THE STRUCTURE OF THE ELEMENTARY TISSUES. 49 



In many places these reticular lamellae are perforated by tapering 

 fibres (Claviculi of Gagliardi), resembling in character the ordinary 

 white or rarely the elastic fibrous tissue, which bolt the neighboring 

 lamellae together, and may be drawn out when the latter are torn asunder 

 (Fig. 56). These perforating fibres originate from ingrowing processes 

 of the periosteum, and in the adult still retain their connection with it. 



Development of Bone. From the point of view of their develop- 

 ment, all bones may be subdivided into two classes. 



(a.) Those which are ossified directly in membrane or fibrous tissue, 

 e. g., the bones forming the vault of the skull, parietal, frontal. 



(b.) Those whose form, previous to ossification, is laid down in hy- 

 aline cartilage, e. g., humerus, femur. 



The process of development, pure and simple, may be best studied in 

 bones which are not preceded by cartilage '' membrane-bones " (e. g., 

 parietal) ; and without a knowledge of this process (ossification in mem- 

 brane), it is impossible to understand the much more complex series of 

 changes through which such a structure as the cartilaginous femur of 

 the foetus passes in its transformation into the bony femur of the adult 

 (ossification in cartilage). 



Ossification in Membrane. The membrane, afterwards forming 

 the periosteum, from which such a bone as the parietal is developed, 

 consists of two layers an external fibrous, and an internal cellular or 

 osteo-genetic. 



The external one consists of ordinary connective tissue, being com- 

 posed of layers of fibrous tissue with branched connective-tissue corpus- 

 cles here and there between the bundles of fibres. The internal layer 

 consists of a network of fine fibrils with a large number of nucleated 

 cells, some of which are oval, others drawn out into a long branched 

 process, and others branched : it is more richly supplied with capillaries 

 than the outer layer. The relatively large number of its cellular ele- 

 ments, which vary in size and shape, together with the abundance of its 

 blood-vessels, clearly mark it out as the portion of the periosteum which 

 is immediately concerned in the formation of bone. 



In such a bone as the parietal, the deposition of bony matter, which 

 is preceded by increased vascularity, takes place in radiating spiculae, 

 starting from a " centre of ossification/' and shooting out in all direc- 

 tions towards the periphery. While the bone increases in thickness by 

 the deposition of successive layers beneath the periosteum, in-growths 

 of the osteogenetic layer of the periosteum take place, and it is by the 

 action of their osteoblasts that bone is secreted at a centre of ossification. 

 The osteoblasts, being in part retained within the primary bone trabec- 

 ulae thus produced, forming bone corpuscles. It is doubtful what part 

 the finely fibrillar part of the osteogenetic in-growth takes in the forma- 

 tion of the trabeculse, probably it supplies the reticular matrix of the 

 4 



