The general framework of the body is built up mainly of a series of bones, supplemented in certain regions by pieces of cartilage; this bony and cartilaginous framework constitutes the skeleton (fig. 264).
In comparative anatomy the term skeleton has a wider application, for in some of the lower animals hard, protecting and supporting structures are developed in association with the common integument. In such animals the skeleton consists of an internal or deep skeleton, termed the endoskeleton, and an external or superficial, termed the exoskeleton. In the human subject the exoskeleton is very rudimentary, its only important representatives being the nails and the enamel of the teeth, and therefore, in human anatomy, the term skeleton is confined to the endoskeleton; this is divisible into an axial part, which includes the bones of the head and, trunk, and an appendicular part, which comprises those of the extremities or limbs. The bones available for study in articulated skeletons and as separate entities have been subjected to a process of maceration, by which they are denuded of all the structures attached to them, viz. muscles, ligaments, periosteum and articular cartilage. Subsequently they are allowed to dry for a prolonged period, with the result that the fat in the marrow drains away and what is left of the marrow itself shrivels up, leaving the bone clean, dry and easy to handle.
Function of bones
Bones provide the central axis and give form and shape to the body. Many of them are adapted to give support to the weight of the body, but they may fulfill some other function in addition, e.g. the thigh bones support the weight of the body in standing, walking and running, but they also provide the level which are essential for locomotion. Other bones give protection to underlying or contained structures, e.g. the cranium protects the contained brain, and the sternum and ribs overlie the heart and lungs and accord them to function as levers for the production of movements they must be connected by muscles, and moveable joints or articulations must be present where individual bones come into contact with one another.
Bones are divisible into four classes: long, short, flat, and irregular.
The long bones are found in the limbs, where they form levers; each consists of a shaft and two ends. The shaft is tubular, with a central cavity termed the medullary cavity; the wall consists of dense, compact substance of considerable thickness in the middle part of the shaft of the bone, but becoming thinner towards the ends; projecting into the medullary cavity is some spongy substance, scanty in the middle of the body of the bone but plentiful towards either extremity. The ends are usually expanded for purposes of articulation and muscular attachment; they consist of spongy substance covered by thin compact bone, and are usually developed from one or more secondary or epiphysial centres of ossification. The medullary cavity and the spaces in the spongy substance are filled with marrow (medulla ossium).
The short bones – Where a part of the skeleton is intended for strength and compactness combined with limited movement it is constructed of a number of short bones, as in the carpus and tarsus. These bones consist of spongy substance surrounded by a thin crust of compact bone.
The flat bones – Where the principal requirement of the skeleton is to protect delicate structures or provide broad surfaces for muscular attachment the bones are expanded into plates, as in the skull and the shoulder-blades, and are composed of two thin layers of compact bone separated by a variable quantity of spongy substance. In the cranial bones the layers of compact bone are known as the tables of the skull ; the outer table is thick and tough, the inner thin, dense and brittle. The intervening spongy substance is called the diploe, and this, in certain regions of the skull, undergoes absorption, and air-filled spaces, termed sinuses, are left between the tables of the skull.