An accurate enumeration of the different pieces which constitute the human skeleton is not so easy a task as might first appear; for bones that were distinct in the foetus coalesce with age, whilst others, as the first teeth, last for a time, and then permanently disappear.
Weber reckons 217 bones, namely :— 27 of the head, 58 of the trunk, 132 of the upper and lower extremities ; and if to these be added 32 teeth, 5 bones of the tongue, and 2 orbicular bones between the malleus and incus, the number amounts to 256.
But in this calculation the occipital and sphenoid bones are counted as one piece, under the name of os spheno-occipitale. The temporal bone, formed by the confluence of five distinct elements, is reckoned also as a single bone. The five sacral vertebrae and the four or five coccygeal vertebrae are enumerated only as two pieces, under the titles of sacrum and coccyx. The ribs and the sternum are described as unconnected with the dorsal vertebrae. Each os innominatum, though composed of ilium, ischium, and pubis, is called a single bone. The difficulty in determining what constitutes a bone accounts for the great difference in the calculations made by the most accurate observers. Meckel reckons 253 bones, and Soemmering 260. Mr. Quain makes the number 197; but he omits the ossicula auditus, the teeth, the os hyoides with its appendages, and the sesamoid bones, as "either accessories or connected with special organs." Adherence to the different centres of ossification in the foetus does not render the task more easy. For, in the first place, the process of ossification in man is not complete until between the twentieth and thirtieth year; and during the whole of that period pieces, originally distinct, are undergoing a blending or coalescence with others. In the second place, an exogenous spine or process, an integral part of a single bone, may be formed by an ossifying centre, developed in some part of the primitive cartilaginous basis.
That these points may be satisfactorily elucidated, it becomes necessary clearly to understand the type upon which a vertebrate animal is formed; we are thence led rightly to understand the signification of the different bones, whether simple or compound, which enter into the composition of the skeleton ; to separate those which have become confluent, and to refer to their proper segment, or system, those which have undergone modifications, or been subject to displacement.