THE STRUCTURE OF THE ELEMENTARY TISSUES. 61 



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, afterward 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 layer is made up of ordinary connective-tissue, being 

 composed of layers of fibrous tissue with branched connective-tissue 

 corpuscles here and there between the bundles of fibres. The internal 

 layer consists of a network of fine fibrils with a large number of nucle- 

 ated cells with a certain addition of albuminous ground or cement sub- 

 stance between the fibrous bundles, some of which are oval, others 

 drawn out into long branched processes: it is more richly supplied 

 with capillaries than the outer layer. The relatively large number of 

 its cellular elements, 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, which is represented then when ossi- 

 fication commences by the species of fibrous connective tissue with many 

 cells above indicated, 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 directions toward the 

 periphery. These primary bony spiculae consist of the fibres of the tis- 

 sue which are termed osteo genetic fibres, composed of a soft transparent 

 substance called osteogen, in which calcareous granules are deposited. 

 The fibres are said to exhibit in their precalcified state indications of a 

 fibrillar structure, and are likened to bundles of white fibrous tissue, to 

 which they are similar in chemical composition, but from which they 

 differ in being stiffer and less wavy. The deposited granules after a 

 time become so numerous as to fill up the substance of the fibres and 

 bony opiculae result. Calcareous granules are deposited also in the in- 

 terfibrillar matrix. By the junction of the osteogenetic fibres and their 

 resulting bony spiculae a meshwork of bone is formed. The osteo- 

 genetic fibres, which become indistinct as calcification proceeds, are 

 believed to persist in the lamellae of adult bone. The osteoblasts, being 

 in part retained within the bone trabeculae thus produced, form bone 

 corpuscles. On the bony trabeculae first formed, layers of osteoblastic 

 cells from the osteo-genetic layer of the periosteum are developed side by 

 side, lining the irregular spaces like an epithelium (fig. 65, b). Lime- 

 salts are deposited in the circumferential part of each osteoblast, and 

 thus a ring of osteoblasts gives rise to a ring of bone with the remaining 



