46 L. BOLK 



In Macacus and Gorilla the origin of the muscle reaches to the 

 median line, so that there is but a small triangular part of the 

 outer surface of the bone uncovered by the muscle, while in 

 Chrysothrix a narrow strip on both sides of the median line re- 

 mains free from the origin of the muscle. It requires no special 

 argument to show that the forces executed by the contracting 

 muscle upon the frontal bone must give rise to a system of tra- 

 jectories in it, able to withstand the strains on its outer surface. 

 And it is important to draw attention to the fact that the fibers 

 of the muscle are directed perpendicularly to the median line and 

 consequently also with regard to the frontal suture, the forehead 

 being directed horizontally immediately behind the superciliary 

 arch. This condition surely favors the formation of trajecto- 

 ries crossing the median line and causing the frontal suture to 

 disappear, as really occurs in all monkeys and apes. In man 

 the condition is greatly changed, though a small part of the 

 frontal bone is still participating in the formation of the tempo- 

 ral fossa, as shown in figure 9. There are two circumstances by 

 which the relation between the temporal muscle and the frontal 

 bone became altered from that obtainmg in monkeys. Firstly, the 

 frontal bone in man is much larger, and the surface of it occupied 

 by the origin of the temporal muscle is considerably smaller in 

 man than in apes. The pressure of the muscle upon the outer 

 face of this bone in man cannot be a very strong one, hence its 

 influence upon the inner structure surely is of little importance. 

 In this respect the condition in man is getting closer to that in 

 Prosimiae. 



The second circumstance peculiar to man is the well-pro- 

 nounced curve of his frontal bone. By this curve the greater 

 part of this bone rises vertically above the orbits. In apes, as 

 pointed out, the fibers of the temporal muscle are directed per- 

 pendicularly to the whole length of the fronta^ suture. In man 

 th's condition is altered, for in consequence of his strongly curved 

 forehead the greater part of the frontal suture is situated in front 

 of the anterior border of the temporal muscle, and moreover is 

 directed nearly parallel to this border. 



