STRUCTURE OF BONE. 



173 



Fig. 45. 



' 



nor are they composed of solid osseous substance. They are made up 

 of cancellated structure, as it is termed ; that is, of osseous lamellae and 



: fibres interwoven together (like those of areolar 

 tissue, on a larger scale) so as to form a multitude 



; of minute chambers or cancetti, freely communicating 

 with each other, and with the cavity of the shaft ; 

 whilst the whole is capped with a thin layer of solid 

 bone. Again, in the thin flat bones, as the scapula, 

 we find the two surfaces composed of solid osseous 



; texture, with more or less of cancellated structure 

 interposed between the layers. And in the thicker 



: flat bones, as the parietal, frontal, &c., this cancel- 



j. lated structure becomes very distinct, and forms the 



[ diploe ; this, however, is sometimes deficient, leaving 



' a cavity analogous to the canal of the long bones : 

 whilst the plates which form the surfaces of the bone 

 (the external and internal tables of the skull), re- 

 semble in their thickness and solidity, as well as in 

 the intimate structure presently to be described, the 



j shaft or hollow cylinder of those bones. Finally, we 



' frequently meet (especially in the Ethmoid and Sphenoid bones) with 



I thin lamellae of osseous substance, resembling those which elsewhere 



i form the boundaries of the cancelli ; these consist of but one layer of 



, bony matter, and show none of the varieties previously adverted to ; they 



are not penetrated by vessels, but are nourished only by their surfaces ; 

 and they consequently exhibit to us the elements of the osseous struc- 

 ture in their simplest form. It will be desirable, therefore, to commence 

 with the description of these. 



290. Wh^n a thin natural lamella of this kind is examined, it is found 

 to be chiefly made up of a substance which is nearly homogeneous, 

 sometimes exhibiting indistinct traces of a fibrous arrangement ; this, 

 however, may be generally resolved by prolonged boiling, into an assem- 



i blage of minute granules, varying in size from l-6000th to 1-14, 000th 



Fig. 46. 



Extremity of Os femo- 

 ris, showing, cancellated 

 structure :^0t, thin layer 

 of bone, in contact with 

 the articular cartilage ; 

 b, cancel! i. 



Lacunas of Osseous substance, magnified 500 diameters: a, central cavity; &, its ramifications. 



of an inch, which are more or less angular in shape, and seem to cohere 

 by the medium of some second substance, which is dissolved by the 

 boiling. They are composed of Calcareous salts, apparently in chemical 

 union with the Gelatine that forms the basis of the osseous substance. 

 In the midst of this granular substance, a number of dark spots are to 

 be observed, the form of which is very peculiar. In their general out- 

 line, they are usually somewhat oval ; but they send forth numerous 

 radiating prolongations of extreme minuteness, which may be frequently 





