56 



ZOOLOGY. 



so keep the cartilage small in amount in spite of its 

 continued increase, Eventually, when no more growth in 

 length is necessary, the whole of the cartilage is absorbed, 

 and the epiphyses unite with the main bone. 



The work of osteoclasts does not end with the disappear- 

 ance of cartilage from any region of the bone. The first- 

 formed bone is in its 

 turn absorbed to a 

 very great extent, the 

 marrow-cavity being 

 thus formed. At first, 

 too, the osteoblasts and 

 their matrix are all 

 arranged in one series 

 of concentric circles (or 

 rather cylinders) ; this 

 primitive bone around 

 the marrow- cavity is 

 in its turn partly ab- 

 sorbed, and a new series 

 of minor cavities the 

 Haversian canals- ex- 

 cavated in it, around 

 which new osteoblasts 

 are concentrically a - 

 ranged. In mem- 

 brane-bones, also, the 

 Haversian canals are 

 similarly formed. Thus 

 bones, in spite of their 

 apparent stony cha- 

 racter, and indeed by 

 reason of their want 

 of plasticity, are subject to a constant reconstruction which 

 cartilage has no need of. 



1 2. The Teeth of the rabbit are of complex structure 

 (see fig. 17) two tissues, and a secretion that is hardly a tissue, 

 forming them. The secretion is enamel, an extremely hard 

 substance (composed of the same compounds as bone, but 



Fig. 17. STRUCTURE OF TOOTH. 

 (After Klein.) 



