THE STUDY OF MAN. 313 



arches must be obliquely set, hence the jaw is prognathous ; but 

 it is also needful that the supraorbital arcade should be advanced 

 to meet and bear these piers, as the mandibular stroke is always 

 vertical. 



But the inner layer of the skull is molded on the small frontal 

 lobes of the brain, so this forward extension must affect only the 

 much thicker and tougher outer table of the skull, which, at the 

 period of the second dentition, here separates from the inner table, 

 the interval becoming lined by an extension of the mucosa of the 

 anterior ethmoidal cell. In this way an air-space, the frontal 

 sinus, is formed, whose development is thus directly correlated to 

 the two factors of brain development and size of the teeth. If the 

 frontal lobes are narrow in a macrodont skull, then the founda- 

 tions of the outer or malar piers of the orbital arch must be ex- 

 tended outward as well as forward, the external angular process 

 becoming a prominent abutment at the end of a strong, low- 

 browed supraorbital arch, whose overhanging edge gives to the 

 orbital aperture a diminished vertical height. 



The crania of the two most macrodont races of mankind, Aus- 

 tralian and African, differ in the relation of the jaw to the frontal 

 bone. In the microcephalic Australian the maxillae are founded 

 upon the under side of the shelf-like projection of the outer table 

 of the frontal, which juts out as a buttress to bear it. On the 

 other hand, the nasal processes of the mesocephalic negro ascend 

 with greater obliquity to abut on the frontal, and have, by their 

 convergence, crushed the nasal bones together and caused their 

 coalescence and diminution. 



The crania of the two most microcephalic races present dis- 

 tinctive features of contrast along the same lines. The Bush- 

 man's skull is usually orthognathous, with a straight forehead 

 and a shallow f ronto-nasal recess ; while the Australian skull is 

 prognathous, with heavy overhanging brows. These conditions 

 are correlated to the rnesodontism of the Bushman and the macro- 

 dontism of the Australian respectively. 



In the course of the examination of the relations of brain de- 

 velopment to skull growth some interesting collateral points are 

 elicited. The frontal bone grows from lateral symmetrical cen- 

 ters, which medially coalesce, union taking place usually between 

 the second and sixth years of age. It has been noticed by anthro- 

 pologists that metopism, as the anomalous non-union of the halves 

 of this bone has been termed, is rare among microcephalic races, 

 occurring only in about one per cent among Australian skulls. 

 Increased growth of the frontal lobes as the physical accompani- 

 ment of increased intellectual activity interposes an obstacle to 

 the easy closure of this median suture, and so in such races as the 

 ancient Egyptian, with a broader forehead, metopism becomes 



TOL. XLII. 21 



