DEVELOPMENT OF BONES. 69 



in their groups, respectively, but one piece; while those which 

 are to be united by ligament, and consequently to be moveable, 

 as the femur, the tibia, the clavicle, and so on, are respectively 

 insulated. In the cartilaginous state the bones have neither cells 

 nor medullary cavities, and consist in a solid, homogeneous mass, 

 the form of which is sufficiently definite; and has its surface 

 covered by periosteum. 



The flat bones of the cranium seem to be an exception to the 

 general rule of a preliminary cartilaginous state, and are com- 

 monly thought to be such. Their appearance is delusive, from 

 the cartilage being extremely soft and thin, and concealed by 

 the pericranium on the one side, and the dura mater on the 

 other; but a careful dissection enables one to distinguish it from 

 this double envelope.* 



III. The osseous matter begins to be deposed when the rudi- 

 ments of the bone have become entirely cartilaginous, with the 

 exception of a few mucous points. In certain bones this change 

 is observable about the commencement of the second montht 

 after conception: J. F. Meckel has placed it about the eighth 

 week. The colour of the cartilage first becomes deeper; and, 

 in the region where ossification is to commence, is of a well 

 marked yellow. The blood vessels, which before this carried 

 only the transparent part of the blood, now dilate, so as to admit 

 the red particles, and a red point is perceived, called the punc- 

 tum ossificationis, from its receiving the first calcareous deposite. 

 This deposite is always near the very centre of the cartilaginous 

 rudiment, and not at its surface: the portion of cartilage nearest 

 to it is of a red colour; but, a little farther off, opaque and hol- 

 lowed into canals. The ossification increases on the surface of 

 the cartilage, and in its interstices, by continual deposites, which 

 are always preceded by that condition just mentioned. The 

 canals of the cartilage transmit the blood vessels, and are large 

 at the beginning of ossification; but, as the process advances and 

 is completed, they diminish gradually, and finally disappear. 



The progress of ossification is somewhat modified in the three 

 classes of bones. 



* Bichat, loc. cit. t Beclard, loc. cit. Bichat, loc. cit. 



