THE POISE OF THE HEAD 119 



human condition by the final phases of the recession of 

 the snout region. 



With the question of the curves of the vertebral column 

 we need deal but briefly, for the subject is one which 

 finds ample discussion in every work upon anthropology. 

 In pronograde quadrupeds the backbone rises as one 

 long, low-pitched arch from the point where it is sup- 

 ported by the fore-limb, to a maximum in the dorso-. 

 lumbar region, and then falls again to the point where it 

 is supported by the hind-limbs. The weight of the trunk 

 is carried from an arch which is supported upon pillars 

 (limbs) at its two extremities. It is this arch which in 

 some animals has a springing point at its centre. In 

 front of the anterior supporting pillar the spine bends 

 up again for the carriage of the head. This time the 

 arch is reversed, for while the curve of the back is convex 

 upon its dorsal side, the curve of the neck is convex 

 ventrally. Again, behind the posterior supporting pillar 

 the spine is also bent upwards ; here, at the sacro-vertebral 

 angle, the bending dorsalwards is more acute; but from 

 this point, the curve is slightly downwards once more, 

 the posterior or sacral arch being like the dorsal arch in 

 miniature, but generally still more flattened (see Figs. 35 

 and 36). 



In arboreal animals these curves are also well marked, 

 and the changes which they undergo are quite definite. 

 Arboreal uprightness, in the sense of the assumption of 

 a sitting posture, has a well-marked influence upon the 

 primitive curves. Some monkeys, as they sit up, spend 

 the greater part of their time with the trunk supported 

 vertical upon their ischial prominences, and in these 

 animals the dorso-lumbar curve tends to be, not one 

 long pitch as in the quadrupeds, but an arch subdivided 

 into an anterior sharper curve over the thoracic part of 

 the body, and a more gradual curve over the abdominal 

 part. The dorso-lumbar curve tends to be concentrated 

 as a dorsal curve, while the lumbar region is scarcely 



