THE SKULL AS A WHOLE 



145 



Supernumerary, Wormian, 1 Sutural, or Epactal Bones (Ossa Triquetra). 



In addition to the constant centres of ossification ^f the skull, additional ones are occasion- 

 ally found in the course of the sutures. These form irregular, isolated bones, interposed between 

 the cranial bones, and have been termed Wormian bones, or ossa triquetra. They are most 

 frequently found in the course of the lambdoid suture, but occasionally also occupy the situation 

 of the fontanelles, especially the posterior and, more rarely, the anterior. Frequently one is 

 found between the anterior inferior angle of the parietal bone and the greater wing of the sphe- 

 noid, the epipteric bone, or the pterion ossicle (Fig. 113). They have a great tendency to be 

 symmetrical on the two sides of the skull, and they vary much in size, being in some cases not 

 larger than a pin's head, and confined to the outer table; in other cases so large that one pair of 

 these bones may form the whole of the occipital bone above the superior curved lines. 



FIG. 113. Wormian bones. 



Graniology. 



Skulls vary in shape and size, and the term craniology is applied to the comparative study of 

 these variations. By means of exact measurements and their correlation, skulls may be classi- 

 fied in various ways. 



I. According to capacity, measured by means of shot, mustard seed, etc. 



1. Microcephalic, with a capacity of less than 1350 c.c. (e. g., Australians, Andamanese). 



2. Mesocephalic, with a capacity of from 1350 c.c. to 1450 c.c. (e. g., Negroes, Chinese). 



3. Megacephalic, with a capacity of over 1450 c.c. (e. g., Europeans, Japanese, and Eskimos). 

 -To facilitate regional description and to compare the normse of one skull with those of 



another, the skull is placed in such a way that a plane passing through the inferior margin of the 

 orbit and the superior margin of the external auditory meatus shall be horizontal (the horizontal 

 line of the Frankfort agreement). Various linear and arc measurements are made between 

 definite and easily localized points on the surface of the skull, and, although previously described, 

 are here tabulated for convenience of reference. They are divided into two groups: (1) those 

 in the mesal plane, and (2) those on either side of it. 



The Points in the Mesal Plane are: 



Mental Point. The most prominent point of the chin. 



Alveolar Point, or Prosthion. The central point of the anterior margin of the upper alveolar 

 arch. 



Subnasal Point. The middle of the lower border of the anterior nasal aperture, at the base of 

 the nasal spine. 



Nasion. The central point of the frontonasal suture. 



Glabella. The point in the middle line at the level of the superciliary ridges. 



Ophryon.The point in the middle line at the level where the temporal lines most nearly 

 approach each other. 



Bregma. The meeting point of the coronal and sagittal sutures. 



Obelion. A point in the sagittal suture on a level with the parietal foramina. 



Lambda. The point of junction of the sagittal and lambdoid sutures. 



1 Wormius, a physician of Copenhagen, is said to have given the first detailed description of these bones. 



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