1.Besides a head and a trunk or body,the higher animals have four limbs.Birds have two wings and two legs,quadrupeds have four legs,and we have two legs and two arms.But in their frame-work these different kinds of limbs are very much alike.
2.The arm joins the body at the shoulder,and the shoulder itself is formed of two bones-the collar-bone in front,and the shoulder-blade behind.The collar-bone does not go round the neck,as its name might lead you to think.It is nearly straight,and has one end joined to the top of the breast-bone,just below the throat,and the other end to the top of the shoulder.
3.The shoulder-blade is a broad flat bone which rests on the ribs behind.Its outer corner meets the end of the collar-bone and forms the top of the shoulder.The arm hangs from this corner of the shoulder-blade,and is also joined to the collar-bone.The collar-bone gives the square shape to the shoulder when looked at from the front.
4.Between the shoulder and the elbow there is only one bone in the arm,but between the elbow and the wrist there are two.When you hold out your arm,and turn the palm of the hand first upwards and then downwards,you can feel those two bones twisting round each other.
5.The bones of the hand are arranged so as to give it great strength and yet allow of very free movement.In the wrist there are eight small bones,set in two rows across.They are very firmly bound together,but their large number allows the wrist to bend much more freely than if there were only one or two.
6.Next come the bones of the hand itself.In the body or palm of the hand there are five long bones-one for each finger,and one for the thumb.Then each of the fingers has three bones,and the thumb has two.Thus we have as many as twenty-seven pieces in the frame-work of the hand and wrist alone.
7.At its lower end the back-bone is fastened to a broad and strong mass of bone,to which the lower limbs are also joined.The bones of the leg and foot are very like those of the arm and hand.They are larger and stronger,as they have the whole weight of the body to carry,and the joints do not move so easily.
8.The joints of the limbs are very beautifully formed.The bones are bound together by bands of a very strong substance,somewhat like the sinews which you find in meat.The ends of the bones which rub against each other are very smooth,and are covered with a substance called cartilage or gristle,which is much softer than the rest of the bone.There is a kind of oil which is made by the body itself at the places where it is needed,and some of this oil is constantly poured over the parts of the bone which touch each other.
9.These joints are just like the hinges we make for ourselves:the parts are firmly joined,quite smooth,and well oiled,so that they move easily.Indeed,they are so like hinges that we call many of them hinge-joints.The elbow-joint is a hinge-joint.
10.The shoulder-joint is of a different kind.The top of the upper arm bone is round like a ball,and it movesin a shallow cup or socketin the end of the shoulder-blade.Thus you can swing your arm round in a circle,and move it up and down,or from back to front.This kind of joint is called a ball-and-socket joint.