232 THE CRANIA OF THE SIMIIDAE (PRIMATES) [SECT. A 



and ultimately downwards, the transition being capable of repre- 

 sentation by the movement of the anterior lever of our diagram 

 (Pr. N., Fig. 153), the active force being indeed the intrinsic 

 cerebral growth. Posteriorly, similar expansion drives backwards 

 and then downwards the occipital wall of the cranium, and 

 with it the foramen magnum in a similar way, capable too of 

 similar representation (see Figs. 150 to 156 inch, pp. 225 

 and 231). 



Expansion upwards produces the bold vaulting of the cranial 

 arc, and in all these respects it is to the human cranium that we 

 are led by the successive stages assumed in the lower and higher 

 quadrupedal monkeys respectively, and then in the Simiidae 

 themselves. In Man the effect is at a maximum, the lower 

 Primates merely suggesting the trend of development. 



But at this juncture, warning must be given of a remarkable 

 secondary change experienced by the hinder parts of the cranial 

 envelope. It is shewn by the position, or more properly the angle 

 of inclination of the foramen magnum. It comes to light when 

 we enquire how the infantile skull compares with that of the 

 adult, and whether the individual recapitulates in his person the 

 ancestral stages of his kind. 



As a matter of fact, when averages are taken, it is found that 

 in children of less than five years of age the typical "human" 

 mode of conformation is more marked than in the adult 1 . In 

 order to make the point clear, we must reconsider once more the 

 inclination of the various segments of the cranial base to one 

 another. The accompanying figure (Fig. 157) shews the positions of 

 the foramen magnum (in relation to the line B. Pr.) in a lowly 

 mammal, in the young child, and in the mature human being 

 respectively. Clearly the latter is tending to regress towards a 

 lower condition as compared with the child. The process leading 

 to the production of the typical human conformation seems to be 

 actually reversed after childhood has been left behind. 



This secondary process or reversal may be compared to the 

 action of " focussing-up " with the fine adjustment of a micro- 

 scope. It was discussed several years ago by Professor Manouvrier. 



1 Bolk, Koninklijke Akademie ran fVetenschappen. Dec. 24, 1909. 



