444 



DR MATTHEW YOUNG. 



a capacity of over 2000 c.c. shown in median sagittal section (fig. 20) should fall 

 short of a value of 180° by about 3°, another specimen (fig. 21) with a cubic capacity 

 of about 1100 c.c. should show a cranial curve of only 156°, while a third specimen 

 (fig. 22), very similar to the last, and of an equal capacity, should show a cranial 

 curve of about 190°. One would have expected that the effect of the increased 

 cranial expansion in the very large skull would have been to bring the anterior part 

 of the cranio-facial axis more into the horizontal than in the case of the smaller skulls, 

 but this is not the case, as is shown by the specimens. The influence of the slope of 

 the plane of the foramen magnum on the parallelism of the anterior and posterior 



Fig. 23. — Impression of median sagittal section of skull No. 41, series K, illustrating Bolk's method of defining the 

 position and slope of the foramen magnum. Spheno-ethmoidal angles 151°. 



F. Fronton. BR = perpendicular dropped on F0 from hasion. 



0. Occipiton. FR 



B. Basion. 



S. Opisthion. ^ RBS = angle by means of which slope of foramen magnum is measured. 



— — = basal index (Bolk). 



base and the resulting size of the cranial curve will be very apparent in the diagrams 

 above mentioned. 



Much research has been devoted to the question of the position and the inclina- 

 tion of the plane of the foramen magnum in mammals. Amongst those who have 

 contributed to the subject are Daubenton and Broca, and the angles they utilised 

 are still associated with their names. A more recent investigator of the same problem 

 is Bolk (42), and I have adopted the methods advocated by him in investigating 

 these points in the present series of skulls. Bolk does not favour the use of angles 

 for defining position, but prefers the use of an index to indicate the location of the 

 foramen magnum, and he obtains the index in the following way : — Having obtained 

 a median sagittal section of the skull and an outline of this on paper, he defines a 

 point on the inner surface of the anterior cranial wall where it becomes continuous 

 with the floor in the mesial plane, which point he names the " Fronton " (F in fig. 23). 



