BONES, THIEB STRUCTURE. 31 



does not exist in all vertebrates, it is only in animals of 

 this branch that true bones occur. Hard parts, it is true, 

 exist in most or many of the animals belonging to the 

 other great branches of the animal kingdom ; but the 

 hard parts of polyps, of star-fishes, and of sea urchins, the 

 horny-like covering of beetles and some other kinds of in- 

 sects, and the still harder covering of crabs and lobsters, 

 the shells of snails, oysters, and clams all are different, in 

 very important particulars, from the bony material or hard 

 parts of the Vertebrates. 



Bones are composed of animal matter more or less ex- 

 tensively impregnated with earthy materials, especially 

 with phosphate and carbonate of lime, and throughout 

 whose substance are minute cavities called lacunse, which 

 send out many much more minute ramifications known as 

 canaliculi. Most bones are traversed by a network of small 

 canals, containing vessels which are supported by connect- 

 ive tissue and fatty matter ; these are called Haversian ca- 

 nals, and they open upon the surface of the bone, and there 

 the vessels which they contain become connected with 

 those of the tough connective tissue which immediately in- 

 vests the living bones, and which is known under the name 

 of periosteum. 



Bones, then, in the living vertebrates, we are to under- 

 stand, are living and vascular animal tissues, as truly so as 

 the flesh or any other tissues of the body, growing and 

 changing by internal additions and modifications in essen- 

 tially the same manner as the other tissues of the body 

 grow and change, and having the same or similar powers 

 of repairing an injury as the other tissues have. 



On the contrary, the shells of snails and clams grow and 

 change only by additions to and modifications of the cir- 

 cumference ; and, if broken, the parts do not really grow 



