(2) =Flat bones= (Ossa plana) are expanded in two directions. They furnish sufficient area for the attachment of muscles and afford protection to the organs which they cover. (3) =Short bones= (Ossa brevia), such as those of the carpus and tarsus, present somewhat similar dimensions in length, breadth, and thickness. Their chief function appears to be that of diffusing concussion. Sesamoid bones, which are developed in the capsules of some joints or in tendons, may be included in this group. They diminish friction or change the direction of tendons. (4) =Irregular bones.= This group would include bones of irregular shape, such as the vertebræ and the bones of the cranial base; they are median and unpaired. Their functions are various and not so clearly specialized as those of the preceding classes. This classification is not entirely satisfactory; some bones, _e. g._, the ribs, are not clearly provided for, and others might be variously placed. STRUCTURE OF BONES[1] Bones consist chiefly of =bone tissue=, but considered as organs they present also an enveloping membrane, termed the =periosteum=, the =medulla= or =marrow=, =vessels=, and =nerves=. The architecture of bone can be studied best by means of longitudinal and cross-sections. These show that the bone consists of an external shell of dense =compact substance=, within which is the more loosely arranged =spongy substance=. In typical long bones the shaft is hollowed to form the =medullary cavity= (Cavum medullare). [Illustration: FIG. 1.—FRONTAL SECTION OF LARGE METATARSAL BONE OF HORSE, POSTERIOR PART. ] [Illustration: FIG. 2.—SAGITTAL SECTION OF LARGE METATARSAL BONE OF HORSE. _S.C._, Compact substance; _S.s._, spongy substance; _C.m._, medullary cavity; _F.n._, nutrient foramen. Note the greater thickness of the compact substance of the inner and anterior parts of the shaft. ] The =compact substance= (Substantia compacta) differs greatly in thickness in various situations, in conformity with the stresses and strains to which the bone is subjected. In the long bones it is thickest in the middle part of the shaft and thins out toward the extremities. On the latter the layer is very thin, and is especially dense and smooth on joint surfaces.