The term skeleton ( to dry,) is applied to the assemblage of those parts which, though not extra-vascular, as was once supposed, are yet sufficiently hard and dense to retain, in the dried state, some of their primitive form and character. Thus defined, the skeleton will comprise not only the cranium, the vertebral column, and the extremities, but likewise the general covering of the body, e.g. the scales of fish, the hard integument of the crocodile, the bony scales of the armadillo, the horns, hoofs, nails, spines, and hair of animals, the epithelial covering of man. We recognise, therefore, an internal or endo-skeleton, and an external or exo-skeleton ; the former being represented by the internal bony framework, the latter by the hard structures of the integument, under every variety of form and development. But to these may be added,
I. The bony, cartilaginous or fibrous capsules, which receive the nerves of special sense, and are hence termed sense-capsules ; namely, the olfactory, the optic, and the acoustic. The olfactory sense-capsule is known as the cellular structure of the ethmoid bone; the optic sense-capsule as the eyeball, which, though permanently fibrous in man, is ossified in birds, reptiles, and in some fish. The acoustic sense-capsule is the petrous part of the temporal bone.
II. Parts of the skeleton which support important viscera, e.g. the cartilaginous or bony rings or arches supporting the branchial apparatus in fish, or the respiratory apparatus in warm-blooded animals, and to such is applied the term splanchnic skeleton.
I propose saying but little in the following pages of either the exo-skeleton or the splanchnic skeleton. It is more convenient to reserve their consideration for a future occasion. The sense-capsules are, in most animals, intimately connected with the endo-skeleton, and the two may be described together.
Greatly as the different members of the vertebrate sub-kingdom, vary in form and physical characters, they are yet moulded after one common type, and admit, in the systems of organs of which they are composed, the closest comparison one with another.