(A ball-and-socket joint.)]

Some joints, as those of the skull, are immovable; some, as those of the
spine, may be moved a little; and others more or less freely, as those of
the limbs. In machines, the parts which move upon each other need to be
oiled, to keep them from wearing out; but the joints of our bodies oil
themselves with a thin fluid, called _synovia_. This fluid resembles the
white of an egg, and comes from a smooth lining inside of the joints. The
ends of the bones which form joints are covered by gristle or _cartilage_,
and are fastened together by very strong, silvery white bands, called
_ligaments_. A sprain is caused by overstretching or tearing some of these
ligaments.

* * * * *

QUESTIONS ON THE LIMBS AND JOINTS OF THE BODY.

What is the trunk of your body?--"All the body but the head and limbs."

Which are your limbs?--"My two arms and my two legs."

How many limbs have you?--"Four."

How many parts has your arm?--"Two parts: my upper arm and my fore-arm."

How many parts has your leg?--"Two parts: my thigh and my lower leg."

How many joints has your arm?--"Three joints: my shoulder joint, my elbow
joint, my wrist joint."