DEVELOPEMENT OF HONE. 21 



thin layer of cartilage. The periosteum of the bones of the skull is 

 termed pericranium ; and the analogous membrane of external 

 cartilages, perichondrium. 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 supplying these membranes that the changes 

 required by nutrition occur in bones, and the secretion of medulla 

 into the interior is effected. The medullary canal of long bones, 

 and the cells of other bones, are filled with a yellowish oily substance 

 the medulla, which is contained in a loose cellular tissue formed 

 by the medullary membrane. 



Developement of Bone. The earliest trace of skeleton in the human 

 embryo is observed in tRe presence of semi-opaque lines, which 

 are seen through the transparent embryonic mass. This trace is 

 composed of a consistent granular jelly, and constitutes the gelatinous 

 state of osteo-genesis. In the second or cartilaginous state, the 

 semi-opaque jelly becomes dense, transparent, and homogeneous, the 

 change taking place from the surface towards the centre, and con- 

 stituting cartilaginification. In the third stage, the cartilage is tra- 

 versed by vessels carrying red blood, which proceed from the fibrous 

 investment and ramify in its interior. The cartilage immediately 

 surrounding these vessels, becomes opaque and of a yellowish red 

 colour. In the fourth stage,* the earthy constituents are attracted 

 from the blood by the opaque cartilage, which becomes altered in 

 character, and shoots into the transparent cartilage in the form of 

 reddish gray fibres, which communicate with each other at acute 

 angles and constitute an areolar osseous tissue. This is the state of 

 ossification. The succeeding changes are those of condensation and 

 the formation of cells, the Haversian and medullary canals. 



Cartilaginification is complete in the human embryo at about the 

 sixth week ; and the first point of ossification is observed in the 

 clavicle at about ihe seventh week. Ossification commences at the 

 centre, and thence proceeds towards the surface ; in flat bones the 

 osseous tissue radiates between two membranes from a central 

 point towards the periphery, in short bones from a centre towards 

 the circumference, and in long bones from a central portion, diaphysis, 

 towards a secondary centre, epiphysis, situated at each extremity. 

 Large processes, as the trochanters, are provided with a distinct 

 centre, which is named apophysis. 



The growth of the bone in length takes place at the extremity of 

 the diaphysis, and in bulk by fresh deposition on the surface ; while 

 the medullary canal is formed and increased by absorption from 

 within. 



The period of ossification^ is different in different bones ; the order 

 of succession may be thus arranged : 



* The spot at which this stage commences is called the punctum ossificalionis. G. 

 t Burdach, Physiologic. 



