o Anatomy of the Rabbit. 



The way in which bones are formed on a cartilage basis serves to 

 xplaih many pecuHarities of the adult skeleton. In the embryonic 

 ondition the cartilage rudiments are associated to form a com- 

 lete but primitive skeleton. In many cases the replacement of 

 base elements by bone is not direct, certain readjustments being 

 ecessary, both for purposes of growth and on account of the much 

 lore special functional requirements of the adult skeleton. 



In the embryonic condition the cartilage rudiments are en- 

 losed by a connective tissue sheath, equivalent to the periosteum 

 f a bone, but described as the perichondrium. The osteoblasts 

 f this layer are concerned with the formation of bone material, 

 oth in the interior (endochondral bone) and on the surface 

 .ntramembranous bone). The formation of endochrondral 



bone is preceded by certain changes 

 which take place in the interior of the 

 cartilage. In the latter, in certaifi 

 areas, known as the centres of ossifi- 

 cation, the matrix becomes partly 

 '. rru. dissolved, the cells enlarged and 

 ultimately broken down. These 

 changes are associated with a deposi- 

 tion of calcareous material, or calci- 



FiG. II. Outline sketch of the prox- i2j.-i i-ii 



al end of the femur of a young IlCatlOn, by whlCh the portlOn of the 



imal: c.f., principal epiphysis for the .. ' , . ^ 



ad of the femur. The accessory Cartilage Undergoing transformation 



iphyses are for the great (tr.ma.), . ., i i t 



ser (tr.mi.), and third (tr.t.) tro- IS temporarily strengthened. Into 



inters. , . , . ,, ,- , 



this area the active cells of the 

 ;richondrium are carried through the agency of vascular in- 

 -owths, the periosteal buds, and the result of their presence is 

 le deposition of bone material in association with the remaining 

 jrtions of the matrix. This condition is partly illustrated in the 

 istal epiphysis of the humerus shown in Fig. 13, A, the figure being 

 om a vertical section of the elbow-joint of a four-day-old rabbit. 



In the long bones the formation of the first or main centres of 

 isification takes place in the shaft, and there are formed afterwards 

 xessory or epiphysial centres for the extremities. A divided 

 :tremity, such as the proximal end of the femur (Fig. 11), may 

 )ssess several such centres — a principal one for the chief epiphysis 

 • actual extremity of the bone and several subsidiary centres for 



