STRUCTURE OF THE ELEMENTARY TISSUES. 47 



(a.) Those which are ossified directly in membrane, e.g., the bones 

 forming the vault of the skull, parietal, frontal. 



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

 cartilage, .</., '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 



a 



FIG. 49. Lamellae torn off from a decalcified human parietal bone at some depth from the sur- 

 face, a, a lamella, showing reticular fibres ; 6, 6, darker part, where several lamellae are superposed ; 

 c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in 

 the lower part, a, a, of the figure. (Allen Thomson.) 



changes through which such a structure as the cartilaginous femur of the 

 foetus passes in its transformation into the body femur of the adult (ossi- 

 fication in cartilage). 



Ossification in Membrane. The membrane or periosteum from 

 which such a bone as the parietal is developed consists of two layers an 

 external fibrous, and an internal cellular or osteogenetic. 



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

 posed 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 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 elements, their varia- 

 bility 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, 



