the same time light. In Fig. 39 you see the thigh-bone and the bone of the arm. In both the shaft is hollow, while the large ends are chiefly made up of cells. In birds it is very necessary that the bones should be light while they are strong, so that they may not be burdensome in flying. In them, therefore, the bones are very hollow. They are so for the same reason that the stalks of tall grass and of grain are hollow. If these were solid, and therefore more slender, they would break very easily as the wind bent them over. And in constructing buildings, the architect very well knows that a hollow pillar has more strength than the same quantity of wood in a solid form.

6. The bones have a great variety of shapes according to their different uses. You see this to be true as you look at the skeleton in Fig. 40. In the skull, which holds the brain, you see the bones so shaped and arranged as to form a somewhat round box. At the under part of this box in front are bones of various shapes to accommodate the organs of four of the

Why are the bones not solid? How are the long bones at their ends! Why are the shafts of these bones hollow? Why are they very hollow n birds? What is said of the stalks of plants and the pillars of build ings!

sexves, seeing, hearing, tasting and smelling. Then there is the bone of the lower jaw, shaped something like a horse shoe. The bones of the chest form a barrel-shaped cavity for the heart and lungs. The spinal column, k, made up of twenty-four bones, extends the whole length of the body as its main pillar. To this are fastened the slender ribs which are joined to the flat breast-bone in front by means of cartilages. The spinal column, you observe, stands firmly supported upon a thick bone which is wedged in between two broad flaring bones, 7 and m. This bowl-form collection of bones, called the pelvis, supports the contents of the abdomen. You observe the large bones of the thigh and leg, which are made for firmness, and the lighter bones of the upper extremity, which are fitted for quickness and variety of motion. I will now notice some of the bones more particularly.

7. In Fig. 41

FIG. 41.

you see the bones of the head-that is, all of them that are in sight in front. There are, twenty-two bones in the head, but some of them are out of sight. Fourteen of these belong to the face. Eight belong to the cranium, that is, the part of the skull that holds the brain. Of these observe particularly the large bone of the forehead, a, called the frontal bone, the o

What is said of the shapes of bones? Describe the skeleton as represented in Fig. 40. How many bones are there in the head? How many of them belong to the cranium? Describe these as seen in Fig. 41

parietal bone, b, and the temporal bone, or bone of the temples, c. There is a bone in the rear, forming the back of the cranium, as the frontal bone forms the front. These bones, with two others on the under part of the cranium, make the round box that holds the brain.

8. The cranium is made round because it will not break as easily as it would if it were of some other shape. This is one reason also why it is made up of so many different bones, instead of being one solid, tight box. If a blow be received on the head, these bones give a little upon each other, as it is expressed, and so they are not often broken. They give more in the child than in the adult, because, besides being less brittle, they are less tightly put together. It is well that it is so; for if it were not, the skull would often be fractured, in the frequent falls which the child has.

9. The bones on the top of the head are fastened together by what are called sutures. They are locked together by little teeth of bone, which shoot by each other, as you see in Fig. 42. Here b is the suture. across the top of the head between the two parietal bones: a a is that between the two parietal bones and the frontal bone in front; and ec is that between the parietal bones and the bone at the back part of the cranium. In the young child these joinings by suture are not formed, and in the infant the bones are quite apart in some places, especially at the upper part of the forehead, as you can perceive by the touch.

Why is the cranium round? Why is it made of so many bones! What are the sutures of the skull? What is said of the joinings of the bones of the skull in the child?

FIG. 42.

a

10. The bones of the cranium, together with their coverings, are well fitted to guard the delicate brain from injury by the blows to which the head is so much exposed. When a blow is received, its force is broken by the hair, the skin, and the muscles, that extend over the bones. And then the bones themselves, as I have before said, give a little upon each other. It requires therefore a very hard blow to break the skull.

11. The spinal column is the most wonderful part of the bony machinery of the body, because it serves so many different purposes. It is the great pillar of the body, and yet it is a chain of twenty-four bones, that can be bent considerably, especially in some parts of it. And besides, there is a canal moving through all this chain of bones, in which lies securely the spinal marrow, an organ as delicate and as essen

How is the delicate brain guarded against injury? Mention the sev. eral uses of the spinal column? Of how many vertebræ is it composed!