DE. J. CLELAND ON THE YAEIATIONS OF THE HUMAN SKULL. 
161 
wards must be accomplished either by movement of the vertebral column so as to make 
the upper surface of the atlas look more and more backwards, or by gradually increasing 
prominence of the anterior extremities of the occipital condyles. There seems to be no 
evidence that the change is in the position of the atlas ; on the contrary, the arch of that 
vertebra appears to preserve a horizontal position throughout life ; but the increasing- 
prominence of the anterior extremities of the condyles is sufficiently great to catch the 
eye, and is what will now be more explicitly proved. 
A flat surface laid on the occipital condyles, and on the skull behind the foramen 
magnum, having been assumed as horizontal, the degree of elevation of the condyles is 
easily estimated by measuring the angle which the long axis of the foramen magnum 
forms with that plane. But the tilting up of the skull is complicated by the increase in 
cranial curvature which proceeds with growth. Were there no alteration of position 
of the back part of the skull in the progress from infancy to adult life, the increase in 
cranial curvature would turn the face more and more downwards ; it may be necessary 
therefore to distinguish tilting up which involves only the back part of the skull from 
such as involves the whole cranium. With a view to this distinction, as fair an index of 
the position of the face as can be found is the line of orbital length, and therefore not 
only the angle at which the horizontal line lies to the long axis of the foramen magnum 
has been measured (column 77), but also the angle at which it lies to the line of 
orbital length (column 78). 
In infants’ skulls the foramen magnum forms the smallest angle with the horizontal 
line, while the cranial curve being as yet imperfect, the line of orbital length forms a 
larger angle with the horizontal than it does in many adults. But it is to be recollected 
that in infants there is at first little attempt at balance, the erect posture being not yet 
assumed. In the children’s skulls the average angle of the foramen magnum with the 
horizontal remains smaller than that of any of the adults except the Hindoos, while 
the angle of the line of orbital length has sunk lower than the average of adult males 
in any nation. 
Among adults there seems to be least tilting round in the Hindoos, and next to them 
in the Australians. In the Esquimaux and Kafirs the angle of the foramen magnum 
with the horizontal line is not very great ; but owing to the small cranial curvature, that 
of the line of orbital depth is greater than in any of the other nations. In the Irish, 
French, and Scotch, on the other hand, the angle of the foramen magnum with the 
horizontal is high, the curvature being great. In the Germans, where the curvature is 
greatest, the tilting up of the foramen magnum is not quite so great ; but this may be 
accounted for by lightness of the face-bones and by the greatness of the parietal breadth 
as compared with the frontal breadth in some of them, a circumstance which throws 
more weight behind the condyles. 
It appears very distinctly from the accompanying Table that the female skull is much 
less tilted back on the condyles than the male, being in this, as in various other respects, 
more child-like than the male skull ; and in the two apparent exceptions to this rule, 
the Kanaka and Kafir, a full explanation is found in the high cranial curvature. 
