297 



CHAPTER XXVI. 



OSSEOUS SYSTEM OF MAMMALIA. 



§ 175. General Characters of the Skeleton. — The osseous tissue 

 and the bone-cells characteristic of it in the higher members of the 

 Mammalian class are shown in Vol. I. p. 23, figs. 14, 15. In the 

 Lyencephala ( Ornithorhynchus, Echidna, Kangaroo, Kat, Beaver, 

 Sloth, Hedgehog, Moley the Haversian canals resemble those of 

 Birds in their smaller relative size, as do the bone-cells in the 

 number and peculiar branchings of their canaliculi, compared 

 with higher Mammalia\ in these the radiated disposition of the 

 canaliculi, concomitantly with the shorter and wider form of the 

 cells, becomes more marked, as shown in fig. 14, Vol. I. In 

 the larger Cetacea the bone-cells have a larger size and less 

 regular shape, and send off long branching canaliculi.^ The 

 osseous tissue in Mammals is less dense and compact than in 

 Birds: the long bones have medullary cavities, as a rule, re- 

 latively larger than in Rejjtilia, smaller and with thicker walls 

 than the homologous pneumatic cavities in Birds. In the Cetacea 

 and the Sloths, recent and extinct, the long, like the other, bones 

 are solid, the central tissue being cancellous : in the Sirenia the 

 bone of the thick ribs is dense and compact throughout: the 

 hardest bone in the present class is that which is accordingly 

 termed * petrosal,' especially in the Whale- tribe, in which its 

 specific gravity reaches 2*433, that of ivory being 1*744. 



The proportion of the Mammalian skeleton which is pneumatic 

 is noticed in Vol. I. p. 25. The vertebral bodies and the limb- 

 bones have the articular surfaces, in the grooving state, supported 

 on distinct plates, called ' epiphyses,' Avhich usually coalesce with 

 the rest of the bone, at maturity. Examples of the exoskeleton 

 are seen in the Armadillos and their huge extinct congeners the 

 Glyptodons: small detached bony nodules were also developed 

 in parts of the thick tegument of the Megatherioids.^ The 

 lacrymal is properly a mucous scale-bone. The bone of the heart 



' ccxciii. pis. xi. and xii. ^ lb. p. 151. ^ Burmcistcr, MS. 



