CHAPTER XVIII 



THE POISE OF THE HEAD AND THE CURVES OF 



THE SPINE 



The differences seen in the disposition of the cervical 

 spinous processes in the Anthropoids and in Man are due 

 tO; and involve, yet another factor which may be termed 

 the poise of the head upon the vertebral column. 



This subject has been so fully investigated by com- 

 parative anatomists that little need be said here con- 

 cerning it. 



The skull is hinged to the foremost vertebra by its two 

 condyles, and between these occipital condyles and the 

 articulating surfaces of the first vertebra the human 

 movement of nodding the head to and fro takes place. 

 The position of the two condyles relativel}' to other 

 anatomical features of the skull varies enormous] v in 

 animals adopting different life postures. The condyles 

 may be situated right at the hinder end of the skull, 

 they may be just beneath the hind end, or they may be 

 situated some distance forward along its base. In 

 pronograde quadrupedal animals, such as the dog, the 

 head is jointed to the vertebral column by condyles 

 situated at the extreme hind end of the skull; the nose 

 is directed forwards in line with the vertebral column, 

 and the skull is braced in position by a strong ligament — 

 the ligamentum nuchse — and by muscles passing from the 

 vertebrae to the back of the cranium (see Fig. 38). In 

 animals which are not purely pronograde quadrupeds, an 

 alteration takes place; the poise of the head becomes 

 changed and the site of the condyles shifts upon the skull. 

 In arboreal animals this change becomes very evident, 



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