2O2 Anatomy of Skeleton 



coronal border to about half-way down the temporal surface. Below this the sphenoid 

 articulates by its greater and lesser wings as far as the ethmoidal notch, taking all the 

 articular surface except that on the external angular process, which articulates with the 

 malar. The ethmoid lies against the inferior aspect of the margins of the notch and 

 also articulates with the lower border of the nasal spine, between the two nasal roof 

 areas. Arranged round the spine antero-laterally are, from within and in front out- 

 wards and backwards, articular surfaces for the nasal, maxilla, and lachrymal. The 

 nasal bones lie together along the front and upper border of the spine. 



Development. 



The bone is developed in membrane, in which two main centres appear in the eighth 

 week, in the region of the future frontal eminences. Small additional centres, appearing 

 later and fusing with the main halves, have been described for the nasal spine, the tro- 

 chlear spine, the external angular process, and the back border of the orbital plate : 

 it seems doubtful whether these are to be considered as of usual occurrence. The bone 

 is in two separated halves at birth, but these come into contact in the first year, fuse in 

 the middle region of the metopic suture in the second year, and are completely joined 

 by the seventh or eighth year. There may be total absence of fusion, as already 

 mentioned, or the metopic suture may remain unclosed below, forming a metopic 

 fontanelle, and Wormian bones may appear here or rarely in the upper part.* 



The frontal sinuses develop as extensions from the nasal cavity, and are first 

 indicated in the bone in the first or second year, but grow slowly up to puberty. After 

 this they increase rapidly, and continue to grow until a late period (see later, under 

 Nasal Cavities). 



PARIETAL. 



A four-sided bone situated in the back and side of the head and articulating 

 with its fellow in the middle line above (sagittal suture). The bone is a little broader 

 above than below. 



The outer surface, convex, is crossed by the temporal ridge about a third or more 

 of its height up : the ridge is curved and usually shows two lines, the lower for the 

 Temporal muscle and the upper for its covering aponeurosis. Above the ridge the 

 bone is covered by Occipito-frontalis ; the lower part is covered by and gives origin 

 to the Temporal muscle. Observe the differences in the surface texture in these two 

 areas. 



The parietal eminence is the most prominent part of the convexity, presenting a 

 smaller curve than elsewhere : it is situated above the temporal ridge behind the centre 

 of the bone, and marks the site of earliest ossification. 



The parietal foramen, which may be absent, unilateral, or very large, is near the 

 upper edge about an inch or less in front of the upper and posterior angle. At birth 

 the position of the foramen is marked sometimes by a transverse slit in the bone (sub- 

 sagittal suture of Pozzi), which enables one part of the bone to ride slightly over the 

 other during parturition, thus aiding the moulding of the head. The slit is obliterated 

 afterwards by extending ossification, and the foramen is only kept open by the persis 

 tence of small vessels that may run through it. Some phylogenetic interest is attached, 

 moreover, to the foramen in that it has been supposed to represent in the human skull 

 the place where, in lower vertebrates, the median eye reaches the surface. 



* The writer has only once seen such a bone in the upper part of the suture, at the bottom of a gum- 

 matous ulcer in a hospital patient. 



