ON METOPISM 41 



stress will determine the arrangement of the cancellous tissue 

 in the frontal bone. And the variations in the arrangement and 

 the course of the pressure and tension lamellae in different ani- 

 mals, without doubt is caused by the variable relation between 

 the frontal bone and the Musculus temporalis. If the muscle 

 arises largely from the frontal bone the internal structure of the 

 anterior region of the cranial vault will be largely influenced 

 by the same. It is obvious that in such a case the frontal and 

 sagittal suture are primarily subject to this influence, as their 

 course is perpendicular to that of the fibers of the muscle. 



I think this idea is sufficient to demonstrate why in lower Pri- 

 mates the frontal suture persists, while in the higher Primates it 

 regularly disappears. For the stress of the masticatory muscles 

 tends to compress the skull in a transverse direction and the vault 

 of the skull will withstand this force by a system of trajectories, 

 running on a frontal plane. Now it is not difficult to understand 

 that it is of advantage that the trajectories do not meet with an 

 open suture in their course. And so the fate of the metopical 

 suture in Primates will depend upon the topographical relation 

 between the temporal muscle and the frontal bone. If the muscle 

 arises from the frontal bones a system of pressure and tension 

 lamallae will be developed in it crossing the median line and hence 

 necessitating the union of the tw^o primary frontal bones. If 

 on the contrary, the bone remains free from the dynamical in- 

 fluence of the muscle, there is no reason for the union of the two 

 bones. 



In figure 1 an attempt is made to elucidate the above described 

 idea by means of a very simple scheme. It represents a frontal 

 section of the anterior part of the vault of the skull, wdth the 

 temporal muscle on both sides. The direction in which the 

 vault will be narrowed by the stress of the contracting muscle 

 is indicated by two arrows. It is obvious that in order to with- 

 stand this stress pressure trajectories will be developed in the 

 vertical parts of the vault, under the direct influence of this 

 force. The compression in the indicated direction will produce 

 a tension in the top of the vault. And while in the vertical 

 parts of it the cancellous tissue will arrange itself in pressure 



