92 HISTOLOGY 



various times after birth, or, in the tibia, shortly before birth; they unite 

 with the diaphyses usually between the eighteenth and twenty-second 

 years, when the bones have acquired their full length. At that time noth- 

 ing is left of the original cartilage except the layer of articular cartilage 

 which covers the joint surfaces. Details in regard to the time when 

 ossification begins in the various bones, the number of centers involved 

 (for many bones have more than the three which have here been de- 

 scribed), and the time when these join the main bone, will be found in text- 

 books of anatomy, and, together with many references to important 

 studies of bone development, in Bidder's "Osteobiologie" (Arch. f. mikr. 

 Anat., 1906, vol. 68, pp. 137-213). 



Structure of Bone in the Adult. The properties of adult bone are essen- 

 tially those of its matrix, which consists of organic and inorganic constitu- 

 ents intimately blended, and perhaps chemically combined. Of the in- 

 organic matter, over 80% is calcium phosphate, Ca 3 (POOa; the remainder 

 includes chlorides, carbonates, fluorides and sulphates of calcium, sodium, 

 potassium and magnesium. In order to cut sections of bone, this in- 

 organic matter must be removed, and decalcification is usually accom- 

 plished by placing the specimen, after it has been preserved, in dilute nitric 

 acid (3-5%) for several days or weeks. The matrix then has the con- 

 sistency of cartilage. Its organic portion, which remains, is composed 

 chiefly of collagen, together with osseo-mucoid. The collagen occurs in 

 very fine white fibrils which are gathered in bundles, arranged in thin 

 layers or lamella. Within these layers the fibers occur in parallel sets 

 which tend to cross one another at right angles, thus producing a lattice 

 work. These "decussating fibers" are seen only in special preparations 

 in which a lamella has been peeled off, so that it can be examined in sur- 

 face view. The calcareous matter is said to be deposited in the cement 

 substance between the fibers, and not within them. Coarser uncalcified 

 fibers are found in embryonic bone and in certain situations in adult bone 

 for example, at the sutures and the places where tendons are inserted. 

 They also extend into the bone from the periosteum (Fig. 79), constitut- 

 ing the "perforating fibers" (Sharpey's fibers). The perforating fibers of 

 the bones of the skull are entirely collagenous. These bones in the adult, 

 together with the entire skeleton at birth, contain no elastic fibers; but in 

 other bones of the adult elastic fibers accompany the perforating fibers 

 (Schulz, Anat. Hefte, Abt. i, 1896, vol. 6, pp. 117-153). 



The periosteum consists of two layers. It has an outer layer of dense 

 connective tissue, rich in blood vessels and containing also lymphatic 

 vessels and nerves. It blends with the surrounding looser connective 

 tissue and in places with fasciae and tendons. The inner layer has few 

 vessels but contains an abundance of elastic fibers. They are chiefly 

 parallel with the long axis of the bone, but in the periosteum of the bones 



