46 ANATOMY OF THE RABBIT 



was already present in many of the oldest known fossil vertebrates. 

 In most vertebrates, the cartilage rudiments later are largely re- 

 placed by bone and membrane bones are added. In many cases 

 the replacement of these elements by bone is not direct, certain 

 readjustments being necessary both for purposes of growth and to 

 meet the much more special functional requirements of the adult 

 skeleton. 



The way in which replacing bones are formed on the cartilage 

 basis explains many peculiarities of the adult skeleton. In the 

 embryonic condition the cartilage rudiments are enclosed by a 

 connective tissue sheath, equivalent to the periosteum of a bone 

 (p. 24), but described as the perichondrium. Like the periosteum, 

 this sheath contains many osteoblasts, which form bone material 

 both in the interior of the cartilage (endochondral bone) and on 

 its surface (intramembranous bone). The formation of endo- 

 chondral bone proceeds from certain localized areas, known as 

 centres of ossification, into which active cells of the perichondrium 

 are carried by vascular ingrowths, the periosteal buds. In the 

 vicinity of these the cartilage matrix partly dissolves and the cells 

 thereby set free die and disintegrate. The ingrowing buds extend 

 into the spaces thus produced, constituting the primary marrow, 

 and deposit layers of bone-matrix round the irregular calcified 

 spicules of cartilage-matrix which still remain. Such deposition 

 of bone material gradually extends from the centres of ossification 

 through the remainder of the cartilage replacing it. This condition 

 is partly illustrated in the distal epiphysis of the humerus shown in 

 Fig. 26 A, where the area of endochondral bone (eb) appears in the 

 centre of the mass of cartilage. 



In 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. 24), may 

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

 or actual extremity of the bone and several subsidiary centres for 

 its outstanding processes. In the shaft the formation of endochon- 

 dral bone is of short duration. Through the activity of the osteo- 



