
functional craniology: an aid in
interpreting roentgenograms
of the skull
analysis of the form of the skull is
facilitated by considering it to be composed
of a number of functional components.  for
the roentgenologist's purposes, division into
the facial or somatic skeleton and the neu-
ral skeleton is valuable, as is the recognition
of three functionally separate portions
within each calvarial bone: the inner table,
the diploe and the outer table.
the base of the skull is the only por-
tion that is preformed in cartilage.  this ac-
counts for the unusual appearance of the
skull in achondroplasia.  in addition, the
base apparently is more closely related to
facial and general somatic growth than to
neural growth.
calvarial bone forms directly from
membrane within the cerebral capsule.
consequently, the size and shape of the cal-
varia directly reflect the form of the grow-
ing neural mass.
the inner table of the calvaria at all
ages is intimately related to the dura mater
and directly reflects the form of the dura
mater.  consequently, the inner table re-
flects the form of the arachnoid mater and,
usually, of the brain as well.
the outer table of the calvaria serves
not only to protect the brain but also to
anchor many of the intrinsic and extrinsic
muscles of the skull.  its form, particularly
in thick-skulled mammals such as the pig,
is determined more by the demands of these
muscles and of the scalp than by the form
of the brain.
the principal function of the diploe
is to form a lightweight yet rigid separation
of the two osseous tables.  in addition, it
serves as a site of hematopoiesis; for this
reason it may tend to a specific volume at a
given age.
the calvarial sutures oppose separa-
tion of the bones of the calvaria while al-
lowing relative motion between them.
when the calvarial bones are passively car-
ried apart with the expanding cerebral cap-
sule by the growth of the neural mass, their
areas are increased by the deposition of
bone in the suture margins.  the sutures,
however, do not function as epiphyses and
have no innate growth potential: removal
of a suture does not disturb final skull form.
because the calvaria represents ossi-
fication of the cerebral capsule, it is small
when the neural mass is small.  the de-
formity of the skull and the secondary
changes that occur in the air sinuses and
the diploe can be explained in terms of
the functional cranial components.  the op-
posite changes occur when the neural mass
is abnormally large.
the correlation between skull size and
intelligence is only fair.  the presence of a
brain of distinctly abnormal size, however,
may be inferred from plain roentgeno-
grams of the skull.
premature closure of the cranial su-
tures may be due to an abnormal form of
the cranial base and possibly of the primi-
tive chondrocranium that precedes it.  the
abnormality is mediated through the dural
fiber tracts that underlie the major cal-
varial sutures.
