_Fatty or adipose tissue_ is formed by the deposit of fat in the cells of the areolar tissue and is found in most parts where the areolar tissue occurs, though it varies largely in amount in different parts. It is found pretty generally under the skin, fills in inequalities about various organs and about the joints, and exists in large quantities in the marrow of the long bones. In moderate amounts it gives grace to the form and constitutes an important reserve fund. [Illustration: FIG. 3.--Adipose tissue (Leroy): _a_, Fibrous tissue; _b_, fat cells; _c_, nucleus of fat cells; _d_, fatty acid crystals in fat cells.] _Cartilage_ consists of groups of nucleated cells in intercellular substance. It is very firm, yet highly elastic, and serves in the joints to break the force of concussion of the harder and less elastic bones. Except when it occurs at the end of a bone, it is covered with a membrane called the perichondrium, which carries its blood supply. In the nose, ear, larynx and trachea it serves to give shape, to keep the passages open, and to afford attachment for muscles. Most of the skeleton of the fetus consists of cartilage, which later develops into bone. _Bone._--In bone the intercellular tissue is rendered hard by the deposit of mineral salts, the resulting material being of great strength and rigidity. The texture may be close and dense like ivory or open and spongy, the difference lying merely in the fact that the one has fewer spaces between the solid particles than the other. There is usually a hard, compact layer on the exterior of the bone, as that is where the greatest cross-strain comes, especially in the long bones, while within is the cancellous or spongy tissue, which gives lightness to the bone and is capable of withstanding enormous pressure, though it can bear little cross-strain. [Illustration: FIG. 4.--Cross-section of compact bone tissue. (After Sharpey.)] =Structure of Bone.=--The hard substance in bone is always arranged in lamellæ or bundles of bony fibers, which in cancellous tissue meet to form a kind of lattice-work, while in the dense tissue they are generally arranged in rings about the Haversian canals, channels through which the blood-vessels pass through the bone longitudinally. Between the lamellæ are spaces called _lacunæ_, in which lie branched cells, the spaces being connected with each other and with the Haversian canals by numerous tiny canals or canaliculi, by which nutrient material finds its way from the Haversian canals to all parts of the bone. Within the bone is the medulla or _marrow_, which is of two varieties: the yellow, which is largely fat and is found in the long bones of adults, and the red, which is nearly three-fourths water and is found in most of the other adult bones and in the bones of the fetus and of the infant.