178 SKELETON. 



embryo, and at this period has the face scarcely visible. The 

 parietes of this vesicle are formed by a thin membrane, con- 

 sisting of two layers, the external of which is the pericranium, 

 and the internal layer is the dura mater. These layers adhere 

 so closely that they cannot be accurately separated by the 

 knife. 



About the third month of the embryo, or even earlier, ossifi- 

 cation may be seen at several points of the cranium, but more 

 extensively about its base. These points are the centres of os- 

 sification, which progressively increase towards their respec- 

 tive circumferences, by the deposite of new bony matter. 

 Generally the base of the cranium begins to ossify before the 

 vault, and is entirely ossified at birth, with the exception of a 

 few parts, as the clinoid processes and the ethmoid bone. 



The following nuclei of ossification show themselves between 

 the laminae of the foetal cranium, from the third to the fourth 

 month. One at the anterior part, for the centre of either side 

 of the os frontis; one for the centre of each parietal bone, on 

 the upper side of the head; one on the side of the head below, 

 for the squamous portion of the temporal bone; and there are 

 several for the occipital bone. These points extend themselves 

 in radii; and, as the intervals between the latter become wider 

 by their divergence, new radii, as observed elsewhere, are de- 

 posited between them. In some of the bones, the radii, from 

 opposite points, in the progress of ossification before and after 

 birth, meet and coalesce : this occurs in the os frontis and in 

 the os occipitis. 



At birth the contiguous margins of the flat bones simply ap- 

 proach each other, but have not interlocked. These bones con- 

 sist then of but one table, the edges of which are very finely ser- 

 ratedj and thereby show the radii of ossification. The edges are 

 held together by the dura mater, internally, and the pericranium, 

 externally; but the fissure between them is very obvious, and so 

 large that it allows very readily considerable motion and the 

 mounting of one bone upon the other by slight pressure. It is 

 always to be observed that the base of the cranium is an ex- 

 ception to the latter rule, both from the breadth of its articulating 

 surfaces, and from its comparatively advanced ossification. In 

 parturition, therefore, the vault of the cranium, by its mobility, 

 is adjusted to the contoar of the pelvis, but the base does not 



