*The Bone Cells.*—Surrounding the Haversian canals are thin layers of bone substance called the _laminæ_, and within these are great numbers of irregular bodies, known as the _lacunæ_. The walls of the lacunæ are hard and dense, but within each is an open space. In this lies a flattened body, having a nucleus, which is recognized as the _bone cell_, or the bone corpuscle (Fig. 96). It appears to be the work of the bone cells to deposit mineral matter in the walls surrounding them and in this way to supply the properties of hardness and stiffness to the bones. The canaliculi connect with the lacunæ in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines (Fig. 94). [Fig. 96] Fig. 96—*Bone cell* removed from the lacuna and very highly magnified. (From Quain’s _Anatomy_.) *How the Bone Cells are Nourished.*—The bone cells, like all the other cells of the body, are nourished by the lymph that escapes from the blood. This passes through the canaliculi to the cells in the different parts of the bone, as follows: 1. The cells in the surface layer of the bone receive lymph from the capillaries in the periosteum.(79) It gets to them through the short canaliculi that run out to the surface. 2. The cells within the interior of the bone receive their nourishment from the small blood vessels in the Haversian canals. Lymph from these vessels is conveyed to the cells through the canaliculi that connect with the Haversian canals. *Plan and Purpose of the Skeleton.*—The framework of the body is such as to adapt it to a _movable_ structure. Obviously the different parts of the body cannot be secured to a foundation, as are those of a stationary building, but must be arranged after a plan that is conducive to motion. A moving structure, as a wagon or a bicycle, has within it some strong central part to which the remainder is joined. The same is true of the skeleton. That part to which the others are attached is a long, bony axis, known as the _spinal column_. Certain parts, as the ribs and the skull, are attached directly to the spinal column, while others are attached indirectly to it. The arrangement of all the parts is such that the spinal column is made the central, cohering portion of the skeleton and also of the whole body. Besides the general arrangement of the parts of the skeleton, there is such a grouping of the bones in each of its main divisions as will enable them to serve definite purposes. In most places they form mechanical devices for supplying special movements, and in certain places they provide for the support or protection of important organs. In most cases there is a definite combination of different bones, forming what is called the bone group. [Fig. 97] Fig. 97—The human skeleton.