The bone tissue or substance of birds are exceedingly compact and hard, white in color; and some of the bones are pneumatic, or contain air instead of marrow, notably these are bones of the skull, the sternum or breast bone, and the upper bones of the limbs. There are a great variety in the amount of pneumaticity or air possessed by the skeleton of different species, but it is not necessarily determined by the flying power of the animal. CHAPTER II. ARTHROLOGY. What is known regarding the articulations of bones of joints. The several bones which form the skeleton are united by means of certain soft structures, forming a number of articulations or joints, the study of which is termed arthrology. Before considering the different forms of joints, it will be advisable to describe briefly the various tissue, other than bone, which enter into and contributes towards their formation. These are chiefly cartilage, connective and elastic substance or tissues, and fat. In health, one bone never comes directly in contact with another, cartilage or fibrous tissue being always interposed; an exception to this exists in the adult skull, most of the bones of which become firmly united by ossification of the interposed soft material. CARTILAGE.--Cartilage, known also by the familiar name of gristle, is a firm, bluish-white elastic animal substance, somewhat transparent, resilient, and flexible, possessing great cohesive power. That which forms the original basis of the bony framework is termed temporary, and that which persists in the adult, permanent cartilage; the former disappears as it is replaced by bone, but the latter, of which alone we have to treat here, never under normal circumstances become ossified. Cartilage consists of corpuscles or cells, usually embedded in an intercellular substance. The articular cartilage is important as it encrusts the articular surfaces of bones, helping to form joints by supplying smooth, elastic cushions, which diminishes both concussion and friction. CONNECTIVE TISSUE (White fibrous tissue.)--In one form or another this tissue is found in all parts of the body. The chief varieties are the areolar and the fibrous; the former serving as a connecting medium, and support to the various organs, and to the structures of which they are formed. It appears as a loose, transparent mesh, its interwoven bundles forming spaces termed the areolar or cells. Connective tissue contains nerves and blood-vessels, for the supply of neighboring structures as well as for its own nourishment. When healthy it is little sensitive to pain. YELLOW ELASTIC TISSUE.--This differs from the white or connective tissue in being yellow, elastic, and not so tough or strong. Its fibers are usually large but when mixed with the white tissue in tendons the size diminishes.