long bones, besides being hollow, are also spongy. They are generally much larger at each extremity, and the spongy, or cell-like appearance, is there much more apparent. Towards, and at, the middle part, they are smaller, firmer, and contain fewer of these little cells.

All the bones in the body are very hard on the outside. Perhaps the teeth are the most so. The inside of the teeth is not much harder than other bones; but the outside is coated with a substance called enamel, which is very hard indeed, and which preserves them from being softened, or injuriously acted upon, by vitiated states of the saliva; the acids which we take with our food; the corroding effects of the atmosphere—or other causes.

Particular description of the Bones.-You have already been told, that the long, round bones such as the humerus, or bone of the arm, and the femur, or bone of the thigh, are hollow, and contain marrow in their cavities. This marrow nearly, or altogether, fills up the hollow spaces. * These hollow spaces are lined by an exceedingly thin and delicate membrane, which also runs in among the marrow. The same sort of fine membrane also lines the cells contained in the spongy bones. These cells have a small quantity of liquid in them, and none of them appear to be entirely empty.

Most of the bones are pierced through their outsides, with one or more holes of considerable size, through each of which is carried an artery, to convey blood to nourish the bones, and a vein comes out by the same aperture, to bring back the blood when it has fulfilled its office. You may wonder that I should talk about blood

*This is true of the bones of most other animals besides man. The bones of many birds, however, are entirely hollow, and contain air, to assist them in flight.

in the bones. But there is blood in them, though not a great deal. This blood, with its vessels, the nerves, and the membranous linings, together with the marrow, and liquid matters, amount to many pounds in weight: for after the bones of any animal have been thoroughly dried, and all moisture extracted from them, they become diminished to almost half the weight they originally were. The entire bones of the human body, when perfectly dry, weigh from eight to twelve pounds.

When they appear entirely dry, if you burn them in a hot fire for a long time, you will lessen their weight a great deal more: I believe about one-half. What is burnt out is the animal substance, principally composed of gelatine, or a material very similar to glue. The half which remains is principally lime, combined with an acid, forming phosphate of lime, with which some portion of carbonate of lime, or chalk, is mixed.

The great purpose which the Creator doubtless had in view, in giving us such a frame-work of strong bones, was, that it might support and give solidity to the soft and fleshy parts. Suppose now that there were no bones, and that the whole body was a mass of flesh only. Would not the legs give way, and finally be crushed down, under the great weight of the body? would not the arms fail in the duties required of them? Most certainly they would.

important uses for bones, Some of them, however,

But there are several other which might be mentioned. you would not very readily understand till you know a little about muscles and tendons, which are the moving powers. I will, therefore, for the present omit them.

Growth of Bone. We are not born with the bones as hard as they become, after we begin to walk and to run about. At first many of them are very soft, and

large number of them are in several pieces, with cartilage or gristle between them. After a few years they grow firmly together. The bones of the head, in particular, are at first separate; and without doing any injury to the soft and delicate structure of the brain, contained within them, admit of some little motion, overlapping each other, as it were. But, after we become older, and the whole skull has acquired firmness and solidity, it would require a very considerable force to remove them from each other, and the consequences of so doing, if we were enabled to effect it, would be highly dangerous. There is, undoubtedly, life, as it is often called, (though we hardly know what life is) in bones. While we continue in good health, and the functions of the body are duly performed, there is not much feeling in them; though in many cases of disease, they are endued with exquisite sensibility. When the surgeon amputates a limb, the proceeding of sawing through the bone, is the least painful part of the operation, though people in general are apt to imagine that it is highly so..

Vessels in Bones.-There are also many minute blood-vessels and nerves running about in every direction, through very small channels in the interior of the bones. That blood is certainly conveyed through and into bones can be made very evident, by forcing, by means of a suitable apparatus, compositions made of liquid wax and other substances, and coloured to represent blood.

Another method is also used to show that blood circulates through bones. If a rabbit, or other small animal, be fed upon madder-root, in a short time the bones will be found to be tinged with the colouring principle of the madder.

We are now, I think, prepared to go on with our studies on the frame-work of the House.

16

CHAPTER IV.

SILLS OF THE HOUSE.

MY READERS, I dare say, know, that in building a house, strong pieces of timber are laid upon the walls, where openings are to be made, for the purpose of supporting, or, as the builders call it, carrying, the weight of that, which is to be above the opening. These pieces are I called sills, and they not only form a base the necessary uprights may be placed, but they answer the further purpose of holding together both the upper and lower parts of the building, in their true positions.

Situation

upon

which

of the Hip-bones.-The sills of the

House I Live in' are two large irregular bones, placed at the top of what I have called, for convenience sake, the pillars. These two large bones are very firm and strong. explanations of their shape without it, that I will show You will find so much difficulty in understanding my

you a picture of them.

These bones are called in books the ossa innominata. Os is a Latin word for bone; and ossa is its plural, meaning more bones than one. Innominatum, of which innominata is plural, means without a name, or nameless:

and the word innominata makes a tolerable name, though rather long. So if a very young child, found in the

streets, whom nobody knew, should be called Peter Nameless, that word nameless, would imply that no specific name had been given to him.

Structure. I have said that the ossa innominata are very firm and strong. They are so in grown persons— but in a child they are less so, and are in three pieces, each of which has a different name. They are joined together in front, by a firm gristle or cartilage. Behind, however, is a strong wedge-like bone placed between them. Between this last bone, called the sacrum, and each of the ossa innominata, there is also a very strong gristle; but it is not so thick or strong as the one I have just mentioned, as situated in the front. The ossa innominata and sacrum together, make a kind of cup, or deep bowl-open at the bottom it is true, but still bowl-like in its shape. This bowl is called the pelvis.

Hip-joint.—The manner of fastening, the thigh-bone, or femur, to the deep hollow of the innominatum, is very remarkable. I shall give a particular account of it, with an engraving, farther along in the book; so that a few words must answer for the present.

The hollow where the femur is fastened, is shaped like the inside of an egg-shell, with the small end broken off, and has received the name of acetabulum, from its supposed resemblance to the cup with which the ancients measured vinegar. The round end of the femur is fastened into this deep cavity, by a very large and strong cord of a sinewy nature. The shoulder is often dislocated, or detached from its situation; but this hollow is so deep, and the cord so strong, that nothing but very great violence will break it, or slip the femur out of its place.

An abuse.—I have said that these two great bones

C