30 



Anatomy of the Rabbit. 



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

 explain many peculiarities of the adult skeleton. In the embryonic 

 condition the cartilage rudiments are associated to form a com- 

 plete but primitive skeleton. In many cases the replacement of 

 these elements by bone is not direct, certain readjustments being 

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

 more special functional requirements of the adult skeleton. 



In the embryonic condition the cartilage rudiments are en- 

 closed by a connective tissue sheath, equivalent to the periosteum 

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

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

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

 (intramembranous bone). The formation of endoch/mdral 



bone is preceded by certain changes 

 which take place in the interior of the 

 cartilage. In the latter, in certain 

 areas, known as the centres of ossifi- 

 cation, the matrix becomes partlv 

 dissolved, the cells enlarged and 

 ultimately broken down. These 

 changes are associated with a deposi- 

 tion of calcareous material, or calci- 

 fication, by which the portion of the 

 cartilage undergoing transformation 

 is temporarily strengthened. Into 

 this area the active cells of the 

 perichondrium are carried through the agency of vascular in- 

 growths, the periosteal buds, and the result of their presence is 

 the deposition of bone material in association with the remaining 

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

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

 from 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 

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

 accessory or epiphysial centres for the extremities. A divided 

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

 possess several such centres— a principal one for the chief epiphysis 

 or actual extremity of the bone and several subsidiarv centres for 



Fig. 11. Outline sketch of the prox- 

 imal end of the femur of a young 

 animal : c.f., principal epiphysis for the 

 head of the femur. The accessory 

 epiphyses are for the ureat (tr.ma.), 

 lesser (tr.mi.), and third (tr.t.) tro- 

 chanters. 



