892 BEPOET— 1892. 



mately of uniform size iu both microdonts and macrodonts, aa the range of motion 

 of the jaw ditfers little in different races ; but when the skull is smaller on account 

 of the smaller size of the brain which it contains, the temporal crest ascends higher 

 on the side-wall. In the average Englishman the temporal crests at their points 

 of greatest approximation anteriorly across the brows are 112 mm. apart, but iu 

 the Australian they are only separated by 103 mm. : the interstephanic distances 

 in these two are respectively 132 and 114 mm. 



The more powerful stroke of the mandibular teeth upon the anvil of the upper- 

 jaw teeth in macrodonts renders necessary a proportionally stronger construction 

 of the bases of support for the upper alveolar arch. In any skull this arch requires 

 to be solidly connected to the wall of the brain-case to which the shock of the 

 impact is ultimately transmitted, and in order to protect from pressure the deli- 

 cate intervening organs of sight and smell, the connection is accomplished by the 

 reversed arches of the infraorbital margins with their piers, malar and maxillary, 

 founded on the frontal angular processes. These foundations are tied together by 

 the strong supraorbital ridge, so that the whole orbital edge is a ring, made up of 

 the hardest and toughest bone in the skeleton. 



A twofold modification of this arrangement is required in the macrodont skull ., 

 The bony circum-orbital ring becomes stronger, especially along its lateral piers ; 

 and also as the alveolar arch is longer, and consequently projects farther forward, 

 its basis of support must be extended to meet and bear the malar and maxillary 

 piers. But macrodonts are often microcephalic, and therefore the frontal region of 

 the skull must be adjusted to form a foundation for this arch. In the average 

 English male skull, held with its visual axes horizontal, a perpendicular drop])ed 

 from the anterior surface of the fronto-nasal suture will cut the plane of the 

 alveolar arch between the premolar teeth or through the first premolar. In an 

 Australian skull the perpendicular cuts the horizontal plane at the anterior border 

 of the first molar teeth. 



It is obvious, therefore, that to ensure firmness the piers of the arches must be 

 obliquely set ; hence the jaw is prognathous ; but it is also needful that the supra- 

 orbital 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 moulded 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 taljle, 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 foundations of the outer or malar piers of the orbital arch must be extended 

 outwards as well as forwards, the external angular process becoming a prominent 

 abutment at the end of a strong low-browed supraorbital ai"ch, whose overhanging 

 edge gives to the orbital aperture a diminished vertical height. 



The crania of the two most macrodont races of mankind, Australian and 

 African, differ iu 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 projec- 

 tion 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 mesocephiilic 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 distinctive features of 

 contrast along the same lines. The Bushman's skull is usually orthognathous, 

 with a straight forehead and a shallow fronto-nasal recess, while the Australian 

 skuU is prognathous with heavy overhanging brows. These conditions are 

 correlated to the mesodontism of the Bushman and the macrodontism of the 

 Australian respectively. 



In the course of the examination of the relations of brain development to skull 

 growth, some interesting collateral points are elicited. The frontal bone grows 

 from lateral symmetrical centres, which medially coalesce, union taking place 



