33
Orientation in line with the margin bb of the metal plate b b . Over the edge of the wedge a the
basion of the skull to be oriented is placed, and moved up and down until the nasion falls
estimate^of jn i jn e w jth the upper sloping edge of the outer side of the frame. The skull may then
tionPof the be fixed in this position and the frame withdrawn from the metal plate b b , leaving the skull
so oriented that the basi-nasal line forms with the horizontal an angle of 270.
When such a method is adopted, the basi-nasal line becomes the base on which to erect
the outline of the cranial vault, whilst it also serves as the platform from which to suspend
the skeleton of the face. As a matter of fact, such a method is frequently employed in
the comparison of skulls, the suggested improvement being that the basi-nasal line is
now placed in the approximate position which it occupies in life with the head erect.
That being so, we are in a much better position to estimate the different relations of
the several parts of the skull to each other, since the extremities of this line, viz. the
nasion and the basion, are points which are largely employed in the mensuration of the
cranium. Further, should this suggested orientation of the skull be adopted, then at
last we have an easy method of estimating the varying degrees of prognathism; since
a perpendicular let drop from the nasion, when the basi-nasal line is disposed at an angle
of 2 70, will give us a means of measuring accurately the projection of the alveolar point
in front of that vertical, and, since the same conditions will be applicable in all cases,
the results are more likely to be in accord with what one observes on the living man
than is now the case.
When the skull is oriented in this manner it will be found that it corresponds pretty
closely to the position suggested by Cleland, in which the most projecting part of the
occipital condyles and the posterior boundaries of the foramen magnum both occupy
the same horizontal plane.
The same observer has pointed out that the position of the skull on the vertebral
column varies with growth. Assuming that in the erect position the head is very nearly
balanced on the summit of the vertebral column, it follows that as the relative
proportions and weight of the facial part of the skull increase during childhood and
adolescence so it becomes necessary that the head be more uptilted. This effect, Cleland
suggests, is probably produced by a gradually increasing prominence in the anterior
extremities of the occipital condyles, which alters the plane of articulation with the atlas.
Any attempt to ascertain the position in which the skull balances on a rigid rod
introduced through the foramen magnum and touching the inner surface of the cranial
vault, as suggested by Sir Charles Bell, is of course of no value, since the proportional
weights of the anterior and posterior parts of the head may be, and doubtless are, entirely
different from the proportionate weight of the corresponding parts of the macerated skull.
Moreover, such a method is only applicable when the mandible is present; when the lower
jaw is wanting, the balance of the cranium is of course very much altered, a fact which
is frequently overlooked.
The orientation of the skull on the basi-nasal line recalls the method suggested by
Aeby, who laid stress on the benefit of having a line which serves as a common base
to calvaria and face alike. This advantage the present system undoubtedly possesses,
and though we are aware that the situation of the fronto-nasal suture is liable to
alterations in position due to the recessing of the nasal bones within the frontal, and
though the nasion cannot strictly be regarded as corresponding to the precise position
of the vomerine plate in relation to the face, yet, despite these objections, it appears
to us to be in most respects superior to the other planes which have hitherto been suggested.
As already stated, the variations in the disposition of the lower orbital margins are Orientation
liable seriously to affect the constancy of the Frankfort-Munich plane, whilst Broca’s J,obtalna
alveolo-condylic plane seems open to similar objections, since the longer the face,
dependent on increased vertical height of the superior maxilla, the greater will be the tionofthe
upward tilt of the skull, thus imparting an appearance of excessive prognathism to the ‘
face where none such necessarily exists, for, as will be shown hereafter, prognathism
depends not on the length of the upper jaw alone, but also on its splay on the cranial
base. It is true that the two conditions may occur in combination, but it is also
noteworthy that a long upper jaw may be so disposed as to project but little, whilst
a short superior maxilla may be so placed as to form a marked protrusion. In any
case it is surely a mistake to orient the skull on points which are situated on the calvaria
and on the face respectively, for the relation of these points must constantly vary, since
the proportions of those parts of the skull on which the points are situated are liable
to alteration. It appears much better to employ a base common to both parts of
the skull and so secure a means of estimating correctly the growth in either direction
from that base line; for if the balance theory of the head be sound, it follows that undue
face projection is counterbalanced by excess of occipital development, and not by an
upward tilting of the skull such as occurs when it is oriented in Broca’s plane.
Not less troublesome, and certainly much more confusing, is the estimation of the ^ ^ o d Sof
facial projection to which the term prognathism is applied. The word implies a pro- determining,
trusion of the jaw ; but how this forward thrust of the facial skeleton is to be estimated,
or by what standard it is to be measured, has hitherto been a matter of much difficulty,
and owing to the variety of methods employed much confusion has arisen. Two
factors in the growth of the upper jaw must be considered : (i)the variations in the length
of its vertical diameter, which are estimated with sufficient accuracy by the measurement
of the distance from the nasion to the alveolar point, and (2) variations in its
horizontal length, which are conveniently expressed by taking the palatal length. In
addition to these diameters the splay of the jaw upon the cranial base, which is to some
extent estimated by taking the basi-alveolar length, must be considered. This measurement
includes, therefore, not only the length of the jaw in a direction slightly oblique
to the horizontal plane, but also the distance through which the jaw is thrust forward
from the basion; the latter distance is occasionally given in some systems of measurement
and is estimated by taking the distance from the basion to the posterior nasal spine,
but owing to the frequency with which the basi-alveolar length is recorded in most
craniometrical inquiries, that measurement may be conveniently employed to denote
the forward projection of the alveolar point, irrespective of the horizontal diameter of
the jaw itself, which is sufficiently expressed by the palatal length. Such being the
case, it is evident that the disposition of the facial skeleton, minus the mandible and
the occasional projection of the nasal bones, falls within a triangular area, the sides
of which correspond to the basi-nasal, the nasi-alveolar, and basi-alveolar lengths, and
since for our purpose we have accepted the basi-nasal line as the base common to the
face and calvaria alike, it follows that the form of the triangle will vary according to
the varying lengths of the nasi-alveolar and basi-alveolar measurements. Given the
three sides of the triangle, it is of course possible to construct the triangle and so
estimate accurately the position of the alveolar point. A difficulty, however, at once
arises when any attempt is made to estimate the position of this point in reference to
a vertical plane, for the position of the alveolar point and the vertical will vary according