A broken limb should be held in place by splints, or a plaster cast, to enable this process to go on uninterruptedly, and also lest a sudden jar might rupture the partially mended break. For a long time, the new portion consists largely of animal matter, and so is tender and pliable. The utmost care is therefore necessary to prevent a malformation. THE JOINTS are packed with a soft, smooth cartilage, or gristle, which fits so perfectly as to be airtight. Upon convex surfaces, it is thickest at the middle, and upon concave surfaces, it is thickest at the edge, or where the wear is greatest. In addition, the ends of the bones are covered with a thin membrane, the _synovial_ (_sun_, with; _ovum_, an egg), which secretes a viscid fluid, not unlike the white of an egg. This lubricates the joints, and prevents the noise and wear of friction. The body is the only machine that oils itself. The bones which form the joint are tied with stout ligaments (_ligo_, I bind), or bands, of a smooth, silvery white tissue, [Footnote: The general term _tissue_ is applied to the various textures of which the organs are composed. For example, the osseous tissue forms the bones; the fibrous tissue, the skin, tendons, and ligaments.] so strong that the bones are sometimes broken without injuring the fastenings. II. CLASSIFICATION OF THE BONES. For convenience, the bones of the skeleton are considered in three divisions: the _head_, the _trunk_, and the _limbs_. 1. THE HEAD. THE BONES OF THE SKULL AND THE FACE form a cavity for the protection of the brain and the four organs of sense, viz.: sight, smell, taste, and hearing. All these bones are immovable except the lower jaw, which is hinged [Footnote: A ring of cartilage is inserted in its joints, something after the manner of a washer in machinery. This follows the movements of the jaw, and admits of freer motion, while it guards against dislocation.] at the back so as to allow for the opening and shutting of the mouth. THE SKULL is composed, in general, of two compact plates, with a spongy layer between. These are in several pieces, the outer ones being joined by notched edges, sutures (su'tyurs,), in the way carpenters term dovetailing. (See Fig. 4.) FIG. 4. [Illustration: _The Skull._--1. _frontal bone;_ 2, _parietal bone;_ 3, _temporal bone;_ 4, _the sphenoid bone;_ 5, _ethmoid bone;_ 6, _superior maxillary (upper jaw) bone;_ 7, _malar bone;_ 8, _lachrymal bone;_ 9, _nasal bone;_ 10, _inferior maxillary (lower jaw) bone._]