24 ANATOMY OF VERTEBRATES. 



an end; but there is a slower growtli going on over the entire peri- 

 phery of the bone, which is covered by a membrane, called the 

 ' periosteum.' In this membrane, the vascular system of a l)one, 

 except the vessel supplying the marrow-cavity, imdergoes the 

 amount of subdivision which reduces its capillaries to dimensions 

 suited for penetrating the pores leading to the vascular canals, 

 figs. 11 and 15, a, a. 



Thus bone is a living and vascular part, growing by in- 

 ternal molecular addition and change, and having the power of 

 repairing fracture or other injury. The shells and crusts of 

 molluscous and crustaceous animals are unvascular ; they grow 

 by the addition of layers to their circumference, may be east off 

 wlien too small for the growing body, and be reproduced of a 

 more eonformal)le size. AVhen fractured, the l^i'oken parts may 

 be cemented togctlicr by newly superadded shell-substance from 

 without ; but are not unitable by the action of the fractured sur- 

 faces from witliin. 



Extension of parts, however, is not the sole process wliicli 

 takes place in the growth of bone ; to adajjt a bone to its destined 

 (jffice changes are wrought in it by the remo\al of parts pre- 

 viously formed. In fishes, indeed, we observe a simple unmodi- 

 fied increase. To whatever extent the bone is ossified, that jiart 

 remains, and conscfpiently most of tlic bones of fishes are solid or 

 spongy in their interior, except where tlie ossification has been 

 restricted to the surface of the primary gristly mould.' The bones 

 of the heavy and sluggish turtles and sloths, of tlie seals, and of 

 the whale-tribe, are solid. But in the acti\ e land quadrupeds, the 

 shaft of tlie long bones of the limbs is hollow, the first formed 

 osseous substance being absorbed, as new bone is licing deposited 

 from without. The strength and lightness of the limli-bones are 

 thus increased after the well-known principle of the hollow cohunn, 

 which Galileo, by means of a straw picked up from his ])rison 

 floor, exemplified, as an evidence of design, in refutation of a 

 charge of Atheism brought against him by the Inquisition. The 

 bones of birds, especially those of powerful flight, are remarkalde 

 for their lightness. The osseous tissue itself is, indeed, more 

 compact than in other animals; but its quantity in any given 

 bone is much less, the most admiralile economy being traceable 

 throughout the skeleton of ))irds in tlic advantageous arrange- 



' In this case, cxomplified in bones of the Lophiiis, Gi/roxiciis-, ami the lower 

 Batracliia, fossilisatioii, wliicli aflVcts only the ossilied ciust. lonves tlie a). pea ranee, 

 throngh the solution of the unossiiied cartilat;e, of a wide inedullarv cavity, wliieb 

 might mislead the raltcontologist in his inferences. 



