16 Anatomy of the Rabbit. 



Cartilage bones, on the other hand, are the characteristic elements of 

 the skeleton. They are known as replacing bones because they are 

 formed on a cartilage basis, the latter, as indicated above, representing 

 the primary cartilage skeleton of the embryo. In the course of develop- 

 ment, except in certain regions where the cartilage persists throughout 

 life, the cartilage material is replaced by bone, which tends to surround 

 and also invade it. 



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 complete but 

 primitive skeleton. In many cases the replacement of these elements by 

 bone is not direct, certain readjustments being necessary, both for pur- 

 poses of growth and on account of the much more special functional 

 requirements 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, 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 forma- 

 tion of endochondral bone is preceded 

 by certain changes which take place in 



X rr .X-—/( A^ *■ mter i° r °f tne cartilage. In the 



latter, in certain areas, known as the 

 '■ttu. centres of ossification, the matrix 



becomes partly dissolved, the cells en- 

 larged and ultimately broken down. 

 These changes are associated with a 

 fig. to. Outline sketch of the proximal deposition of calcareous material, or 

 P ^„cta^epn?h u y ^ ?<2°S£? htTof \t calcification, by which the portion of the 

 femur. The accessory epiphyses are for cartilage undergoing transformation is 



the great (tr.ma.). lesser (tr.mi.). and ., , ,, , t , .-, • 



third (tr.t.) trochanters. temporarily strengthened. Into this 



, area the active cells of the perichondrium 

 are carried through the agency of vascular ingrowths, 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. 12, the figure being that of 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 ossifica- 

 tion 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. 10), 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 endochondral bone is of short duration. Through 

 the activity of the osteoblasts lying directly in the perichondrium, or 

 later the periosteum, a process of formation of intramembranous bones 

 goes on during the whole period of growth, and the result of the peripheral 



