GROWTH OF BONES. 23 



growing body, and be reproduced of a more conformable 

 size. When fractured, the broken parts may be cemented 

 together by newly superadded shell-substance from with- 

 out; but are not unitable by the action of the fractured 

 surfaces from within. 



Extension of parts, however, is not the sole process 

 which takes place in the growth of bone; to adapt a bone 

 to its destined ofl&ce, changes are wrought in it by the 

 removal of parts previously formed. In fishes, indeed, 

 we observe a simple unmodified increase. To whatever 

 extent the bone is ossified, that part remains, and, conse- 

 quently, most of the bones of fishes are solid or spongy 

 in their interior, except where the ossification has been 

 restricted to the surface of the primary gristly mould. 

 The bones of the heavy and sluggish turtles and sloths, 

 of the seals, and of the whale-tribe, are solid. But in the 

 active land quadrupeds, the shaft of the long bones of the 

 limbs is hollow, the first-formed osseous substance being 

 absorbed as new bone is being deposited from without. 

 The strens^th and lio^htness of the limb-bones are thus 

 increased after the well-known principle of the hollow 

 column, which Galileo, by means of a straw picked up 

 from his prison floor, exemi^lified, in refutation of a charge 

 of Atheism brought against him by the Inquisition. The 

 bones of birds, especially those of powerful flight, are re- 

 markable for their lightness. The osseous tissue itself is, 

 indeed, more compact than in other animals ; but its quan- 

 tity in any given bone is much less, the most admirable 

 economy being traceable throughout the skeleton of birds, 

 in the advantageous arrangement of the weighty material. 

 Thus, in the long bones, the cavities analogous to those 

 called " medullary" in beasts, are more capacious, and 

 their walls are much thinner. A large aperture called 



