3 i2 THE POPULAR SCIENCE MONTHLY. 



meso- or microdontal races in weight. Thus that of a male Aus- 

 tralian exceeds that of the average Englishman in the proportion 

 of 100 : 91. 



To work this heavier jaw more powerful muscles are needed. 

 In the average well-developed Englishman with perfect teeth the 

 weight of the fleshy portion of the great jaw-muscles, masseters 

 and temporals, is 60 grammes, while the weight of those as ascer- 

 tained in two Australians was 74 grammes. 



Correlated with this greater musculature a sharper definition 

 of the areas for the attachments of the jaw-muscles is required. 

 The muscular fascicles are approximately of uniform size in both 

 microdonts and macrodonts, as the range of motion of the jaw 

 differs 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 ap- 

 proximation anteriorly across the brows are 112 mm. apart, but in 

 the Australian they are only separated by 103 mm. ; the interste- 

 phanic 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 delicate intervening organs of sight and smell, 

 the connection is accomplished by the reversed arches of the in- 

 fraorbital margins with their piers, malar and maxillary, founded 

 on the frontal angular processes. These foundations are tied to- 

 gether 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 circumorbital 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 maxil- 

 lary piers. But macrodonts are often microcephalic, and there- 

 fore 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 dropped 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 insure -firmness the piers of the 



