17 



Now what is the function of these supra-orbital ridges? To find 

 the answer tlie researcher should ascertain the part plajed by these 

 ridges in the structure of the skull as a whole, and what is their 

 topographical relation to their immediate surroundings. This may be 

 done quickest by making a sagittal section that extends along the 

 axis of the orbit, through the ridge and the adjoining part of the 

 skull. The image resulting fiom it is represented for Gorilla in fig. 1. 



(^/Cfc^CCCTDCr- 



Fig. 1. 



What does this figure teach ns? First of all that, properly speaking, 

 the term supra orbital ridge is not quite fit and that this formation 

 cannot be compared with the occipital-, and the sagittal ridge also 

 characterizing the skull of Gorilla. For, in reality, of this so-called 

 supra-orbital ridge the lateral portions form the roof of the orbits, 

 while the central part forms the roof of the nasal cavity. If, there- 

 fore, the supra-orbital ridge should be removed, nearly the whole 

 content of the orbita would be deprived of the overlying osseous wall 

 and would consequently come to lie immediately under the skin. 



Direct observation of the topographical relation, therefore, leaves 

 no manner of doubt about the function of the so-called supra-orbital 

 ridge, it is namely the indispensable osseous wall of the orbita at the 

 top. It is not a crest like the crista sagittalis and the crisia 

 occipitalis, but it is an indispensable wall of a cavity in the skull. 

 But if this is a fact the origin of the superorbital ridge must be 

 closely allied to general growth-phenomena of the skull after the 

 early childhood of the ape. For we stated that, notwithstanding the 

 absence of the supra- orbital ridges in the child-ape, still also here 



2 



Proceedings Royal Acad. Amsterdam. Vol. XXV. 



