412 DR MATTHEW YOUNG. 



According to Cunningham (48), different degrees of extension of the parietal and 

 occipital regions, as well as differences in the growth of the frontal area, modify the 

 position of the bregma. The difference in growth of the frontal region, as we shall 

 see later, is brought out in the Scottish series examined by comparing the mean 

 glabella-bregma chord found therein with those found in the Australian and Tasmanian 

 types, and I believe that this difference in growth has an influence on the size of 

 the zentrum angle, while, at the same time, as will be demonstrated later on, 

 growth of the occipital region of the vault is an important factor,' not only 

 as affecting the zentrum angle, but also the relative positions of the bregma, and 

 of the lambda, and the size of the upper glabella angle. 



The mean value of the upper glabella angle in the Tasmanian is greater than in 

 the Scottish skull, and the glabella-bregma length is considerably shorter on the 

 average in the former than in the latter. The result of these various factors is that 

 the zentrum angle in the Scottish skull is on the average greater than in the 

 Tasmanian skull. 



In the Australian skull, as calculated from the series of 90 dioptrographic tracings, 

 the mean glabella-bregma line is relatively shorter than the glabella-basion line, the 

 upper glabella angle is practically equal to that in the Tasmanian, and these factors 

 apparently so balance one another than the mean zentrum angle is approximately 

 equal in the two complete series, namely, 89'45° in the Australian and 89 "6° in the 

 Tasmanian. 



It is interesting to note in connection with this angle that in the skulls of the 

 anthropoid apes of different ages as figured in median outline by Klaatsch, with the 

 basion-bregma and glabella-lambda horizontal lines inserted, at an early age in the 

 chimpanzee and orang-utan the zentrum angle is 89° and 90° respectively, thus 

 approaching the angle found in the human subject ; the younger the animal the 

 closer is the approximation in the angle, while in the young gorilla it is 84°. With 

 increasing age the basion-bregma line slopes more and the upper end moves forward 

 relatively to the lower end (Klaatsch), so that the zentrum angle is considerably 

 smaller in the adult than at an earlier stage. The explanation of the alteration in 

 the size of the angle is, without doubt, that it is due largely to the displacement 

 backwards of the foramen magnum and so of the basion, the mid-point of its 

 anterior margin, that takes place in all primates, including man, in their passage 

 from the juvenile to the adult stage, and which is due to the increase in length of 

 the basi-sphenoid and basi-occipital that takes place at that time (Bolk). 



Another factor that undoubtedly influences the size of the angle at different ages 

 is the fact that, as pointed out by Cleland (49), " the base line continues to elongate 

 after the arch has acquired its permanent characters," which is fully confirmed by a 

 comparison of the juvenile and adult skulls in the present collection. 



It is found that the chimpanzee shows the largest zentrum angle when it has 

 lied the adult state and is considerably larger than that found in the gorilla and 



