170 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



be found remaining, and presenting an appearance as if developed into a cluster 

 of minute cells ; whilst its prolongations form the ranaliculi, from which the 

 nuclear matter seems afterwards to disappear altogether. This view is sup- 

 ported by several considerations; amongst others, by the fact of the existence 

 of such stellate nuclei in many Vegetable cells (Fig. 14); and by the corre- 

 sponding appearances witnessed by Professor Owen in the formation of the 

 Cementum of Teeth, a structure identical with bone, and produced by the 

 calcification of the capsule ( 216). Others, again, regard the lacunae and 

 their radiating prolongations as themselves constituting cells ; and examples 

 are not wanting of similar forms in bodies known to have this character, as 

 the pigment-cells of the skin of Batrachia (Fig. 90, c). Dr. Sharpey, on the 

 other hand, states as the result of his observations, that the concentric layers 

 within the Haversian canals are formed by a process analogous to the intra- 

 membranous ossification ; namely, by the calcification of successive layers of 

 fibres, generated in the blastema, and possibly derived from the granular cells. 

 These fibres, being arranged in a reticular manner, may here and there include 

 an entire cell or cell-nucleus, the presence of which may determine the posi- 

 tion of a lacuna; whilst the canalicula may result from the apposition of the 

 minute apertures, existing between the other reticulations of the decussating 

 fibres. This view seems to derive confirmation from the appearances pre- 

 sented by very thin shreds of the gelatinous matrix, left after the removal of 

 the calcareous matter by acid ; for these, according to Dr. S., are plainly com- 

 posed of transparent fibres, resembling those of the white fibrous tissues, in- 

 tersecting one another at acute angles, and forming a network, in the meshes 

 of which are minute perforations, that are nothing else than transverse sections 

 of the canaliculi. 



20J. In the formation of a long bone, we usually find one centre of ossifi- 

 cation in the shaft, and one in each of the epiphyses; in the flat bones, there 

 is one in the middle of the surface, and one in each of the principal processes. 



The ossification usually proceeds to a considerable 

 extent, however, in the main centre, before it com- 

 mences in the extremities or processes; and these 

 remain distinct from the principal mass of the bone, 

 long after this has acquired solidity. During the 

 spread of the ossifying process, the cartilaginous 

 matrix continues to grow, like cartilage in other 

 parts; but after the bony deposit has pervaded its 

 entire substance, in the manner just described, a 

 change takes place in the method adopted. The 

 osseous laminae, that subdivide the whole texture, 

 are removed by absorption from the interior of the 

 shaft, so as to leave the great central medullary 

 cavity; whilst, on the other hand, they receive pro- 

 gressive additions in the external portion, which is 

 thus gradually consolidated into the dense bone, 

 that forms the hollow cylinder of the shaft. This 

 consolidation is effected by the deposit of a series 

 of concentric laminae, one within another, on the 

 lining of the Haversian canals. The bone con- 

 tinues to increase in diameter, by the formation of 

 new layers upon its exterior ; anil Dr. Sharpey has 

 pointed out that these layers are formed, not (as usually stated) in a cartila- 

 ginous matrix, but in the substance of a membrane, consisting of fibres and 

 granular cells, and exactly resembling that in which the flat bones of the roof 

 of the skull are developed. The Haversian canals, too, of these new layers 



[Fig. 68. 



Scapula of a Fcetus at the 

 seventh month ; showing the pro- 

 gress of ossification. Natural 

 size. The light parts are epiphy- 

 ses as yet cartilaginous. From 

 the Museum of King's ColU-ge, 

 London.] 



