THE FACE AND THE CRANIUM 91) 



to the front as the elongated muzzle shrinks between 

 them. 



As the eyes begin to take up a forward position, a bar 

 of bone forms behind them and intervenes between them 

 and the space at the side of the skull in which the muscles 

 of the jaw lie. In the Tree Shrews, the orbit has already 

 commenced its separation from the temporal fossa; in 

 Tarsius spectrum, the separation is complete, as we might 

 imagine from the shortness of its face, but in all the other 

 Lemurs the orbit and the temporal fossa communicate 

 freely. In all the Monkeys and Anthropoid Apes, as 

 in Man, the orbital cavity is an entirely separated com- 

 partment surrounded by bony walls and containing the 

 eyeball and its associated muscles, nerves, and vessels. 

 The bringing of the eyes to the front of the face and their 

 lodgement in separated bony orbital cavities has, in all 

 probability, far-reaching effects; but it must be looked 

 upon only as a part of the general process of change in 

 head formation brought about by hand-feeding in arboreal 

 life. Another factor not to be disregarded is the change 

 to which we will allude more fully later the alteration 

 of the head poise. Some consideration of this change is 

 inseparable from a study of the recession of the snout 

 region. When the face becomes so short that the whole 

 skull is balanced upon its condyles, a complete change 

 takes place in the axis of the principal movements of 

 the head upon the trunk, and a greatly increased range 

 is given to these movements. The arboreal Primates 

 may nod their heads backwards and forwards, as in the 

 human method of saying " Yes." This movement takes 

 place between the condyles of the skull and the first 

 cervical vertebra, and it is the primitive movement of 

 raising and lowering the head common to all Mammals. 

 But they also have an enormously increased power of 

 turning the head from side to side, in the human method 

 of signifying " No." This movement takes place between 

 the first and second, as well as, to a lesser extent, between 



