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OSTEOLOGY 



DEVELOPMENT AND GROWTH OF BONE' 

 The primitive embryonal skeleton consists of cartilage and fibrous tissue, in 

 which the bones develop. The process is termed ossification or osteogenesis, and 

 is effected essentially by bone-producing cells, called osteoblasts. It is customary, 

 therefore, to designate as membrane bones those which are developed in fibrous 

 tissue, and as cartilage bones those which are preformed in cartilage. The princi- 

 pal membrane bones are those of the roof and sides of the cranium and most of the 

 bones of the face. The cartilage bones comprise, therefore, most of the skeleton. 

 Correspondingly we distinguish intramembranous and endochondral ossification. 



In intramembranous development the process begins at a definite center of 

 ossification where the cells (osteoblasts) surround themselves with a deposit of 



bone. The process extends from this center to the 

 periphery of the future bone, thus producing a net- 

 work of bony trabeculse. The trabeculse rapidly 

 thicken and coalesce, forming a bony plate which 

 is separated from the adjacent bones by persistent 

 fibrous tissue. The superficial part of the original 

 tissue becomes periosteum, and on the deep face of 

 this successive layers of periosteal bone are formed 

 by osteoblasts until the bone attains its definitive 

 thickness. 



In endochondral ossification the process is funda- 

 mentally the same, but not quite so simple. Osteo- 

 blasts emigrate from the deep face of the perichon- 

 drium or primitive periosteum into the cartilage and 

 cause calcification of the matrix or ground-substance 

 of the latter. Vessels extend into the calcifying area, 

 the cartilage cells shrink and disappear, forming 

 primary marrow cavities which are occupied by pro- 

 cesses of the osteogenic tissue. There is thus formed 

 a sort of scaffolding of calcareous trabeculse on which 

 the bone is constructed by the osteoblasts. At the 

 same time perichondral bone is formed by the osteo- 

 blasts of the primitive periosteum. The calcified 

 cartilage is broken dowm and absorbed through the 

 agency of large cells called osteoclasts, and is re- 

 placed by bone deposited by the osteoblasts. The 

 osteoclasts also cause absorption of the primitive 

 bone, producing the marrow cavities; thus in the 

 case of the long bones the primitive central spongy 

 bone is largely absorbed to form the medullary cavity 

 of the shaft, and persists chiefly in the extremities. 

 Destruction of the central part and formation of subperiosteal bone continue until 

 the shaft of the bone has completed its growth. 



A typical long bone is developed from three primary centers of ossification, 

 one for the diaphysis or shaft and one for each epiphysis or extremity. Many 

 bones have secondary centers from which processes or apophyses develop. 



The foregoing outline accounts for the growth of bones except in regard to 

 length. Increase in length may be explained briefly as follows: Provision for con- 

 tinued ossification at either end of the diaphysis is made by a layer of actively 

 growing cartilage — the epiphyseal cartilage — which intervenes between the diaph- 



'Only a brief general statement of osteogenesis can be made here; details must be sought 

 in embryological literature. 



Fig. 3. — Left Femur of Young Pig, 

 External View, to Show Di- 

 vision OF A Long Bone into 

 Shaft (s) and Extremities. 

 Proximal extremity consi.sts of 

 two parts, head (A) and trochanter 

 major (t. m.), which have separate 

 centers of ossification. Distal extrem- 

 ity consists of trochlea (/) and condyles 

 (c); e.I., epiphyseal cartilages; s./., 

 supracondyloid fossa. 



