anterior and lateral. The distance between
them increases the space in which the centre
of gravity can oscillate, without being carried
so far forward as to pass beyond a perpen-
dicular, from the cotyloid cavity to the base
* The linec c', in the diagram passes through the
centre of the sacro-iliac joint, at which point, a line
drawn from the cotyloid fulcrum of the bent lever
falls perpendicularly upon the line of action of the
weight, transmitted obliquely in the sacral axis,
and this consequently, the effective point of power
of the bent arm/, e.
of support at the feet" (p. 514. vol. i. Anat.
JDescrip.). Now these experiments of Weber
prove that the centre of gravity is directly
over the cotyloid support, and cannot oscil-
late between these two articulations. The
only oscillation of the line of gravity which
can take place without falling is along the
length of the basis of support the feet.
Soon after, in reference to the sitting po-
sition, he says, " the tuberosities of the ischia
being a little anterior to the cotyloid cavities,
and near the front of the pelvis, the centre
of gravity tends to pass behind the base of
support; and the body easily falls backward
in that position." Now, the tuberosities of
the ischia, in the erect posture, are consider-
ably behind the line of gravity, or transverse
vertical plane, which crosses at or near the
ischio-pubic ramal suture ; and though, in
the sitting posture, they are brought a little
nearer the line of gravity, yet a much more
satisfactory reason of the trunk more easily
falling backwards than forwards, is because of
the support of the hams in front, and the eleva-
tion of the coccyx behind above the plane of
support.
Again, by the projection of the sacrum and
tuberosities of the ilia behind the sacro-iliac
joints, another lever, less powerful than the
foregoing, is formed, having also the cotvlo-
femoral supports for its fulcrum, and 'the
spinal column for its weight, the anterior arm
of this lever being the cotylo-sacral arch
(fig. 87. a', c'), and the posterior, the over-
hanging tuberosities of the ilia and projecting
sacrum ( c', d'}. Measuring from the centre
of the sacro-iliac articulation, the anterior
arm is 1^ inch in direct length, and the
posterior about 2 inches and a half. The
power in this lever resides in the power-
ful muscles which pass from the sacral and
iliac bones posterior to the sacro-iliac joint,
to the osseous spinal projections and append-
ages above viz. the longissimus dorsi and
sacro-lumbalis, and its action is shown in the
increase of the pelvic inclination on the change
from the sitting to the standing position ; the
principal movement taking place in the sacro-
lumbar joint. It acts to the most advantage
when the centre of gravity of the trunk, from
which it is derived, is thrown in advance of
the cotyloid fulcrum (a a'), so that the lever,
though apparently one of the second order
(i. e. in which p and w are on the same side
of the fulcrum), is in reality one of the first
order, [in which the fulcrum may be between
them, and supports both the power, p, and
the weight, w. Hence, in the drooping of the
trunk forwards in old age, the action of
these muscles contributes, to produce the in-
creased obliquity of the pelvic lever in the
manner before described. This may be made
more evident by inspection of the diagram
(fig. 86. B), which is taken from a small brass
model made to illustrate this point. It will be
seen that the pressure on the cotyloid fulcrum,
F, could not be w p, as in the second
order of. levers, but must necessarily be
w-fp, and therefore in the first order of
levers. P, in this case, may be represented
142
PELVIS.
by the line P c, and the line of gravity of
the trunk, ca, and is doubtless considerably
increased by the resisting tension of the an-
terior abdominal muscles acting through the
extensors of the thigh on the femoral sup-
ports in the line c b, b d.
In the foregoing dispositions of the pelvic
structure, the office fulfilled by the sacrum is
so compendious and important as to call for
particular attention to the shape and position
of that bone, and the manner in which it is
articulated with the ilia, so as to be at once
firm and strong as a keystone, yielding as a
spring, and moveable as a jointed bone. And
Fig.
we shall find all these requirements beauti-
fully provided for.
First. The sacrum is wedge-shaped, when
viewed at its anterior aspect, narrowing from
above downward, especially along the surface
immediately between the lower portions of its
iliac articular surfaces in the plane b, e (Jig.
88. A), which are inclined to each other at
an angle varying from 15 to 30, and aver-
aging about 20. When viewed from above at
its base, as in fig. 89. page 144.), it also pre-
sents a wedge shape with the base directed ante-
riorly, the lateral masses of the base becoming
narrower from before backwards ; so that the
88.
A, diagram of the sacral auricular facet, natural size and placed as in the erect position of the body, with
the lines of tension and pressure, a, centre of action of the posterior deep ligaments; dfe, arc of sacral
groove, forming a quadrant with the lines of tension, ad, a e; d e, chord of the arc; d f, e f, chords
of the semi-arcs ; d g e, sacro-vertebral angle ; d b e, triangle forming the sacral " voussoir ;" b and [e,
position of sacral " joggles."
B, diagram of a transverse section of the pelvis in the line of the sacral axis, posterior view'; a c b, angle
of vertical sacral wedge ; d, e, depth of sacro-iliac articulations ; /, interosseous sacro -iliac ligaments.
sacrum appears to be a double wedge, having
its broadest part at the border of junction of
the base with the anterior surface, and tapering
from this point, both upwards and backwards
and downwards and backwards.
Hence it has been stated by Cruveilhier to
be liable to dislocation downwards and for-
wards, from the want of bony support in that
direction. But the sacrum, in the erect po-
sition of the body, is placed, not vertically,
but obliquely, with its base directed more
forwards than upwards, and its anterior sur-
face more downwards than forwards, so that
the upper limb of the auricular surface is
E laced nearly vertically, and the lower nearly
orizontally, as seen in fig. 88. A. The di-
minished breadth of the base of the sacrum
posteriorly is due to the bevelling of the
lateral surfaces for the implantation of the
deep posterior and interosseous ligaments, as
seen in fig. 88. (B, e),the bone not being here
in apposition with the overhanging iliac tuber-
osities, the area of absolute contact being
confined to the auricular surfaces themselves.
Again, the increased breadth of the anterior
surface at the auricular angle c (fig. 89.),
will be found, on careful inspection, to depend
upon the presence of the pointed projections
on each side, before described as received
into corresponding depressions on the au-
ricular surfaces of the ilia, which latter, being
circumscribed below by a raised border, cause
the sacrum to bite on the ilia here to a consider-
able extent, forming what is called, in engi-
neering nomenclature, a "joggle" to the sacral
" voussoir." By the iliac support thus re-
ceived, the position of the sacrum is well
protected against pressures, coming either
directly downwards in the line d, b (fig. 86. A),
or downwards and forwards in the direction of
PELVIS.
143
the line a, b ; b being placed in the diagram
upon the sacral projection. By measuring in
eight sacra, the distances hetween the upper
extremities of the auricular facets on each side,
at the point d, marking them off on paper,
and then taking the distances in like manner
at c, which corresponds to the lateral notch
opposite the second sacral vertebra (see fig.
78. A, b.), I found the line d, c to coincide
pretty nearly with the mean direction of the
superior vertical limb, and with the superior
half of the central curved groove d, e ; and to
fall in a plane which was inclined to the one on
the opposite side, so as to form with it an
angle varying from 15 to 25, and in all cases
directed downwards.
Now, the sacrum is so placed in regard to
the cotylo-sacral arch, and the line of pressure
from above, that the angle formed by the
surfaces of the base and anterior face is the
narrowest point of a rapidly-expanding arti-
cular wedge placed antero-posteriorly. This
is better seen by a lateral view of the auricu-
lar facet, with the bone in the natural oblique
position, as in Jig. 88. A. The facet will
then be seen to have its angular projection
pointing downwards and forwards in the
direction of the cotylo-sacral arch in the line
a b, and its two limbs diverging so as to pre-
sent a broad surface of articulation with the
ilia in the lines b d, be, forming with d e,
the triangular " voussoir " d b e. The depth
of the keystone is the greatest distance
between the anterior border of the superior
limb, d, and the inferior extremity of in-
ferior limb, e (fig. B), and is about 2^ inches
in the adult male. The wedge shape formed
by them is well seen in the posterior view of
a transverse section along the sacral axis, as
in fig. B, where the lines a c, b c show the
obliquity of the wedge, and form an angleaci,
of from 20 to 35. In a direction downwards
and backwards, then, the sacrum has, like an
artificial " voussoir," or key stone, its broad end
directed upwards towards the point of pressure.
But, as Cruveilhier has justly observed,
forces acting in the curve of the lumbar
vertebrae are partly counteracted by the
elastic spring-like yielding of the lumbar and
sacro-lumbar fibro cartilages ; and by the
lumbar curve they are, at the same time, di-
rected backwards as well as downwards (viz.,
at first in the direction of the linerf, f (fig.
88. A), and then in that of / e, which latter,
produced to meet the vertical line d, b, at g,
forms an angle d g e, of about 1 17, coinciding
with the average sacro-vertebral angle in the
direction of the sacral axis), thus tending to
drive the broad end of the sacral " voussoir "
between the narrower iliac intervals ; and
so, in relation to the direction of the prin-
cipal forces acting on the pelvic arch, the
sacrum becomes a true keystone.
Another arrangement which would tend,
from the obliquity of the bone, to counteract
any forward displacement, is the sudden in-
version of tha vertical sacral wedge at the ex-
tremity of the lower limb of the auricular
surface (fig. B, e), opposite the third sacral
bone, at which point, we have mentioned in
the description of the sacrum, the anterior
surface becomes suddenly broader from above
downwards ; so that here the sacrum by an-
other "joggle" again bites on the iliac. A
third disposition preventive of this displace-
ment has been pointed out by Mr. Ward, in
the superiority of breadth of the posterior
over the anterior surface of the sacrum, op-
posite the point c. (fig. A.), the middle of the
inferior articular limb in many instances.
Behind and above the angular projection
on the sacral facet is an elongated depression
or groove, which passes along the centre of
both limbs of the auricular surface, and re-
ceives a 'reciprocal elevation on the iliac
articular surface (fig. 89./) Now this ridge
on the iliac surface evidently bites in its turn
on the sacrum, and presents another obstacle
to anterior displacement in the superior limb,
as well as to downward displacement at the
inferior limb. The surfaces thus applied to
each other, being so curved, give a greater
extent of apposition than if they were plane,
and, at the same time that they allow of a
limited yielding of the sacrum to pressure,
keep the surfaces continually in contact. And
we shall find that, although the general shape
of the articular surface is rendered angular by
the "joggle" b (fig. 88. A), the groove and
corresponding iliac ridge form a regular cres-
centic curve, or segment of a circle d,f, e,
of which, in fact, the central internal projec-
tion of the tuberosity of the ilium above at
a is the centre. Now it is to this prominence
that the powerful deep posterior and interos-
seous sacro-iliac ligaments are mainly fixed
above ; and it is by being suspended by and
moving on them in the radius a,f, that the
sacrum slides on the ilia downwards and back-
wards in the direction of this groove on the
reception of force from above.
That this motion, though limited, does
take place, and in this direction, may very
readily be proved, on the detached pelvis, by
striking directly downwards on the upper
extremity of three or four lumbar vertebrae
cut off with it. The impulse will be almost
entirely felt at the tip of the coccyx, in a
direction upwards and backwards. That
portion of it which is directed immediately
downwards is checked by the powerful liga-
ments above mentioned, and is but little felt
at the sacral promontory.
If a section of the whole pelvis, in the
direction of the cotylo-sacral arch is made,
as in the next figure, a very important element
in the mechanism of the sacro-iliac articula-
tion is brought to view ; viz., the deep pos-
terior and interosseous sacro-iliac ligaments,
(d, e.) These ligaments are continuous one
into the other, becoming shorter down wards, as
the distance between the bones becomes less.
They narrow also antero-posteriorly, so as
finally to be received into the retiring angle
formed by the limbs of the articular facet, at
which point they are seen in the transverse
section in the sacral axis in fig. 88. B, /.
They are attached, externally, to the central
144-
PELVIS.
prominence on the inner surface of the iliac backwards, and are curved also a little out-
tuberosities, a, a' y which project upwards and wards, the better to resist inward traction,
Section of the pelvis and heads of the thigh bones, made in the direction of the cotylo-sacral arch, a little below
the pelvic brim ; shoAving the antero-posterior sacral wedge, the suspending olfice and oblique direction
of the posterior sacro-iliac ligaments, and the wary section of the joint, a, iliac tuberosities; b, c,
antero-posterior sacral wedge; d, deep posterior sacro-iliac ligaments; e, interosseous ligaments;
/, auricular groove; c, sacral joggle; g, c, cotylo- sacral rib. (Drawing made from a recent section.)
following the lesser curve of the iliac crest.
This thickened central portion of the tubero-
sities is placed above the angle of the articular
facet, in the line of direction of the cotylo-
sacral arch produced upwards through it. In
the accompanying figure, the section, made al-
most in the plane of the pelvic brim, cuts di-
rectly through it. Passing downwards and in-
wards, the powerful fibres of these ligaments
are attached to the upper external part of the
posterior surface of the sacrum, b; and they
suspend the sacrum between them some-
what in the manner of a suspension bridge,
of which the iliac tuberosities are the sus-
pending buttresses. This arrangement evi-
dently considerably adds to the yielding elas-
ticity of the sacro-iliac joint, and does much
to lessen the concussions passing through
it. It is evident also that it is in these
ligaments that the most powerful preven-
tive to anterior and downward displacement
of the sacrum resides; for this could not
take place without absolute rupture of their
numerous . fibres, resisting, as they do, all
motion of the sacrum, except, in the limited
sweep of the radii they form, a motion which
exactly coincides with the movement of the sa-
crum proved by the experiment just mentioned.
But these ligaments, from their oblique
direction inwards, at the same time that
they resist downward pressure, pull with a
corresponding force the sacrum and the ilia
more closely together, and render, by this
constantly tightening and bracing process, all
the before-mentioned provisions for resisting
displacement more effective, and, by a gradu-
ally increasing resistance, overcome the impel-
ling force. To illustrate this effect, it may be
mentioned that the effect of placing too much
weight on the crown of an artificial arch
is to cause the line of pressure (c, a, c,Jig. 90.
A), which ought to pass through the centres
of the " voussoirs " perpendicular to their
joints, to rise above the " cxtrados " at the
apex, a, and to be brought within the inner
surface or " intrudes" of the arch, b, on each
side ; and this causes the voussoirs, a d, d c,
to turn on each other at the edges nearest
the line of pressure ; and in consequence the
crown of the arch sinks and opens below, b,
PELVIS.
145
and the haunches rise and open above, d y d.
The sacro-iliac joints, being the haunches of
Fig. 90.
A, diagram of a yielding arch, a, extrados ; 6, in-
trados ; d, d', the haunches ; c, a, c', dotted line
of pressure.
B, parallelogram of forces of sacro-iliac posterior deep
ligaments, a, c, vertical or sustaining force;
c, d, lateral or tightening force ; b, c, diagonal
direction of ligaments.
the cotylo-sacral arch, have in like manner a
tendency to separate above and behind when
the pressure on the sacrum is increased ; and
this tendency is counteracted by the strong
posterior sacro-iliac ligaments. By the law
of the resolution of forces, this tightening ac-
tion of the sacro-iliac ligaments may be ex-
pressed by the opposite sides, a 6, c d, of a
parallelogram (fig. B), of which the line of
direction of the ligaments, b, c, forms the dia-
gonal, and the remaining sides, a c y b d, the
sustaining power.
Lastly. Because of the oblique position of
the sacrum with regard to the ilia, forces
acting on the lumbar vertebra have a ten-
dency to throw the base or sacral promon-
tory downwards, and to tilt the apex with the
coccyx upwards, as is seen in the experiment
of striking the separated extremity of the
lumbar vertebrae before alluded to, by the im-
pulse felt at the sacral promontory. It will
be better understood by reference to the
drawing and diagram of a model made to re-
present the action (Jig. 91. A. and B). " The
tendency of the sacrum is to turn round the
axis of the sacro-iliac joints in the curve d f e,
(fig. 88. A.), and round the centre a {fig. 91.
</). To counteract this tendency of the base
downwards and forwards, the strong ilio-lumbar
ligaments (a) pass backwards and outwards
from the last lumbar vertebra to the crest of
the ilium, upon which it obtains a long and
broad hold. And to resist the tilting upwards
of the apex of the sacrum, are attached the
extremely powerful sacro-sciatic ligaments (),
which aid also the oblique sacro-iUac ligaments
to resist backward displacement.
Thus are constituted two strong yet elastic
springs on each side acting upon the con-
cavities of the lumbar and sacral curves, which
have, perhaps, the most powerful influence
of any that have been before mentioned, in
breaking the force of shocks and concussions
passing along the bones of the trunk and
lower extremities. The importance of this
office of the sacro-sciatic ligaments is seen in
their great strength, and in the consolidation
of the lower sacral vertebras to which they are
attached. The forward direction of the base
of the sacral wedge when taken antero-
posteriorly, as seen by looking from above,
facilitates this elastic yielding of the sacral
spring, as it evidently could not take place if
Fig. 91.
A, vertical section of the os innominatum, with the sacrum and ligaments attached, made in the line of the
ischio-sacral support, a, ilio-lumbar ligament ; b, sacro-sciatic ligaments ; c, iliac tuberosity. (Drawn
from a recent section.)
B, model of the mechanism of the same structures. <?, d represents the yielding motion of the posterior
deep sacro-iliac ligaments.
ep sacro
Supp.
146
PELVIS.
the double sacral wedge had a small diameter
directed forwards as well as downwards, in
which case it would be prevented by the ilia
from moving in that direction at all.
Thus the pelvic supports of the trunk are
a peculiar and admirable combination of the
arch and the suspension bridge. Under heavy
weights, the preparatory tension of the pelvic
muscles, as the psoae, pyriformes, and great
glutei, will, by more closely approximating the
sacrum and ilia, produce the conditions of the
arch. But in sudden shocks, the strain will
fall more immediately upon the ligamentous
suspensory structures, as the sacro-iliac, sacro-
sciatic, and ilio-lumbar ligaments, more calcu-
lated, by their resiliency, to break their force
gradually, and finally overcome them.
The thick, strong, and elastic fibro-cartilagin-
ous pads inserted between the opposing os-
seous surfaces of the sacro-iliac and pubic
symphyses may be mentioned also, as con-
tributing to deaden and break the force of
shocks passing through the pelvic arches ; and
these being generally, as we have seen, arranged
in two layers allowing of limited sliding motion
between them, are better calculated to resist
sudden shocks passing obliquely than single
discs, such as the vertebral, which are chiefly
disposed to resist pressure passing directly.
The posterior projection of the iliac tubero-
sities protects the sacrum, deeply situated be-
tween them, from direct force tending to
produce anterior dislocation, while their in-
ternal direction prevents that bone from slip-
ping backward between them.
The great breadth and size of the sacrum
in the human pelvis it being proportionally
larger than that of any other animal indi-
cate its importance as the basis of support to
the spine, and the crown of the pelvic arch ;
and, in connection with the admirable mecha-
nical and architectural arrangements just de-
scribed, present a wide contrast to the pelvic
structures of animals ; and prove the erect
position to have been designed for the habi-
tual expression of the dignity of man.
The thigh of man, when standing, forms one
line with the trunk, and makes an obtuse angle
with the posterior arm of the pelvic lever ; but
in quadrupeds it is directed much more for-
wards, and forms an acute angle with both the
ilia and the spine. In quadrupeds, the thighs are
much closer together and more pressed upon
the flanks, and, even when they rest on their
haunches, they naturally support themselves
on their fore legs. This is even seen to a
great extent in the apes and monkeys, so
difficult is it for them to maintain the centre
of gravity in an erect posture. The extensors
and flexors of the thigh on the pelvis are also
more developed in man, in order to sustain
with more firmness the erect posture. Hence
the greater breadth of the hip and buttock,
and the bulk of the thigh. The breadth of
the pelvis also gives a greater leverage to
these powerful muscles.
In walking, the human pelvis is thrown
alternately, on each side, upwards, forwards,
and sideways, as the leg on that side is lifted ;
the trunk keeping its centre of gravity over
the bearing leg by swaying regularly to that
side, the pelvic hoop being at the same time
drawn over the supporting leg by the powerful
abducting muscles, the glutei.
On account of the greater width of the
pelvis and trochanters in the female, the
centre of gravity oscillates through a greater
space, and takes longer time to pass over from
one leg to the other, and hence the greater
amount of undulation in their gait, especially
when running.
Mechanism of the human pelvis in regard to
parturition. As a containing cavity, when
completed by its muscular and fascial struc-
tures, the pelvis offers a basin-shaped struc-
ture with a somewhat triangular superior
aperture, the sides of which are formed by the
psoae muscles, and the base by the pubes ; and
with a moveable floor, formed by fasciae and
the levator ani muscle, and perforated by the
rectum and generative organs. Its walls are
interrupted laterally by the sacro-sciatic and
obturator foramina, which are filled by soft
and yielding muscular and ligamentous struc-
tures, and give way considerably to pressure
from within, enlarging the pelvic diameters
opposite to them. They afford, in common
with the superior and inferior openings, the
outlets for the nerves and vessels passing from
the lumbar and sacral plexuses and iliac
trunks to the inferior extremities and peri-
neum. The inferior outlet also transmits the
external communications of the pelvic viscera.
These are, the bladder supported by the
pubis ; the rectum, supported by the sacrum
and coccyx ; and the internal organs of gene-