THE FRAMEWORK OF THE BODY 29 



rounding a central cavity except at the ends, where the interior of the 

 bone is filled with a spongy open network of bone substance. In life, both 

 the central cavity of the shaft and the meshes of the spongy bone at the 

 ends are filled with a soft tissue called bone marrow, and the outer surface 

 of the bone is enclosed in a sheath of dense connective tissue called the 

 periosteum. 



The disposition of the bone substance in the form of a cylinder with 

 internally reinforced ends, instead of a solid rod, gives the bone the 

 greatest mechanical strength attainable for the quantity of structural 

 material used. The strength of the bone is also materially increased by 

 the arrangement of its finer structural elements. The inner and outer 

 walls of the shaft are formed of circumferential layers, or lamellae, and 

 the spongy bone at the ends of the shaft is not haphazardly arranged but 

 has its partitions aligned in definite planes and curves. A study of these 

 details of structure in relation to the work done by the bone shows that 

 they form a pattern giving maximum resistance to the stresses to which 

 the bone is exposed. The same structural efficiency is found in the arrange- 

 ment of the materials in other bones of the skeleton. 



All of the materials that are utilized in bone, cartilage, and ligament 

 belong to the group of tissues called sustentative. These tissues are notable 

 for being composed largely of cell products rather than of cells. In the 

 living body the sustentative cell products everywhere contain the living 

 cells that produce them. In ligaments the cell products are elongated and 

 very strong fibers, some elastic and others nonelastic. In cartilage the cell 

 product is a tough, usually semitransparent material that under the 

 microscope can be seen everywhere to enclose the cartilage-producing 

 cells. In bone the matrix consists both of calcium phosphate, which makes 

 the bone hard, and of a strong and tough organic material that pervades 

 the mineral parts. One can take two fresh bones from a recently living 

 animal, burn one to remove the organic parts, and place the other in 

 acid to remove all minerals. The bone treated with fire is dry, hard, and 

 brittle; that treated with acid is a tough, flexible structure of organic 

 material. Both retain the recognizable shapes and dimensions of the 

 original bones. The accompanying illustration (Fig. 3.1) shows more or 

 less diagrammatically the relationship of the matrix (the cell product in 

 bone and cartilage) to the cells that produce it. The matrix is penetrated 

 by nerves and blood vessels that supply the living cells; in bone these 

 occupy the channels called the Haversian canals. 



The divisions of the skeleton. The complete human skeleton is shown 

 in Fig. 3.2. Two chief regions or divisions of the skeleton may be recog- 

 nized, each with minor subdivisions. The axial skeleton comprises the 

 head and backbone (skull and vertebral column), together with the ribs, 

 the sternum, and the cartilages that connect the front, or ventral, ends 



