36 LECTURE II. 



The thinnest-walled and hollowest pneumatic bone of the bird of 

 flight was first solid, next a marrow-bone, and finally the case of an 

 air-cell. The solid bones of the Penguin, and the medullary femur 

 of the Apteryx and Dinornis, are arrested stages of that course of 

 development through which the pneumatic wing-bone of the soaring 

 Eagle had previously passed. 



In proceeding after the foregoing survey of the general nature, 

 chemical constitution, development, growth, and structure of the 

 osseous system, to the description of the skeleton in the vertebrate 

 animals, there next remains to define a hone ; and the endeavour to 

 do this has not been the least difficult part of my task, with re- 

 ference to the applicability of the definition to the vertebrate series 

 in general. 



To the human anatomist the question — what is a bone? — may ap- 

 pear a very simple, if not a needless one : he will most probably reply 

 that a bone is cmy single piece of osseous matter entering into the 

 composition of the adult skeleton ; and, agreeably with this definition, 

 he will enumerate about 260 bones in the human skeleton. 



Soemmering, who includes the thirty-two ^teeth in his enumeration, 

 reckons from 259 to 264 bones ; but he counts the os spheno-occipitale 

 as a single bone, and also regards, with previous anthropotomists, the 

 OS temporis, the os sacrum, and the os innominatum, as individual 

 bones ; the sternum, he says, may include two or three bones, &c. 

 (xiii. p. 6.) : but, in Birds, the os occipitale is not only anchylosed to 

 the sphenoid, but these early coalesce with the parietals and frontals ; 

 and, in short, the entire cranium proper consists, according to the 

 above definition, of a single bone. Blumenbach, however, applying 

 the human standard, describes it as composed of the proper bones of 

 the cranium consolidated, as it were, into a single piece (xjv. p. 56.). 

 And in the same spirit most modern anthropotomists, influenced by 

 the comparatively late period at which the sphenoid becomes anchy- 

 losed to the occipital in Man, regard them as two essentially distinct 

 bones. In directing our survey downwards in the mammalian scale, 

 we speedily meet with examples of persistent divisions of bones which 

 are single in Man. Thus it is rare to find the basi-occipital confluent 

 with the basi-sphenoid in mammalian quadrupeds ; and before Ave quit 

 that class we meet with adults in some of tlie marsupial and monotre- 

 matous species, for example, in which the supra-occipital, " pars 

 occipitalis proprie sic dicta," of Soemmering, is distinct from the con- 

 dyloid parts, and these from the basilar or cuneiform process of the os 

 occipitis : in short, the single occipital bone in Man is four bones in 

 the Opossum or Echidna; and just as the human cranial bones lose 

 their individuality in the Bird, so do those of the Marsupial lose their 



