composed of one great bone, or of different bones anchored solidly together, the body would be perfectly rigid; since motion is necessary to life, flexible connections between some of the bones are absolutely essential. Our movements are actually made by means of muscles, but nearly all of them become effective through the motions of bones to which the muscles are fastened. The bones are often very irregular in shape; careful study shows that the irregularities are due either to provision for the contact of one bone with another in the joints, a contact that must allow in most cases for motion of one on the other, or to provision of places to which muscles can be fastened in such a way as to make their pull effective. It is, of course, out of the question for us to examine the skeleton bone by bone. Figures are given of a number of typical bones: all that we can do in addition is to mention some of the interesting features of the skeleton. The skeleton of the head is called the skull; its chief features are the brain case, the eye sockets, and the parts about the nose and mouth. The brain case is made up of eight bones firmly joined together by saw-tooth margins to make up a roughly spherical box which holds the brain, and protects this delicate and vitally important organ from all injury except the most severe. There are a number of small openings out from the brain case through which nerves pass, and one large opening below and at the back through which the spinal cord merges into the brain. The bones which make up the sides of the brain case are much thickened just behind the ears. A hollow extends from each ear into the bone, and within this hollow, securely protected from harm, is the actual organ of hearing. There are extensions of the hollow backward which are not occupied by any organs, and which communicate with the cavity of the ear. These sometimes become infected from the ear, causing the condition known as mastoiditis. Not only is this condition excruciatingly painful, but on account of the thin layer of bone which separates it from the brain itself it is highly dangerous. For this reason any ear trouble should be carefully watched lest it develop into mastoid trouble. [Illustration: A SIDE VIEW OF THE SKULL]