56 THE HUMAN SPECIES 



and the more perpendicular position of the forehead are con- 

 sidered as secondary sexual characters, whereas among the lower 

 races the distinction between male and female is much slighter. 



The human skull, in addition to its relatively superior size, 

 has other distinctive characters : 



On the inner surface of the skull we are struck by the great 

 size of the basal angle and the downward direction of the 

 occipital foramen which in the other vertebrates tends rather 

 backwards. This situation of the occipital foramen gnables the 

 human head to be freely balanced on the vertebral column ; it 

 is caused by the skull and brain being pressed in this direction 

 during an early phase of embryonic development. In all other 

 animals, up to the anthropoids, the frontal half of the skull is 



FIG. 13. Skull of a newly-born Orang. 

 (From Selenka.) 



FIG. 14. Skull of a human embyro of 

 ten months. (From Selenka). 



preponderant, and this has' led to a proportionately greater 

 development of the cervical muscles (see Figs. 13 and 14). In 

 man the inwardly visible basal angle corresponds with the out- 

 wardly visible inclination of the planum nuchae ; this inclines at 

 a sharper angle in man than in animals, a fact due to the 

 greater development of the brain. Hence the higher develop- 

 ment of the frontal lobes in the human brain causes the greater 

 breadth of the interorbital septum and the great size and 

 extent "of the ethmoidal cells. In the ape the frontal lobes 

 are smaller and narrower and the interorbital septum is there- 

 fore narrow, the ethmoidal cells being either entirely absent, 

 or but very slightly developed. 



The sutura transversa occipitalis, which makes its appear- 

 ance very early in the foetus, sometimes fails to close completely, 



