BONE. 



corpuscles or elementary cells, united together by a soft amorphous matter. 

 Very soon, however, they become cartilaginous, and ossification in due time 

 beginning in the cartilage and continuing to spread from one or from several 

 points, the bone is at length completed. 



But, while it is true with respect to the bones generally that their ossifi- 

 cation commences in cartilage, it is not so in every instance. The tabular 

 bones, forming the roof of the skull, may be adduced as a decided example 

 to the contrary ; in these the ossification goes on in a membranous tissue 

 quite different in its nature from cartilage ;* and even in the long bones, 

 in which ossification undoubtedly commences and to a certain extent pro- 

 ceeds in cartilage, it will be afterwards shown that there is much less of the 

 increment of the bone really owing to that mode of ossification than has, 

 till lately, been generally be- 

 lieved. It is necessary, there- Fig. XLIX. 

 fore, to distinguish two spe- 

 cies or modes of ossification, 

 which for the sake of brevity 

 may be called the intramem- 

 branous and the intracartila- 

 ginous. 



Ossification in membrane. 

 The tabular bones of the 

 cranium, as already said, 

 afford an example of this 

 mode of ossification. The 

 base of the skull in the em- 

 bryo is cartilaginous ; but iu 

 the roof, that is to say, the 

 part comprehending the pari- 

 etal, the upper and greater 

 part of the frontal, and a 

 certain portion of the occipital 

 bones, we find (except where 

 there happen to be commenc- 

 ing muscular fibres) only the 

 integuments, the dura mater, 

 and an intermediate mem- 

 branous layer, which differs 

 from cartilage in its intimate 

 structure as well as in its 

 more obvious characters, and 

 in which the ossification pro- 

 ceeds. 



The commencing ossifica- 

 tion of the parietal bone, 

 which may be selected as an 

 example, appears to the naked 

 eye in form of a net-work in 

 which the little bars or spicula of bone run in various directions, and meet 

 each other at short distances. By and by the ossified part, becoming 



* This fact was pointed out and insisted on by Dr. Nesbitt, who distinguishes the two 

 different modes of ossification, and so far his riews are quite correct. See his human 

 Osteogeny, Lond. 1736. 



Fig. XLIX. PARIETAL BONE OP AN EMBRYO 

 SHEEP. SIZE OP THE EMBRYO, 2^ INCHES. 



The small upper figure represents the bone of the 

 natural size, the larger figure is magnified about 12 

 diameters. The curved line, a, b, marks the height 

 to which the subjacent cartilaginous lamella ex- 

 tended. A few insulated particles of bone are seen 

 near the circumference, an appearance which is quite 

 common at this stage. 



