THE SKELETON. 33 



the long bones of Mammalia, takes place chiefly at the cai'tilaginous 

 ends beneath the epiphyses. This is proved by boring holes, or intro- 

 ducing shots, at definite distances in the diaphyses of growing bones, 

 and examining the perforations a week or a fortnight after the ex- 

 periments. The interval between the holes next the ends of the 

 bone is found much increased, whilst that between those nearer the 

 middle is but little, if at all, changed. All these experiments concur 

 to prove that the growth of a bone is not by uniform and general 

 extension, but by accelerated increase at particular parts. 



But extension of parts is not the sole process which takes place in 

 the growth of bone : to adapt the bone to its specific ofiice changes 

 are wrought in it by the absorption of parts previously formed, espe- 

 cially in the higher classes of Vertebrata. In fishes we observe a 

 simple unmodified increase ; but in some species, ossification com- 

 mences at the periphery of the animal mould or basis, and is always 

 limited to a thin outer crust of the bone, the rest remaining carti- 

 laginous or gelatinous. In some of the higher cartilaginous fishes, 

 for example, an osseous crust is formed upon the periphery of certain 

 cartilages, in the form of prisms, which contain oval calcigerous cells, 

 but without conspicuous radiated tubes. Such bones in a dried or 

 fossil state seem to have had large internal or medullary cavities ; but 

 they were filled by the unossified animal basis. To whatever extent 

 the bone of a fish is originally ossified, such it remains, and con- 

 sequently most of the bones of fishes are solid or spongy in their 

 interior. 



The bones of the Chelonia are likewise solid ; a coarse diploe fills 

 the interior of the long bones of the extremities ; and we find a 

 similar structure in the bones of the Cetacea and of the Seal tribe. 

 Among terrestrial mammals the inactive Sloths and their great 

 extinct congeners, the Megatherium and Mylodon (xii. p. 83.), 

 have the long bones of the extremities solid ; whilst the agile Ru- 

 minant shows each diaphysis in the condition of the hollow column, 

 both the strength and lightness of the bones being increased by the 

 progressive absorption of the first-formed substance, as new bone is de- 

 posited from without. The condition which is illustrated by this section 

 of the femur of the Nilghau (Prep. 856 c), is common, in fact, to the 

 long bones of all land mammals, except the Tardigrades above specified. 

 The Saurian and most of the Batrachian reptiles have likewise the 

 cavity in each long bone, called " medullary," from its containing a 

 cellular tissue filled by a fine, light, oily matter or marrow. Even 

 the ribs of the large Ophidians have their medullary cavities : and 

 the bodies of the vertebrae of some lizards and of the great extinct 

 Poikilopleuron are similarly excavated. The medullary cavities of the 



VOL. II. D 



