42 FORMATION OF BONE. 



periods in other bones, in many not till long after birth, and is 

 not completed in all the bones of the body till near the period 

 of puberty. 



In the metamorphosis of cartilage to bone, the white and 

 homogeneous cartilage which forms the mould of the bone, 

 becomes hollowed out so as to present irregular cavities,* 

 which subsequently form canals lined by a vascular membrane 

 and filled with a viscous fluid, which extend to the centre of 

 its structure. One of these canals forms subsequently the 

 nutritious foramen. The cartilage becomes opaque and 

 yellowish round this spot, the vessels convey red blood, 

 numerous red points are formed in the structure, and ossification 

 commences at the centre of the bone ; never upon the surface. 

 In the long bones a bony ring is first formed in the centre, and 

 the vascular canals extend themselves in the direction of the 

 extremities — in the flat and thick bones, in radii, attended by a 

 redness in the cartilage, nearest the seat of ossification, and a 

 diflxised yellowness beyond it. From these canals the ossific 

 material is deposited, and the central point of ossification 

 grows, till the bone is completed. As the bony portion 

 advances in growth, its redness diminishes, and the vascular 

 canals which are at first large, decrease in size, so as to become 

 in the adult bone microscopical. The ossescent or provisional 

 cartilage of the bone, is solid and has in no instance any cavity 

 in its centre. The ring of bone which, as before observed, is 

 the first step of development in the long bones, has a cavity 

 in its centre which is subsequently destined to lodge the 

 medulla. In the flat bones, and especially those of the cranium, 

 ossification commences between the second and third months 

 of foetal life. Those of the cranium are formed between the 

 pericranium and dura mater, and their cartilaginous mould is 

 so thin and soft, that Howship and Beclard have denied its 

 existence. The vascularity commences in them at a central 



* According to the German anatomists, see page 44, the hollowing of these 

 canals, is produced by an aggregation of the cartilaginous corpuscles into a series 

 of linear ranges, between which the vessels shoot that convey the earthy mate- 

 rial of the bone. — p. 



