BONE, the organ which in higher animals forms the basis of the fabric of the body. Many of the creatures placed at the bottom of the animal scale, composed of soft gelatinous matter, and buoyant in water, need no solid support ; but all animals that possess solid organs, and whose body rests upon particular points, must have some substance of a dense and inflexible nature to afford to their various tissues and structures the requisite resistance and support. The substances that serve this purpose are various, hut the most common are the salts of lime, sometimes the carbonate, some times the phosphate, and at other times both combined in different proportions. Carbonate of lime constitutes the solid basis of many of the compound zoophytes and the corals. It also constitutes the principal part of the fabric of the shells of Mollusca. It is found also in the external skeleton of the Crustacea,as the crabs and lobsters, but in this instance the phosphate of lime is also present, and predominates. It is in the skeletons of the Vertebrate animals that we find the phosphate of lime greatly preponderating. This is characteristic of bone.
When an animal possesses bone as the solid support of it indicates a high degree in the scale of organisation. Bone is an elaborate structure found in no class below the Vertebrate. Even the lowest order of this, which is the highest class of animals, is wholly destitute of it ; for it ia not found in large tribes of fishes, the shark, the sturgeon, the ray, &c. In these the lesehigbly-organised substance called cartilage is substituted, and accordingly these fishes are called cartilaginous in contradistinction to the osseous; and in all classes below the cartilaginous fishes the dense and inflexible substance which sus tains the soft parts of the body, and which affords points of resistance for the action of those parts, consists either of shells or of some modification, and not of true organised bone.
In general the structure which performs the office of bone in the lower animals is placed on the exterior of the body, and often indeed forms its external envelope ; true bone, on the contrary, is always placed in the interior. Even when it approaches the surface bone is
always covered by some soft part, as muscle, membrane, skin, &c. Crust, shell, horn—the substances which form the skeleton of the inferior animals—are thus external, the softlaarts being internal; but in the higher animals the skeleton is always internal, and the soft parts, which are sustained by it and which re-act upon it, are external.
The office of bone in the animal economy is chiefly mechanical, and the mechanical purposes to which it is subservient require that it should be of different sizes and forms. In the human skeleton there are commonly enumerated 260 different bones, which present every variety of size and figure. But all these varieties may be reduced to three classes : the long and round, as the bones of the upper and lower extremities ; the broad and flat, as the bones of the skull ; or the short and square, as the separate bones that compose the vertebral column. The long bones are adapted for motion, the flat for protection, and the square for motion combined with strength. Accordingly the long bones, which are adapted to communicate a free range of motion, are moulded into lengthened cylinders, and form so many levers, consti tuting organs of locomotion exquisitely constructed and combined for the accomplishment of their office, as is seen in the fin of the fish, in the wing of the bird, and in the limb of the quadruped. In the employment of the flat bones for the covering of some of the more tender and delicate organs, as the brain and spinal cord, the form of these hones is such as to add to their strength, as is manifest in the vaulted roof of the skull ; while iu the construction of the vertebral column, composed of the short and square bones, which are so adjusted as to afford a limited range of motion with a great degree of strength, many and opposite purposes are effected.