of the Vertebrate Skeleton. 275 



cording to Prof. Humphry, most effectively reduce their length 

 by sucli disease as obliterates the epiphysial lines, while their 

 thickness decreases by cessation of muscular action. 



Growth also has a local morphological aspect. Thus Eden- 

 tates, Cetaceans, Chelonians have the bones of the skeleton 

 solid ; most mammals and most living reptiles have medullary 

 cavities in their long bones, while in most birds these cavities 

 become chambers into which prolongations of the membrane 

 covering the lung extend. It is necessary to remark that 

 Edentates and Chelonians are comparatively inactive animals, 

 and that Cetaceans move in a comparatively unresisting me- 

 dium, so that, however active, their muscular labour is light ; 

 and that birds, as a rule, are far more active than mammals. 

 Now in mechanics there is a law (clearly stated as a mecha- 

 nical law by Mr. Herbert Spencer), the law of the neutral 

 axis, by which, if a substance is exposed alternately to pres- 

 sure in opposite directions, there will be at the outsides alter- 

 nate pressure and tension, and in an internal part (of varying 

 extent according to the substance strained) the neutral axis 

 which experiences compressions only. 



Now we have seen that the alternation of pressure and ten- 

 sion is the condition of growth, and compression the condition 

 of atrophy. Hence it may be inferred that the solidity of 

 bones will be in the inverse proportion to the activity of the 

 muscles which are attached to them ; or, speaking generally, 

 the hoUowness of bones is in direct proportion to the activity 

 of the animal, the compressions at the neutral axis necessarily 

 resulting in atrophy of the bone there. Among flexible trees," 

 the law of the neutral axis is seen in the formation of pith. 



Another special condition of bones, is that in some animals 

 they become composite — that is, develope special and terminal 

 parts or plates called epiphyses, which sometimes subsist 

 throughout life, and are sometimes obliterated as the energy 

 of growth declines. Thus, in the internal skeleton of living 

 Chelonians and Crocodiles I have not noticed any appearance 

 of sej)arate terminal ends ; while if certain bones of crocodiles 

 are compared with others of some marsupial mammals, there 

 will be seen, with a close resemblance of form, separate bone- 

 elements in the mammal, which make the articular ends. 

 Such separate elements may be seen in amphibians, lizards, 

 many mammals, and, rarely, perhaps,. in some birds. Why 

 this difference ? Of course we naturally infer that the kind of 

 pressure and tension which ossified the bone originally sets 

 up in the articular cartilage (or elsewhere) the same kind of 

 action within its substance by the mechanical power of loco- 

 motion. D^. Humphry states that epiphyses appear at the 



