The SKELETON constitutes the framework on which the soft tissues of the body are supported, enabling them to retain their definite position in the body and, in the case of the limbs, affording a strong system of levers by means of which the muscles may change the situation of the body as a whole or of its various parts. To serve such functions the skeleton must be strong yet elastic, and must permit of movement without effort yet also without any serious lessening of its strength. These ends are attained by dividing up the skeleton into numerous constituent pieces, by making these of very strong and hard yet somewhat elastic material (bones and cartilages), and by connecting these bones and cartilages by means of joints or articulations, at which the firm structures can move on one another without resistance and yet securely owing to the fact that they are appropriately attached to each other by strong ligaments. If, then, we wish to study the skeleton, we should not confine our attention to the bones, but should also consider the cartilages, ligaments, and joints that are concerned in maintaining the form of the body and in enabling it to move about.
Cartilages * are tough, but elastic and compressible to a considerable extent. In the adult skeleton they are found completing the skeleton of the thorax (costal cartilages), holding bones firmly together (as in the vertebral column), or filling up intervals between bones (as in certain parts of the skull), coating the articular surfaces of bones (articular cartilages), or interposed between two articular surfaces (inter-articular cartilages). Cartilage occurring in the body is in general of the hyaline variety when covering articular surfaces of bone, but has the structure of one of the varieties of fibro-cartilage in other situations, save when it is taking the plac? of a bone in the skeleton.
Articulations can, as a whole, be divided into synarthrodia! or immovable and diarthrodial or movable. Diarthrodial joints possess a synovial cavity to permit of easy movement, and this cavity may be wholly or partly divided by an inter-articular cartilage or meniscus into two parts. Synarthrodial joints are firmly fixed by interlocking of the bones concerned or by interposition of a thin but strong layer of fibro-cartilage over a large area on each of the bones. A subdivision of this class, where a certain very small amount of motion is permitted between the bones owing to the intervention of a thicker pad of fibro-cartilage, is usually termed an amphiarthrosis.
Ligaments are bands of fibrous tissue that pass over a joint to connect the bones ; naturally they are best developed over diarthrodial joints, where the articulating bones have no connecting medium between them, but depend on outside agents for security. It is evident that, if the ligaments are not to be a hindrance to free movement, they must be so disposed with regard to the joint surfaces, and these must be so shaped with reference to the ligamentous attachments, that any given ligament will either be tense and effective at one part of the movement or will remain tense throughout it ; in the latter case the movement of the articulation can only occur in one plane, and the ligament must be attached practically in the axis round which the movement takes place.
* For detailed histology and for accounts of varieties of tissues consult special works on histology.
The bones constitute the larger part of the skeleton. Bone is extremely hard, but at the same time exhibits a small amount of elasticity and toughness. It differs from mere calcification of cartilage, in that it has a definite organised structure (Fig. i), but it can be looked on as composed of animal or organic matter impregnated with earthy salts : analysis gives (roughly) about one-third or less of animal matter and the rest as mineral matter. The former gives toughness and the latter hardness to the composite result.
Fresh bone possesses a pinkish hue on the surface, which is covered by a fibrous periosteum of variable thickness. Bone exists in two states in the body, termed, according to the appearance it presents, compact and cancellous ; examples of both sorts of bone can be seen in a section along a bone of any size. Complete Haversian systems only occur, of course, in the compact bone, which is made up of a series of the systems (Fig. 2).
The shaft of a long limb bone is composed of compact bone which makes a shell that is hollow internally : this is the medullary cavity, and it is walled directly by the compact shaft-wall in the middle of the bone, but at each end cancellous tissue appears and occupies the cavity. The medullary cavity and the interstices of the cancellous bone are filled with very vascular marrow, of which two distinct varieties are found-(a) yellow marrow, found in the shafts of long bones, and containing about 96 per cent, of fat, and (b) red marrow, occurring in the articular ends of the long bones and elsewhere and containing 75 per cent, of water, with very little fat and a considerable amount of organic solids.
The skeleton comprises an appendicular skeleton of the limbs as well as an axial skeleton of the trunk.
The various bones of these parts are divided, according to their shape and appearance, into short, long, flat, and irregular.