DR. J. CLELAND ON THE VARIATIONS OF THE HUMAN SKULL. 
157 
affected by the size of the whole skull as well as by the proportionate size of the face 
and its projection forwards; and thus neither measurement yields results of much im- 
portance. 
Causes of prognathism . — If the reader take into consideration the various points bear- 
ing on prognathism which have already been considered, he will easily convince himself 
that the causes of that appearance are not, as is usually supposed, confined to the form of 
the face. The statement of the case may not unfairly be put thus : — that a retreating- 
forehead and projecting face are regarded as characters of a savage or degraded form of 
the human skull ; but it has been shown that the result on the profile produced by that 
combination is exactly the same as what is produced by the combination of a vertical 
forehead with a non-prominent face, or, in other words, a large orbito-frontal angle plus 
a small orbito-nasal angle is equal to a small orbito-frontal plus a large orbito-nasal 
angle ; and by studying the direction of the floor of the anterior cranial fossa which 
separates those two angles, it has been further seen that it is far from being true that 
the forehead in the ruder races of men slopes more back on the floor than in cultivated 
nations, and that it is by no means always the case in admittedly prognathous races that 
the orbito-nasal angle is larger than in orthognathous races. Therefore, although pro- 
gnathism sometimes is the accompaniment of a large orbito-nasal angle, it remains to be 
explained how a skull with a small orbito-nasal angle may yet be prognathous, and why 
a skull with that angle large is not necessarily so. 
The first thing to be noted in explanation is that in prognathous races skulls often 
are found which are not prognathous except in their dentition. But besides this there 
is another matter to be taken into account. The division of skulls into orthognathous 
and prognathous is simply an application of the norma verticalis, and a skull is judged 
of according to the apparent prominence of the face when it is laid on a flat table. Such 
apparent prominence may be made to disappear by placing a support beneath the occi- 
pital bone ; and thus it is apparent that the form of the back of the head affects progna- 
thism as estimated by the norma verticalis. The principal circumstance which acts in 
this way is the degree of cranial curvature ; for it is evident that if the fore part of the 
skull be taken as fixed in position, the greater the curvature the lower will the back 
part of the base be brought. Two other causes, namely, shortness and levelness of the 
base, conspire when present along with deficient curvature to increase apparent progna- 
thism : they are not necessarily or even usually present in prognathous races ; but the 
flattened American skull 96 affords a good example of the combination alluded to pro- 
ducing great apparent prognathism with a rather small orbito-nasal angle. In the 
Esquimaux shull 77 an illustration is given of a prognathous appearance dependent on 
deficient curve, notwithstanding the great length of the foramino-optic line and the small- 
ness of the orbito-nasal angle. ^ 
It ought further to be noticed that length of face, including dental sockets, will tilt 
up the fore part of a cranium laid on a table and produce apparent prognathism similar 
MDCCCLXX. 
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