The _periosteum_ is exquisitely sensible, and is the safe-guard to the substance of the bone, which is delicately tender notwithstanding it is not so sensible as the skin that covers it; insomuch that it will become _carious_ on the least exposure to the air, or the attack of any foreign body of matter whatever; whence in wounds and fractures in general, great attention should be paid to the substance of the bone, being very subject to become _carious_ and to _exfoliation_, which is of the greatest consequence; but of this I shall say more in another place. The marrow is principally designed for the nourishment of the bones; which is evident from its being plentiest in young people, when the bones are strongest; and that when it is deficient, they become brittle, and lose their tenacity. The marrow is contained in a cellular substance, partly in vesicles of a nervous texture, and partly bony cells. At the ends of the long bones the texture is more spungy than in the middle, where the cavity is less, but the substance is most compact. Though anatomically there is no perceptible circulation in the bones, yet, that a circulation is actually existing, is evident, from a liquor oozing out from the ends of a fractured bone in the living animal; by which a fractured bone again unites, and this is called the _callus_; and whilst in its liquid state, resembles the white of an egg, which gradually ossifies, and becomes as hard as the main substance of the bone. The number of bones differ somewhat in various subjects; ordinarily they amount to two hundred and fifty-two: In the head sixty-three; in the trunk seventy; in the arms and hands sixty; and in the legs and feet sixty. As the bones are the support of the animal fabric, I have inserted in the next page, a catalogue of the human skeleton, which occasionally may be referred to: _A Skeleton of the Human Body._ BONES in the HEAD. The SKULL. _The Forehead._ Os Frontis 1 _The Hindhead._ Os Occipitis 1 _The Sides of the Head._ Ossa Parietalia 2 _The Temples._ Ossa Temporum 2 _The Basis of the Skull._ Os Ethmoides 1 Os Sphenoides 1 _The Bones of Hearing._ Ossicula 8 Auditus The FACE. _The Upper-Jaw._