1102 



GENERAL ANATOMY OR HISTOLOGY. 



membrane, such as those forming the roof and sides of the skull ; others, such as 

 the bones of the limbs, are preceded by rods of cartilage. Hence two kinds of 

 ossification are described : the intramembranous and the intracartilaginous. 



Intramembranous Ossification. In the case of bones which are developed in 

 membrane no cartilaginous mould precedes the appearance of the bone-tissue. 

 The membrane, which occupies the place of the future bone, is of the nature of 

 connective tissue, and ultimately forms the periosteum. At this stage it is seen 

 to be composed of fibres and granular cells in a matrix. The outer portion is 

 more fibrous, while internally, the cells or osteoblasts predominate ; the whole 

 tissue is richly supplied with blood-vessels. At the outset of the process of bone 

 formation a little network of bony spiculse is first noticed radiating from the point 

 or centre of ossification. When these rays of growing bone are examined with a 

 microscope, they are found to consist at their growing point of a network of fine 

 clear fibres and granular corpuscles with an intervening ground substance (Fig. 

 631). The fibres are termed osteogenetic fibres, and are made up of fine fibrils dif- 

 fering little from those of white fibrous tissue. Like them, they are probably 

 deposited in the matrix through the influence of the cells in this case the osteo- 



Union of 

 adjacent 

 spicules. 



Osteoblasts.<:' 



Osteogenetic 



fibres. 



Calcific deposit 



between the 

 fibres. 



Bony 

 spicules. _ 



FIG. 631. Part of the growing edge of the developing parietal bone of a fretal cat. (After J. Lawrence.) 



blasts. The osteogenetic fibres soon assume a dark and granular appearance from 

 the deposition of calcareous granules in the fibres and in the intervening matrix, 

 and as they calcify they are found to enclose some of the granular corpuscles or 

 osteoblasts. By the fusion of the calcareous granules the bony tissue again assumes 

 a more transparent appearance, but the fibres are no longer so distinctly seen. 

 The involved osteoblasts form the corpuscles of the future bone, the spaces in 

 which they are enclosed constituting the lacunae. As the osteogenetic fibres grow 

 out to the periphery they continue to calcify, and give rise to fresh bone spicules. 

 Thus a network of bone is formed, the meshes of which contain the blood-vessels 

 and a delicate connective tissue crowded with osteoblasts. The bony trabeculse 

 thicken by the addition of fresh layers of bone formed by the osteoblasts on their 

 surface, and the meshes are correspondingly encroached upon. Subsequently 

 successive layers of bony tissue are deposited under the periosteum and round 

 the larger vascular channels, which become the Haversian canals, so that the bone 

 increases much in thickness. 



Intracartilaginous Ossification. Just before ossification begins the bone is 

 entirely cartilaginous, and in a long bone, which may be taken as an example, the 

 pvocess commences in the centre and proceeds toward the extremities, which for 



