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.

FIG. 18.

[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.