204 /.. W. G. Buchner: 



very elosely uiidtT tlie cranial )-oof. so that the rranial iilaiie. and 

 the ()r))ital roof join uiuler an acute angle."" This anteiioi- point 

 IS calkMl the " Fi-onton.'' 



The posterior cranial point is also obtained on the bisected skull. 

 Foi- its determination the grt'atest distance from the fronton 

 to a point on the posterior ^vall is estimated with tiie aid of com- 

 passes. Bolk fuithei- says. " Witli the lower Primates this pfdnt 

 ■can, as a rule, l)e determined at once."' In man. howevei-. and 

 especially in juvenile skulls, it is not so easily estimateil. Bolk 

 found that very often no definite point could !)e determined for the 

 'Occipiton, as it is termed, foi' he found that infrequently " a 

 fairly large part of the interioi' surface of the skull in the median 

 plane describes a circular arc." Wliere this was the case, the 

 middle of this circular arc was always chosen as tlie posterior 

 point." 



It is obvious that tliis base line, from frojiton to occipiton, as 

 Bolk himself points out, is determined by tlie sliape and form of 

 the skull, and is thus distinguished from other planes, which are 

 more or less dependent on fixed cranial points. From this 1)ase 

 line is determined the ])<'sition, displacement and slope of the 

 foramen, occ'i ptalt nuKjninn. The positio.n he calculated l)y means 

 of an index basalis, whilst the slope is determined by the liasal 

 Angle. The former is constructed by dropping a line at right 

 angles from the fronton-occipiton plane, which cuts the basion. 

 This base line is thus divided into two paits at the point C (see 

 fig. 1). As Fc becomes greater it folloAvs that the basion retreats, 

 iind this is expressed by the formula 



B^cxlOO Till- 



= Index basahs. 



Fo 



By comparing the l)asal indites of tlu' anthropoid and man. Bolk 

 found that a consideral)le differenct' existetl between them, the 

 •average indices of tlie human skulls being I") units less than that 

 .d' the anthropoid. 



In his second ]ia]>ei'. the same author discusses the " slope of the 

 foramen magnum in Pi-imates. The slojte is estimated by again 

 ■employing the f ronton-oeci])it<m base lim'. As l)efiue. a line is 

 jirojected fi-om the base line at right angles, and cutting the l)asion. 

 From this projei'ted line c Ba (tig. 1) is estimated the iiulination 

 of thi. slope ,,f Hh' A-/vn//^// bv ealeulating tl,c> angle wliieh tlie plane 

 c ]}a forms with the basion-opist hioii plane l>a «^p. 



Tt apjiears to me that Bolk's methods ,,f delermining the j.osition 

 and sloj)e of the foramen ocr'i p'llah iikkiiiiuii are beset hx at least 



