150 Prof. Macalister, Variations in the ossification 



Variations in tJie ossification of the Occipital bone. By Prof. 

 A. Macalister, President. 



[Read 4 March 1901.] 



In one occipital bone out of every four some recognisable 

 divergence from the described normal type is to be found specially 

 in the region of the planum interparietale. These are either 

 varieties in degrees of curvature, ridging or other qualities of 

 surface, or varieties in the union of the component elements. At 

 present I propose to deal only with the latter series. 



The portion of the occipital behind the foramen magnum 

 originates partly in cartilage, partly in membrane. The former 

 portion, the supraoccipital, includes all the planum nuchale as 

 far as the linea nuchas suprema, the latter includes all that part 

 of the planum interparietale which extends from the linea nuchas 

 suprema to the lambdoid suture. The supraoccipital region 

 ossifies from a pair of centres which arises about the ninth week 

 close to the region of the protuberance. These unite medially 

 about the twelfth week. Apparently there is not much variation 

 in these except in the extent to which they spread. I have 

 already described the occurrence of intercalary centres between 

 the exoccipital and the supraoccipital, and the os Kerckringii has 

 been long known. When it does not exist a median spur derived 

 from the united supraoccipital centres can be usually found in 

 the foetal skull which enters and, by expanding and uniting 

 with the adjoining exoccipital borders, fills up the infra- occipital 

 fontanel. 



It is the region of the planum interparietale which is most 

 prolific of varieties. My observations lead me to believe that 

 normally there are at least four spots at which ossification begins 

 in this membranous area. These begin about the twelfth week 

 most commonly as a little cluster above the region of the pro- 

 tuberance which generally unite on the median part of the space 

 about the fourteenth week. As the position of these clusters 

 of bony nodular reticulations is inconstant, there is sometimes 

 a difficulty in their analysis, and often they appear to be grouped 

 from the first as a pair of almost confluent nuclei. 



