CRANIUM. 
743 
rence would be to strengthen the part in which 
(from the situation of the organ) these vibra¬ 
tions might, in general, be expected to concur ; 
and this is the contrivance adopted in the cra¬ 
nium, for in the centre of its base there is a qua¬ 
drilateral portion (the body of the sphenoid 
bone) of characteristic massiveness and strength. 
It does not however augment uniformly in 
its substance from above downwards. The 
matter is accumulated in dense lines or ribs, 
which pass to a common centre, and constitute 
thereby a peculiar skeleton or Frame-work of 
surpassing strength, which admits of the intro¬ 
duction of a lighter and more fragile structure 
in the intervening spaces, and resists the shocks 
that arrive through the spine, from behind or 
from above. 
This frame-work is situated almost entirely 
in the base; the only part which is in the 
calvarium being a longitudinal curved line, 
formed by the ethmoidal process of the sphe¬ 
noid bone, the crista galli of the ethmoid, the 
spine of the frontal, tbe thickened commutual 
margins of the parietals, and the superior limb 
of the internal occipital spine. Independently 
i of this curved rib, the calvarium consists of 
four ovoidal domes, two on each side; formed, 
the anterior by the corresponding half of the 
frontal bone, and the posterior by the parietal. 
The summits of these domes are their centres 
of ossification, and their bases abut, partly on 
* the longitudinal rib, and partly on the frame¬ 
work in the base. 
The part to which all the forces tend is the 
body of the sphenoid bone. From its poster ior 
comers there pass backwards two ribs, (the 
petrous processes of the temporal bones,) 
which terminate on the extremities of an arch, 
(the lateral limbs of the internal crucial spine 
of the occiput,) which fis placed horizontally, 
and the convexity of which is turned back¬ 
wards. 
This arch and the two ribs which connect it 
£ to the centre are in the line in which the oc¬ 
ciput would strike the ground in falling back¬ 
wards ; and they further form the brim of the 
pit which contains the cerebellum, so that the 
vibrations of force pass in the interstice between 
that organ and the cerebrum. 
From each side of the body of the sphenoid 
bone there stretches forwards, outwards, and 
upwards towards the temples, a curved rib, 
(the anterior part of the great wing,) and, from 
the anterior part of the body, a transverse rib 
which overlays the former. These and the 
posterior lateral ribs, all of which depart from 
a common centre, constitute the frame-work of 
the base which sustains the ovoidal domes of 
the calvaria. The frontal dome is placed with 
its summit (the frontal depression) looking 
backwards, downwards, and inwards ; its mar¬ 
gin is received, inferiorly on the whole length 
of the anterior transverse, and on the extremity 
of the anterior lateral curved rib ; towards the 
middle line, on so much of the longitudinal 
rib as extends to the parietal bones ; and supe¬ 
riorly, it is applied against a portion of the 
base of the parietal dome. It is against these 
parts that it thrusts, whenever it receives a 
shock on its summit. The parietal dome is 
placed with its summit (tbe parietal depression) 
looking downwards and inwards. Below, it 
is received on the extremities of the lateral 
ribs ; above, it thrusts against the remainder of 
the longitudinal rib ; behind, it falls on the 
corresponding portion of the horizontal arch ; 
and, in front, it antagonizes the frontal. 
It is by the bases of these domes thus 
thrusting against a solid frame-work, that the 
cranium is endowed with the power of re¬ 
sisting lateral shocks whether they approach 
from before or behind; and it is not, as some 
allege, simply by the mobility of the head, 
that it withstands blows, which, if it were 
fixed, would fracture it. 
There yet remains to be noticed an impor¬ 
tant part of this skeleton or frame-work ; that 
which bears upon the spine, and resists the 
force transmitted through it. At the bottom of 
the pit containing the cerebellum, there is an 
elliptical opening (the foramen magnum), the 
margin of which is very dense; this opening is 
provided underneath with two tubercles (the 
articulating processes), by which it rests on the 
vertebral column; from these tubercles a curved 
rib on each side (the lateral process of the oc¬ 
cipital bone and the mastoid of the temporal) 
extends upwards and outwards to the extremity 
of the posterior lateral rib ; the segment of the 
margin of the opening which is anterior to the 
tubercles, is prolonged upwards and forwards, 
in the form of a broad pillar (the basilar pro¬ 
cess), to the back part of the common centre ; 
the segment which is behind the tubercles 
sends off, at its back part, a spine (the inferior 
limb of the internal crucial spine), which ends 
at the centre of the horizontal arcb, at the point 
where the superior longitudinal rib terminates ; 
and this point of confluence of the forces from 
below, from above, and from behind, is strength¬ 
ened by a nodule (the internal occipital protu¬ 
berance). The frame-work of the cerebellar 
cavity is thus connected with that of the general 
cavity; anteriorly, to the body of the sphenoid 
bone ; posteriorly, to the tubercle of the occi¬ 
pital ; and, laterally, to the extremities of the 
petrous processes of the temporal bones. In 
both of them it will be seen that they occupy 
spaces between the grand divisions of the ner¬ 
vous matter, which latter is, therefore, removed 
from the chance of sustaining injury by shocks, 
much more completely than it could have been 
had the parietes been submitted to a progres¬ 
sive augmentation of substance from above 
downwards. As it is, the spaces in which the 
nervous matter reposes are thin and frequently 
diaphanous ; and, were they situated in un¬ 
protected parts, would be perforated by the 
slightest force. 
During a considerable period of life the sub¬ 
ject enjoys additional protection from the slight 
yielding of the bones, and from the cartilage 
which intervenes especially at the base. Pres¬ 
sure applied on the vertex would tend to disjoin 
the parietal bones from each other, and from 
the frontal and occipital bones. This the pe¬ 
culiar nature of the articulations forbids, and 
the longitudinal rib chiefly, and the expanded