20 



THE BONES. 



Fig. 12. 



masses forms layers of bone substance, that finish by constituting the lamellae of 

 spongy tissue at the periphery and medullary tissue at the centre, to fill the 

 areolae of the latter. 



In this way is accomplished the transformation of cartilage into bone. As 

 will have been observed, it is not a mere calcification of the cartilaginous tissue, 

 but a real substitution of bone tissue, presenting the following phases : 1. Peri- 

 chondrial and endochondral calcification in places (nuclei or centres of ossifica- 

 tion). 2. Invasion of the centres of ossification by vessels, rivulation of the 



cartilage. 3. Medullization of the cartilage, and 

 formation of osteoblasts. 4. Development of the 

 bone tissue, appearance of the osteoplasts. 



B. Development of the Fibrous Bones. — The bones 

 originally fibrous are not merely charged with lime 

 salts in order to acquire a bony appearance. They 

 do not pass through a transitory cartilaginous state, 

 but the phenomena of ossification take place in the 

 fibrous tissue in the following manner : — 



The fasciculi of connective tissue become calcified, 

 separate here and there from one another to form 

 spaces, into which blood-vessels enter or osteoblasts 

 appear. To this phase of medullization succeeds the 

 phase of ossification, which is accomplished in the 

 same manner as in the cartilaginous bones. Only 

 the directing lines are represented by connective 

 fasciculi, which later become Sharpey's fibres. 



C. Progress of Ossijication. — Ossification com- 

 mences simultaneously in several parts of the skeleton, 

 and in each of the bones in particular — though not 

 throughout their entire extent at the same time. On 



CARTILAGE AT THE SEAT OF the coutrary, in certain determinate points of the carti- 

 ossiFicATioN, showing at ifc laginous or fibrous mass, osseous tissue is developed 

 ircells^"rirged*m column'! ^^^ich, gradually extending, ends by completely in- 

 each of which is enclosed in a vadiug it. Thcsc poiuts are named nuclei (or centres) 

 sheath of calcified intercellular ^f ossificatiou. The uuclci are primitive or comple- 



substance. , mi i i • , 



mentary. ihe latter, termed epiphyses, are in a 

 fashion added to the bone, and wholly or partly form certain apophyses. 



Although these centres of ossificatiou increase from day to day, they never- 

 theless remain for a long time independent of each other, and are united only by 

 cartilage. When the skeleton is completely developed, the various centres of 

 ossification become joined to each other, and the entire bone forms one piece ; 

 there are no longer any apophyses. 



Up to the present time, attempts to discover the laws which govern the 

 appearance of the centres of ossification have been futile ; the size of the bone 

 counts for nothing ; and the influence of proximity to the centres of circulation, 

 which has been sometimes brought forward (for the sternum, for example, 

 which is never completely ossified), cannot be accepted. The law which 

 presides over the union of the centres of ossification has also been sought 

 for. A. Berard imagined he could formulate it in the following proposition .- 

 Of the two extremities of a long bone, it is always that towards which the nutrient 

 foramen is directed that is first united to the body of the bone. But to Berard's 



