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."