[Illustration: FIG. 28.--THE SKELETON.] CHAPTER III THE SKELETON _Experiment 1._ (At home.) =Is the Arch of the Foot Elastic?=--Wet the foot in a basin of water and, while sitting, place the foot flat upon a piece of paper. Draw the outline of the track. Repeat, but stand with your whole weight upon the foot. Draw track. Conclusion? (Take sketches to school. Which sketch shows the flattest foot?) Devise a method for measuring the length of the foot with and without the weight of the body upon it. What difference? Conclusion? _Experiment 2._ =Composition of Bone.=--Place a bone in a hot fire and let it remain for three or four hours. It will keep its shape however long you burn it; but unless you handle it carefully when you take it out, it will crumble to pieces. If not thoroughly burned, the bone will be black from the carbon of the animal matter still left in it. _Experiment 3._ Obtain a slender bone like the rib of a hog or the leg bone of a fowl, and put the raw bone into a vessel containing strong vinegar or two ounces of muriatic acid and a pint of water. Leave it there for four days. When the bone is taken out, it can be tied into a knot. The acid may be washed off, and the bone preserved in a bottle of alcohol or glycerine. _Experiment 4._ =The Forms of Joints.=--Obtain the disjointed bones of a fowl or small mammal and place them one at a time in their sockets and study the fit and motion of the joints. _Experiment 5._ =Pivot Joints.=--Through what fraction of a circle do the pivot joints in the forearm and neck allow the hand and head to rotate? =Review Questions.=--Where are the bone cells? How does nourishment reach them? How has the mineral part of the bones been deposited? How long may bone cells live? Name animals with outside skeletons. Inside skeletons. No skeleton. =Forms and Uses of Bones.=--The three chief _uses of bones are protection, motion, and support_. In order to fulfill these purposes, the bones must have different sizes, shapes, and positions. The bones are classed by _shape_, as _long, flat, and irregular_. Those whose chief use is _to protect are broad and flat_. The bones which _furnish support are thick and solid_; those designed to _aid in motion are long and straight_. Including six small bones in the ear, there are two hundred and six bones in the adult skeleton.