38 



GENERAL ANATOMY OF THE SKELETON 



which are spaces for air. Canals, or foramina, are channels or openino-s in bone 

 through which nerves or vessels pass. 



Structure of Bone. Bone is a highly specialized form of connective tissue. In reality, it is 

 white fibrous tissue, calcified and structurally modified until it becomes osseous tissue. Bone 

 is not simply a crude mass resulting from the calcification of cartilage or fibrous tissue; ifTs a 

 distinct tissue, of a definite structure, the constituent parts of which arc arranged symmetrically. 

 ~There are two varieties of bone: dense or compact bone (substantia compacta), and can- 

 cellous, loose, or spongy bone (substantia spongiosd). 



Compact bone is dense, and is always found upon the exterior of the bony tissue. Even this 

 apparently compact tissue is porous; it differs from cancellous bone in its greater density and in 



the arrangement of its osseous substance into lamellae. It 

 forms practically the entire shafts of the long bones and 

 constitutes the outer portion of their extremities and of the 

 short, flat, and irregular bones. With the exception of 

 enamel and dentin it represents the hardest substance of the 

 body, is tough and clastic, and much force is required to 

 break it. Compact bone consists of an outer membrane, 

 _tb-eripsteum, internal to which is seen the osseous 

 tissuE** 1 *"" 



The periosteujh (Fig. 2) is a fibrous membrane adhering 

 to the surface or the bone in nearly every part except at the 

 cartilage-covered extremities. When strong tendons or 

 ligaments are attached to the bone, the periosteum is incor- 

 porated with them. By means of the periosteum many 

 vessels reach and enter the hard bone through Yolkmann's 

 canals. This is shown by stripping the periosteum from 

 the surface of living bone, when small bleeding points are 

 seen, each of which marks the entrance of a vessel from 

 the periosteum. It thus becomes obvious that the loosen- 

 ing of the periosteum, by depriving a portion of the bone 

 of its nourishment, may produce necrosis. The membrane 

 is firmly attached to the bone by trabeculse of fibrous tissue, 



Sharpey's fibres (Fig. 3), which penetrate the bone at right angles to its surface, and carry 

 bloodvessels. They do not directly enter the Haversian systems, but only the circumferential 

 and intermediate lamellae parts that are formed by periosteal action. Prolongations from some 

 of these vessels reach the Haversian canals, and even the bone marrow. In the extremities of 



FIG. 3. Fibres of Sharpey from the 

 parietal bone (adult man) isolated by 

 dissociation. (After Kolliker.) 



FIG. 4. Transverse section of compact tissue of bone. Magnified about 150 diameters. (Sharpey.) 



a long bone, vessels from the periosteum penetrate the layer of compact bone and reach the 

 cancellous tissue. ^ In the newborn and in the young the periosteum is composed of three 

 layers: an outer or fibrous layer, containing some bloodvessels, and composed of bundles of 

 white fibrous tissue; a middle or fibroelastic layer, containing some bloodvessels, fibrous 

 tissue, and much elastic tissue; and an inner or osteogenetic layer, which is very vascular and 

 contains numerous cells, which are converted into osteoblasts or bone-forming cells. / 



