1890-91.] Drs Symington & Thomson on Defective Ossification. 275 
bones towards the tip of the nose is also normal. We find, 
therefore, a distinct shortening of the base of the cranium in 
front of the foramen magnum, this shortening being limited 
to the parts normally ossified at birth, by intra-cartilaginous 
ossification. 
Occipital bone . — The upper portion of the supra-occipital (inter- 
parietal bone of comparative anatomists) which is developed in 
membrane is well formed ; its junction with the lower portion of 
the supra-occipital is indicated by the usual fissure extending 
inwards from the margin on either side. All the parts of the 
occipital bone ossified from cartilage, viz., the lower part of the 
supra-occipital, the ex-occipitals, and the basi-occipital, are much 
smaller than normal, and are not separated from one another by 
cartilage as in the normal foetus ; the lower part of the supra- 
occipital being ossified to the ex-occipitals, whereas normally they 
are separated by a layer of cartilage several millimetres in thickness. 
Again, the cartilage between the ex-occipitals and the basi-occipital 
is absent, although there is no osseous union of these bones. 
The changes in the basi-occipital have already been described. 
The foramen magnum is remarkably diminished in size, measuring 
only - 65 cm. in its antero-posterior diameter, compared with 2 - 2 
cm., which we found to be the average of four normal skulls. 
This diminution is, of course, the direct result of the premature 
ossification and arrested development of the four elements of the 
occipital bone which surround the foramen, and by the growth of 
which the foramen increases in size. 
Sphenoid . — As already mentioned, the pre- and post-sphenoidal 
nuclei, together with the basi-occipital, are represented by a single 
osseous mass, the os tribasilare. The pituitary fossa is distinctly 
smaller than usual, its antero-posterior diameter being *6 cm. as 
compared with 1*0 cm. The orbito-sphenoids, or lesser wings, are 
smaller. The alse-sphenoids (greater wings), on the other hand, are 
fully as large as normal ; this suggests that they are not formed 
entirely in cartilage as commonly described, but partly in membrane, 
and the radiating appearance of their outer edge supports this view. 
Ethmoid . — The mes-ethmoid, which is normally cartilaginous at 
birth, is, as we have already mentioned, of normal size. Its 
lateral masses, although well ossified, are distinctly smaller than 
