44 L- BOLK 



ward. The axis of the orbital fossa is making therefore a more 

 acute angle with the median plane. In consequence of the ro- 

 tation of the plane of entrance of the orbit, the insertion of the 

 primitive orbital arch at the frontal bone was shifted from the 

 hindermost border of the bone forward, so that a part of the outer 

 surface of the frontal bone is added to the temporal fossa. By 

 this enlargement of the fossa the temporal muscle was enabled 

 to arise to a smaller or greater extent from the frontal bone. 



The differences between Prosimiae and the higher Primates 

 are clearly shown by the figures 2 to 9. In these figures the lat- 

 eral and superior view of some prosimian and simian skulls 

 is drawn. The course of the main sutures and also the extension 

 of the temporal muscle is indicated. Figures 2 and 3 represent 

 lateral views of the cranium of Avahis sinavensis and of Stenops 

 gracilis respectively. In both it is obvious that the frontal bone 

 is completely excluded from the temporal fossa, and that there 

 are no fibers of the temporal muscle arising from this bone. 

 Hence it is easy to understand that in those crania the frontal 

 suture persists, as is shown in figure 4, representing the superior 

 view of the skull of an Avahis niger. The frontal bone remains 

 free from the dynamical influence of the temporal muscle, its 

 anatomical significance is a restricted one. It functions only 

 as roof of the orbits and the foremost narrow part of the cavity 

 of the skull. In consequence of the absence of forces acting 

 upon this bone, its system of trajectories cannot be strongly de- 

 veloped. Hence there is no reason for both frontal bones to 

 unite. 



Quite the contrary happens in the skulls of monkeys from the 

 Old and New World, as is illustrated by figures 5, 6, 7, and 8. 

 Figure 5 represents a side view and figure 6 a superior view of 

 the skull of Chrysothrix, a platyrrhinic monkey, figure 7 a side 

 V ew of the skull of Macacus, and figure 8 such a one of a female 

 Gorilla. The extension of the temporal muscle and the course 

 of the sutures in the cranial vault are drawn. These figures 

 require but little comment. In all it is clear that the frontal bone 

 participates in the formation of the temporal fossa, and that no 

 small part of the temporal muscle takes origin from this bone. 



