GROWTH OF BONES. 21 



ton, in the sturgeon and armadillo, are explicable only ' 

 with reference to the different media and other conditions 

 under which the two vertebrates were destined to exist. ' 



In no species, and in no system of the skeleton, are 

 bones a primary formation of the animal: they are the 

 result of transmutations of pre-existing tissues, as sub- 

 stances composing animal bodies — e. g. nerve, muscle, 

 membrane, &c. — are called. The inorganic salts, defined 

 in the tabular view of the composition of bone, pre-exist ' 

 in the blood, in the albumen of the egg of the oviparous 

 vertebrates, and in the milk which nourishes the new-born 

 mammal. 



The primitive basis, or "blastema," of bone is a sub- 

 transparent glairy matter, containing a multitude of 

 minute corpuscles. It progressively acquires increased 

 firmness — sometimes assumes a membranous or ligament- 

 ous state, sometimes a gristly state, before its conversion 

 into bone. Its assumption of the gristly state is attended 

 by the appearance in it of numerous minute nucleated 

 cells. As the gristle or "cartilage" hardens, these cells 

 increase in number and size, and are aggregated in rows at 

 the part where ossification is about to begin. These rows, 

 in the cartilaginous basis of long bones, are vertical to its 

 ends — in that of flat bones they are vertical to the peri- 

 pheral edge. The nucleated cells are the instruments by 

 which the earthy particles are arranged in order; and in i 

 bone, as in tooth, there may be discerned, in this prede- • 

 termined arrangement, the same relation to the acquisi- ■ 

 tion of strength and power of resistance, with the greatest j 

 economy of the building material, as in the disposition of i 

 the beams and columns of a work of human architec-; 

 ture. 



Osteine, so formed, is arranged in thin plates, concen- 



