24 ANATOMY OF VERTEBRATES. 



an end; but there is a slower growth 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 bone, 

 except the vessel supplying the marrow-cavity, undergoes 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 cast off 

 when too small for the 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 

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

 faces 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 

 office changes are wrought in it by the removal of parts pre- 

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

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

 remains, and consequently 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. 1 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 strength and lightness 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, 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 remarkable 

 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 admirable economy being traceable 

 throughout the skeleton of birds in the advantageous arrange- 



1 In this case, exemplified in bones of the Lophius, Gyrosteus, and the lower 

 fiatrachia, fossilisation, which affects only the ossified crust, leaves the appearance, 

 through the solution of the unossified cartilage, of a wide medullary cavity, which 

 might mislead the Palaeontologist in his inferences. 



