oblique,_ H, _external straight, and, back of it, the internal straight muscle._]
Since the tendon can not always pull in the direction of the desired motion, some contrivance is necessary to meet the want. The tendon (B) belonging to one of the muscles of the eye, for example, passes through a ring of cartilage, and thus a rotary motion is secured.
[Illustration: _The three classes of Levers, and also the foot as a Lever._]
THE LEVERS OF THE BODY. [Footnote: A _lever_ is a stiff bar resting on a point of support, called the _fulcrum_ (_F_), and having connected with it a _weight_ (_W_) to be lifted, and a _power_ (_P_) to move it. There are three classes of levers according to the arrangement of the power, weight, and fulcrum. In the first class, the _F_ is between the _P_ and _W_; in the second, the _W_ is between the _P_ and _F_; and in the third, the _P_ is between the _W_ and _F_ (Fig. 18). A pump handle is an example of the first; a lemon squeezer, of the second; and a pair of fire tongs, of the third. See "Popular Physics," pp. 81-83, for a full description of this subject, and for many illustrations.]--In producing the motions of the body, the muscles use the bones as levers. We see an illustration of the _first class_ of levers in the movements of the head. The back or front of the head is the weight to be lifted, the backbone is the fulcrum on which the lever turns, and the muscles at the back or front of the neck exert the power by which we toss or bow the head.