BONE, in its ordinary state, is a hard inflexible body, in the human species of a whitish colour, without sensibility or contractility. and very little subject to decay. It serves as a defence and a support to the soft parts of the body, either affording them a solid case, in which they are lodged and protected from injury, as takes place with re spect to the brain and lungs, or as a basis to which the different parts may be attached, as is the case with the muscles. The bones are also fixed points, against which the muscles react when they commence their contrac tions ; they form a system of levers, by which the move ments of the body are effected, and they very essentially contribute to the function of locomotion, by the share which they have in the formation of the joints. The to tal number of bones is about 260, exhibiting every varie ty of figure and size, according to the structure and uses of the parts in which they arc found. They may, howe ver, be arranged under two great divisions, the broad and flat, and the long and round bones ; to the first class be long the bones of the skull, and to the second those of the arms and legs.
These two kinds of bones differ not merely in their ex ternal shape, which may be conceived to be an incidental circumstance, and one of little importance, but likewise in the more essential points of the mode of their growth and their mechanical structure. They also serve very different uses in the animal economy ; the long bones are adapted for the purposes of motion, either enabling us to shift our position from place to place, or to act upon other bodies that are contiguous to us, while the object of the flat bones is simply to protect the parts which they in close.
It would be carrying us far beyond the proper subject of this article to enter into any minute description of the individual bones, but it may be proper to make a few ge neral observations upon this part of the animal fabric, and especially to show how admirably each of its individual parts is adapted to its particular uses. For this purpose we cannot take a better example than that of the upper and lower extremities. In the human subject, the arms are obviously intended, nor for support, but for i prehen sion. They are therefore attached to the trunk n such a
manner as to be easily applied to contiguous bodies in all directions, the upper part admitting of free motion, but at the same time possessing considerable strength, while the extremity is formed of an assemblage of small bones, that are so connected together as to form an apparatus by which we can execute all the necessary offices of life with a degree of quickness and accuracy, which would be al most inconceivable, were we not so familiar with it.
The lower extremities are equally fitted for their spe cific object, the support of the body and its progressive motions. They are so situated as to hear the weight of the body in the most advantageous manner, the feet af ford a firm basis to the pillars which are placed upon them, while its smaller parts possess that degree of mo tion upon each other, which is the best calculated for pro moting this end, without exhibiting that variety of minute actions, which are necessary for the hand and fingers, but would have been inconsistent with the greater firmness that is requisite in the parts destined to support the whole body.
The form and structure of the joints are among the most interesting parts of the animal economy. Techni cally speaking, every part is styled an articulation where two bones unite together, but at present we shall only notice those which are moveable, where the bones are united by ligaments, or other membranous bodies of a flexible nature, so as to he capable of changing their rela tive position. They present a great variety of forms, but they may be all reduced to two divizions, the ball and socket, and the hinge. In the first kind of joint the move able bone is furnished with a round end, which a corresponding hollow in the fixed bone, while in tLe hinge, both the bones are furnished with processes and depressions, which are mutually adapted to each other ; the hip and shoulder are examples of the first, the knee and elbow of the second kind of articulation. It is obvious that the ball and socket admits of motion on every side, while the hinge is moveable in one direction only.