ANATOMY OP VEKTEBRATES. 25 



merit of tlic weighty material. Tims, in the long bones, the cavi- 

 ties analogous to those called 'medullary' in beasts, are more 

 capacious, and their walls are much thinner: a large aperture 

 called the ' pneumatic foramen,' near one end of the bone, com- 

 municates with its interior ; and an air-cell, or prolongation of tlie 

 lung, is continued into and lines the cavity of tlic bone, which is 

 thus filled with rarefied air instead of marrow. The extremities 

 of such air-bones present a light open net-work, slender columns 

 sliooting across iu different directions from wall to wall, and these 

 little columns are likewise hollow. 



In the mammalian class, the air-cells of bone are confined to 

 the head, and are filled fronr the ca^'itics of the nose or ear, not 

 from the lungs. Such cells are called ' fr(jntal sinuses,' ' an- 

 trum,' ' sphenoidal,' and ' ethmoidal sinuses,' in man. The 

 frontal sinuses extend backward over the top of the skull in the 

 ruminant and some otlier quadrupeds, and penetrate the cores of 

 the horns in oxen, sheep, and certain antelopes. The most re- 

 markable developcmcnt of cranial air-cells is presented by the ele- 

 phant; the intellectual pltysiognomy of this huge quadruped being 

 caused, as in tlie owl, not by the actual capacity of the lirain case, 

 but by the vast extent of the pneumatic cellular structure between 

 the outer and iimer plates of the skull-wall. 



In all these varied modifications of the osseous tissue, the cavi- 

 ties therein, whether mere cancelli, or small medullar}' cavities 

 as in the Crocodile, or large medullary cavities as in the Ox, or 

 pneumatic cavities and sinuses as in the Owl, are the result of 

 secondary changes by absorption, and not of the primiti'^'e consti- 

 tution of the bones. These are solid iu their commencement in 

 all classes, and the vacuities are estaldishcd by the removal of 

 osseous matter previously formed, wlulst increase proceeds by 

 fresh bone Ijcing added to the exterior siu-face. 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 bones of the 

 Apteryx, exemplify arrested stages of that course of developement 

 through which the pneumatic wing-bone of the soaring Eagle had 

 previously passed. 



But these mechanical modifications do not exhaust all the 

 chano-es through which the parts of a skeleton, idtimately becom- 

 ino- Idouc, have passed ; they have been previously of a fibrous 

 or of a cartilaginous tissue, or both. Entire skeletons, and parts 

 of skeletons, of vertebrate animals exhiljit arrests of these early 

 stafi-es of developement ; and this quite irrespccti^'e of the grade of 



