THE NEUROSKELETON. 25 



actual capacity of the brain-case, but by tlie enormous 

 extent of tlie pneumatic cellular structure between the 

 outer and inner plates of the skull- walls. 



In all these varied modifications -of the osseous tissue, 

 the cavities therein, whether mere cancelli, or small me- 

 dullary cavities as in the crocodile, or large medullary 

 cavities as in the ox, or pneumatic cavities and smuses 

 as in the owl, are the result of secondary changes by 

 absorption, and not of the primitive constitution of the 

 bones. These are solid at their commencement in all 

 classes, and the vacuities are established by the removal 

 of osseous matter previously formed, whilst increase pro- 

 ceeds by fresh bone being added to the exterior surface. 

 The thinnest-walled and widest air-bone of the bird of 

 flight was first solid, next a marrow-bone, and finally 

 became the case of an air-cell. The solid bones of the 

 penguin, and the medullary femur of apteryx, exemplify 

 arrested stages of that course of development through 

 which the pneumatic wing-bone of the soaring eagle had 

 previously passed. 



But these mechanical modifications do not exhaust all 

 the changes through which the parts of a skeleton, ulti- 

 mately becoming bone, have passed ; they have been pre- 

 viously of a fibrous or of a cartilaginous tissue, or both. 

 Entire skeletons, and parts of skeletons, of vertebrate 

 animals exhibit arrests of these early stages of develop- 

 ment ; and this quite irrespective of the grade of the 

 entire animal in the zoological scale. The capsule of the 

 eyeball, for example, in man, is a fibrous membrane ; in 

 the turtle, it is gristle; in the tunny, and most other 

 fishes, it is bone. The skeletal framework of the little 

 l2iUcelet-fLsh{BrancJiwsioma) does not go beyond the fibrous 

 3 



