DEVELOPMENT AND GROWTH OF BONE 



23 



endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory 

 and probably are concerned in mediating the muscle sense (kinesthesia) 



DEVELOPMENT AND GROWTH OF BONEi 



whiciy^h/?'''*'!? embryonal skeleton consists of cartilage and fibrous tissue, in 

 which the bones develop The process is termed ossification or osteogenesis, ;nd 

 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 ot the face. The cartilage bones comprise, 

 therefore, most of the skeleton. Correspondingly 

 we distinguish intramembranous and endochondral 

 ossification. 



_ In intramembranous ossification the process 

 begins at a definite center of ossification (Punctum 

 ossificationis), where the osteoblasts surround them- 

 selves with a deposit of bone. The process extends 

 from this center to the periphery of the future bone, 

 thus producing a network of bony trabecule. The 

 trabeculae 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 perios- 

 teal bone are formed by osteoblasts until the bone 

 attains its definitive thickness. Increase in circum- 

 ference takes place by ossification of the surround- 

 ing fibrous tissue, which continues to grow until 

 the bone has reached its definitive size. 



In endochondral ossification the process is 

 fundamentally the same, but not quite so simple. 

 Osteoblasts emigrate from the deep face of the peri- 

 chondrium or primitive periosteum into the cartilage 

 and cause calcification of the matrix or ground- 

 substance of the latter. Vessels extend into the cal- 

 cifying area, the cartilage cells shrink and disappear, 

 forming primary marrow cavities which are occupied 

 by processes of the osteogenic tissue. There is thus 

 formed a sort of scaffolding of calcareous trabeculae 

 on which the bone is constructed by the osteoblasts. 

 At the same time perichondral bone is formed by 

 the osteoblasts of the primitive periosteum. The calcified cartilage is broken down 

 and absorbed through the agency of large cells called osteoclasts, and is replaced 

 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, 



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

 in embryological literature- 



Fig. 4. — Left Femur of Young Pig, 

 Lateral View, , to Show Di- 

 vision OF A Long Bone into 

 Shaft (s) and Extremities, 

 Proximal extremity consista of 

 two parts, head (h) and trochanter 

 major (t.m.), which have separate 

 centers of ossification. Distal extrem- 

 ity consists of trochlea (0 and condyles 

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

 supracondyloid fossa. 



