THB RABBIT. 55 



phosphate in their matrix ; but while the latter become true 

 bone-corpuscles, with canaliculi to nourish them, the former 

 by this secretion of phosphate shut themselves off gradually 

 from nutrition. At this point we have a mass of calcified 

 cartilage surrounded by a thin layer of true bone. Next 

 comes a very remarkable change : a great number of very 

 large cells from the perichondrium break through the thin 

 layer of bone and literally eat their way into the calcified 

 cartilage. These devouring cells are called osteoclasts ; 

 accompanying or following them are other cells which are 

 ordinary bone-corpuscles or osteoblasts, and these begin to 

 form true bone among the ruins of the calcified cartilage. 

 Blood-vessels also grow in along with the osteoblasts. Thus 

 cartilage-bone differs from membrane-bone, in the fact 

 that its formation is accompanied by a migration into a 

 region previously occupied by cartilage. There is no change 

 of cartilage into bone : the original cartilage matrix and 

 corpuscles is entirely eaten up, except certain portions to 

 be immediately mentioned, which persist as unaltered 

 cartilage. 



These persistently cartilaginous regions of a cartilage- 

 bone are. firstly, the two ends in the case of bones which are 

 movably jointed or articulated to other bones, as are the 

 limb-bones. Secondly, in many bones the immigration of 

 osteoclasts and osteoblasts takes place at more than one 

 point usually at three, one in the middle and one at each 

 end. These two end-regions of ossification (i.e. bone-forma- 

 tion) are much smaller than the main central one and are 

 called the epiphyses of the bone. Between the epiphyses 

 and the main ossification a narrow band of cartilage per- 

 sists, and the presence of this makes possible the growth of 

 the whole bone in length. Bone-tissue can easily grow in 

 thickness, by additions from the cellular layers of the 

 periosteum, but since the ends usually have a definite form 

 of their own, growth in length cannot take place by mere 

 additions of bone at the two ends there must be inter- 

 calation of new material somewhere between. The actual 

 growth in length is effected by the persistent cartilage, and 

 in proportion as this increases its substance the osteoclasts, 

 like rack-renting landlords, absorb the extra amount and 



