46 DEVELOPMENT OF BONE. 



In the fresh state, bones are invested by a dense fibrous membrane, 

 the periosteum, which covers every part of their surface, with the excep- 

 tion of the articular extremities, the latter being coated by a thin layer 

 of cartilage. The periosteum of the bones of the skull is termed peri- 

 cranium ; and the analogous membrane of external cartilages, perichon- 

 drium. Lining the interior of the medullary canal of long bones, the 

 Haversian canals, the cells of the cancelli, and the cells of short, flat, 

 and irregular bones, is the medullary membrane, which acts as an internal 

 periosteum. It is through the medium of the vessels ramifying in these 

 membranes that the changes required by nutrition occur in bones, and 

 the secretion of medulla into their interior is effected. The medullary 

 canal, Haversian canals, and cells of long bones, and the cells of other 

 bones, are filled with a yellowish oily substance, the medulla, which is 

 contained in a loose areolar tissue formed by the medullary membrane. 



Development of Bone. To explain the development of bone it is ne- 

 cessary to inform the student, that all organized bodies, whether belong- 

 ing to the vegetable or the animal kingdom, are developed primordially 

 from minute vesicles. These vesicles, or, as they are commonly termed, 

 cells, are composed of a thin membrane containing, a fluid or granular 

 matter, and a small rounded mass, the nucleus, around which the cell was 

 originally formed. Moreover, the nucleus generally contains one or 

 more small round granules, the nucleolus or nucleoli. From cells having 

 this structure all the tissues of the body are elaborated ; the ovum itself 

 originally presented this simple form, and the embryo at an early period 

 is wholly composed of such nucleated cells. In their relation to each 

 other, cells may be isolated and independent, as is exemplified in the 

 corpuscles of the blood, chyle, and lymph ; secondly, they may cohere 

 by their surfaces and borders, as in the epiderma and epithelium; thirdly, 

 they may be connected by an intermediate substance, which is then 

 termed intercellular, as in cartilage and bone ; and fourthly, they may 

 unite with each other in rows, and upon the removal, by liquefaction, of 

 the adherent surfaces, be converted into hollow tubuli. In the latter mode 

 capillary vessels are formed, as also are the tubuli of nerve and muscular 

 fibre. One of the properties of cells may also be adverted to in this place ; 

 it is that of reproducing similar cells in their interior. In this case the 

 nucleoli become the nuclei of the secondary cells, and as the latter increase 

 in size, the membrane of the primary or parent cell is lost. 



Bone, in its earliest stage, is composed of an assemblage of these mi- 

 nute cells, which are soft and transparent, and are disposed w T ithin the 

 embryo in the site of the future skeleton. From the resemblance which 

 the soft tissue bears to jelly, this has been termed the gelatinous stage of 

 osteo-genesis. As development advances, the cells, heretofore loosely 

 collected together, become separated by the interposition of a transparent 

 intercellular substance, which is at first fluid, but gradually becomes hard 

 and condensed. The cartilaginous stage of osteo-genesis is now esta- 

 blished, and cartilage is shown to consist of a transparent matrix, having 

 minute cells disseminated at pretty equal distances, and without order, 

 through its structure. Coincident with the formation of cartilage is the 

 development of vascular canals in its substance, the canals being formed 

 by the union of the cells in rows, and the subsequent liquefaction of the 

 adhering surfaces. The change which next ensues in the concentration 

 of the vascular canals towards some one point ; for example, the centre 



