PERIOSTEAL OSSIFICATION. 473 



it is at once perceived that the disposition of the con- 

 stituent parts is entirely that of a bone, for (primary*) 

 medullary spaces and trabecular networks alternate with 

 each other, just as if the observer had before him the 

 medullary spaces and trabeculae of a spongy bone. 

 The substance which forms the trabecular networks, 

 is on the whole dense, and is therefore readily distin- 

 guishable, even with a low power, from the more deli- 

 cate substance which is enclosed by the trabeculae and 

 fills the cavities of the meshes. This latter substance 

 presents, when more highly magnified, a finely striated 

 fibrous appearance. The bands of fibres in part run 

 parallel to the borders of the trabeculae. In these latter 

 the same structures can be seen with a high power, that 

 are usually presented by the bone, namely jagged cor- 

 puscles, distributed with great regularity. 



This structure exactly corresponds to that which we 

 have seen in the development of bone from periosteum ; 

 it is, in short, the plan followed in the growth of bone in 

 thickness. Wherever young periosteal deposits are ex- 

 amined, there is found in .the meshes of the network, 

 formed by the osteoid substance, this primary marrow 

 containing fibres, but no cells, as is the case at a later 

 period. This primary marrow consists of the remains of 

 the periosteum itself (after its proliferation), which have 

 not yet undergone the transformation. The transforma- 

 tion into osteoid tissue advances into the proliferating 

 periosteum in the first instance always in such a way, 



* The primary medullary spaces formed out of periosteum are subsequently 

 all filled with compact bone, and it is by the conversion of this into true mucous 

 medullary tissue, abounding in cells (which afterwards take up fat) that the 

 secondary medullary spaces are formed. Of the primary medullary spaces formed 

 out of cartilage, however, a considerable number do not pass through any 

 such intermediate stage as that just described as occurring in peiiosteal ossifi- 

 cation, but become at once filled with true medullary tissue and are therefore 

 equivalent to the ordinary, secondary medullary spaces. From a MS. Note by the 

 A uthor. 



