4. My leg has two parts:

my thigh, and my lower leg;

and three joints:

my hip joint, my knee joint, my ankle joint.

5. My foot is used in standing, walking, running, skating, and jumping:

my instep, my toes, the sole of my foot, the ball, the hollow, the heel, my toe joints, and my toe nails, which protect my toes.

QUESTIONS FOR THE FORMULA.

1. Which are your limbs?

2. Tell about your arm.

3. Tell about your hand.

4. Tell about your leg.

5. Tell about your foot.

[Ill.u.s.tration: THE ELBOW JOINT.

(A hinge joint.)]

[Ill.u.s.tration: THE HIP JOINT.

(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."

How many joints has your leg?--"Three joints: my hip joint, my knee joint, my ankle joint."

What are joints?--"Bending places."

How many kinds of joints have you?--"Two: hinge joints, and ball-and-socket joints."

What kind of a joint is the shoulder joint?--"A ball-and-socket joint."

Why do you call the shoulder joint a ball-and-socket joint?--"Because at the shoulder the arm may move in any direction."

Tell how the shoulder joint is made.--"The upper end of the bone of the upper arm is rounded and fastened in a hollow place called a socket."

Which of the joints of the arm and hand are hinge joints?--"The elbow joint, the wrist joint, the thumb joint, the finger joints."

Which of the joints of the leg and foot are hinge joints?--"The knee joint, the ankle joint, the toe joint."

Which of the joints of the leg is a ball-and-socket joint?--"The hip joint."

Where is the heel?--"At the back part of the foot."

Where is the ball of the foot?--"On the sole of the foot, behind the great toe."

Where is the hollow of the foot?--"In the middle of the sole of the foot."

Where is the sole of the foot?--"On the bottom of the foot."

Where is the instep?--"Between the ankle joint and the toes."

Where is the lower leg?--"Between the knee joint and the ankle joint."

Where is the thigh?--"Between the hip joint and the knee joint."

Where is the upper arm?--"Between the shoulder joint and the elbow joint."

Where is the fore-arm?--"Between the elbow joint and the wrist joint."

Where are the toe joints?--"Between the parts of the toes."

Where are the finger joints?--"Between the parts of the fingers."

Where is the ankle joint?--"Between the lower leg and the foot."

Where is the knee joint?--"Between the thigh and the lower leg."

Where is the hip joint?--"Between the trunk and the thigh."

Where is the wrist joint?--"Between the fore-arm and the hand."