610 . PROFESSOR SIR WILLIAM TURNER ON 



In addition to the radial lines drawn on the figures, the dimensions of which are 

 given in Table XVII. , lines have been drawn to express other relations. Thus the line s 

 is parallel with the dorsum selke and cuts the plane of the foramen magnum at an 

 obtuse angle, — named in my Challenger Report the foramino-sellar angle. The line o.s. 

 is drawn from the basion through the basi-occipital, and the body of the sphenoid to the 

 sphenoido-ethmoid articulation. It is the basi-occipito-sphenoid axis, and corresponds 

 with the basi-cranial axis of Huxley. The line c.r., or cribriform axis, is in the plane 

 of the cribriform plate of the ethmoid bone, is drawn through the sphenoido-ethmoid 

 and ethmo-frontal sutures, and its length is the distance between these sutures. It is 

 intersected by the basi-cranial axis, and forms with it the sphenoido-ethmoid angle or 

 basi-ethmoid angle of Huxley. If the inclination of the basi-occipito-sphenoid axis were 

 a constant quantity, variations in this angle would express the degree of departure 

 of the cribriform plate from the horizontal plane ; but as the basi-cranial axis is not of 

 uniform obliquity in the human skull, the angle may be modified by its degree of 

 inclination, as w T ell as by that of the cribriform axis. The difference in the angle in 

 the series of crania was not more than 8°. 



A line drawn from the sphenoido-ethmoid angle to the most projecting part of the 

 superior maxilla is the spheno-maxillary line, and it forms with the basi-occipito- 

 sphenoid axis, the spheno-maxillary angle. If this axis had been constant in its 

 obliquity the angle would have been necessarily more open in prognathic jaws, but with 

 this, as with the sphenoido-ethmoid angle, variations in the angle are also produced by 

 modifications in the obliquity of the basi-cranial axis. In determining the value of this 

 angle the obliquity of both the factors, which by their intersection form it, requires to be 

 considered. The maximum difference in this angle in the five crania was 16°. 



Of the three factors which collectively make up the longitudinal circumference, two, 

 viz., the length of the foramen magnum and the basi-nasal diameter, together form the 

 base line as defined by Cleland. The total longitudinal arc constitutes the third factor, 

 and the tables of measurements of the respective skulls give the data from which the 

 relation of the base line to that arc in each specimen can be easily computed. 



In the five male skulls specified in Table XVII. the relation of the base line to the 

 longitudinal arc was as 1 to 2 '78. In a larger series of seventeen male skulls from Fife 

 and Mid-Lothian the relation was as 1 to 2*8. In a series of twenty male Australian 

 aboriginals the base line was to the arc as 1 to 2*72, which gives, therefore, a smaller 

 proportion of arc. In the Scottish skulls the mean length of the base line was 134 "3 

 mm. and that of the arc was 37 6 '5, whilst in the dolichocephalic Australians the mean 

 base line was 139*8 mm. and that of the arc was 380"4. 



I have measured the arc and base line in the skulls of five adult male gorillas, and 

 found the mean base line to be 163"8 mm. and the mean arc 311*6, the proportion of 

 base line to arc being as 1 to 1*9. The increase in the proportion of the base line to 

 the longitudinal arc in the human skull may be regarded, therefore, as marking a stage 

 of approximation to a lower mammalian type. 



