372 



human properties in the skull form one entity. However, in this 

 paper I shall confine myself to ray real snhject. 



My first attempt was to ascertain the essential morphological fea- 

 tures of the prognathous and the oi'thognatiioiis sliuU-type, for the 

 criterion of short or long jaws is inadequate. Witli the aid of Figs 

 1 and 2 these features are easy to establish. 



Fig. 1. 



Fig. 2. 



Fig. 1 shows a median section of a human skull. Fig 2 a similar 

 section of the skull of Lemur, a Prosimia. Three lines have been 

 drawn in both tigiires, viz the axis of the cranial cavity, the axis of 

 the nasal cavity and the axis of the base of the skull. The three 

 lines demonstrate in a simple way the essential features of the 

 orthognathous and the prognathous skull-type. They are the following: 

 In the orthognathous type the axis of the nasal cavity is approxi- 

 mately perpendicular to the axis of the cranial cavity, in other 

 words the nasal cavitj' is situated beneath the cranial cavity ; in 

 the prognathous type, on the contrary, the axis extends more or less 

 in the same direction as the axis of the cranial cavity. As to the 

 axis of the base of the skull, it is flexed in either case, but in 

 opposite direction. In the orthognathous type it is flexed between 

 the basi- and the praesphenoid, an angle is formed with its open 

 side turned anteriorly downwards. It is known in the literature as 

 the sphenoidal angle. In tlie prognathous type the base is flexed 

 between the praesphenoid and the ethmoid. An angle is formed with 

 its open side turned posteriorly upwards. This angle I shall term 

 the ethmoidal angle. 



So it appears that the typical differences between the orthognathous 

 and the prognathous skulls consist in the different situation of the 

 nasal-cavity, either subcerebral or praecerebral, and in the different 

 direction in w^iich the base of the skull is flexed. The length of 

 the jaws I do not consider as a fit criterion. 



