THE PELVIS.
The bony structure of the hips.--The pelvis; _sacrum_ (five
vertebræ welded together); _coccyx_ (the caudal appendage in man
and the monkeys resembling man); the _innominate bones_ (ilium,
pubis, ischium); the cotyloid cavity; the acetabulum and its
notch.--The articulations of the pelvis, sacro-iliac and pubic;
sacro-iliac ligaments; ilio-pubic, or Poupart’s ligament (fold
of the groin); ligaments of the hip-bones.--The pelvis as a
whole.--Comparison of the pelvis in the male and female.
The pelvis is formed by the union of the two hip-bones, one on either
side, with the vertebral column, so as to form the pelvic cavity or
basin, which is bounded behind by the lower portion of the vertebral
column, sacrum and coccyx, laterally by the hip-bones, and in front
by the _symphysis pubis_, the antero-inferior articulation of the
two hip-bones together. It completes the trunk and abdomen in their
lower parts, and serves to articulate with the thigh-bones (femora)
externally. There is a great contrast between the shoulder girdle and
the pelvic girdle. The former is freely movable, and has only a slight
attachment by the sterno-clavicular joint to the axial skeleton: the
latter is immobile, and is firmly adherent to the vertebral column
(sacrum) by means of the sacro-iliac joint and powerful accessory
ligaments.
The _os sacrum_ (Figs. 5, 8, 32, 35, 38), so called because it was
the part of the trunk offered in sacrifice by the ancients to their
gods, is formed of five vertebræ (sacral vertebræ) fused together
by osseous union, the separate portions of which are easily seen on
careful examination. Taken as a whole it forms a pyramid, the base
of which (2, Fig. 35) is turned upwards and forwards, corresponding
to the body of the first sacral vertebra. This sacral pyramid, being
directed obliquely from above downwards and backwards (Figs. 8, 9),
presents a surface called antero-inferior, or rather inferior, on
which we recognise five united vertebral bodies (Fig. 5, page 28); a
postero-superior surface--better called superior--on which we recognise
the rudimentary spinous processes (Fig. 9, page 38) and the laminæ of
these same five vertebræ, these portions being all united together;
and, finally, the lateral surfaces, expanded above to form the
_auricular surface_ for articulation with a similar surface on the
hip-bone. This surface extends over the sides of the first three sacral
vertebræ; and behind it are rough excavations for the attachment of the
powerful posterior sacro-iliac ligaments.
[Illustration: FIG. 32.
SECTION OF THE PELVIS.]
The _coccyx_ (21, Fig. 5), placed below the sacrum, is a rudimentary
caudal appendage, but instead of being, as in the greater number
of mammals, free and movable, it is found in man curved towards the
interior of the pelvis, whose inferior outlet it contributes to close.
Its borders give attachment to powerful _sacro-sciatic ligaments_,
which help to fill up the space between the vertebral column and the
hip-bone on each side.
[Illustration: FIG. 33.
THE HIP-BONE OF AN INFANT: its three primitive pieces:--1,
ilium;--2, its superior border;--5, 6, ischium, its acetabular
part (in 7);--8, pubis, its acetabular part (11).]
The coccyx is formed of a series of four vertebræ, welded one with the
other, and so diminished in size that each of them is reduced to a
small osseous nodule, representing a rudimentary vertebral body.
The _hip-bones_ (_ossa innominata_) are two in number, one on each
side, articulating behind with the sacrum, and uniting with each other
in front (Figs. 35, 38) to form the symphysis pubis. In order to
understand the arrangement of the parts of which a hip-bone consists
it is necessary to notice that this bone is made up originally, in an
infant, of three distinct parts, which are afterwards joined together
as age advances; of these parts, the superior is called the _ilium_;
the two others are inferior; that placed in front is called the
_pubis_, and that behind, the _ischium_ (Fig. 33). The junction of the
three parts is effected at the central portion of the bone, at the
bottom of the great articular cavity of the hip-joint (acetabulum).
The names of almost every part of the hip-bone are derived from its
three constituent portions--namely, the _ilium_, _pubis_, and _ischium_.
[Illustration: FIG. 34.
RIGHT HIP-BONE (external surface).--1, 1, iliac crest;--2,
anterior superior iliac spine;--3, posterior superior iliac
spine;--4, superior curved line;--5, inferior curved line;--7,
anterior inferior iliac spine;--11, ischial spine;--12, great
sciatic notch;--13, acetabulum; and 14, its pit (fovea);--16,
its great notch, directed downwards;--17, spine of pubis;--18,
horizontal ramus of pubis;--19, body and descending ramus of
pubis;--20, 20, tuberosity of ischium;--22, obturator foramen.]
[Illustration: FIG. 35.
PELVIS OF THE MALE.--1, 1, iliac crests;--2, sacrum;--3,
symphysis pubis;--5, 5, acetabula;--6, 6, rami of pubes and
ischia;--7, 7, obturator foramina;--10, 10, iliac fossa.]
Examining first the external surface of the bone, we notice that its
upper part is expanded into a large area called the _dorsum ilii_
(5, Fig. 34), which is crossed by three curved lines limiting the
attachments of the gluteal muscles (4 and 5, Fig. 34). Below this area
the bone is narrowed and presents a circular cavity (13 and 14, Fig.
34), broad and deep, which has received the name of the _acetabulum_ (a
vinegar-cup); its use is to form the articular socket of the hip-joint
for the reception of the head of the femur. The margin of this cavity
is prominent throughout its entire extent, except below, where it is
deficient, and gives rise to the _notch_ of the acetabulum; this notch
is a valuable starting-point to settle the natural position either of
the iliac bone or of the entire pelvis (Fig. 34). If, in fact, this
bone of the pelvis is supposed to belong to a figure in the upright
position, the acetabular notch should be directed downwards, as shown
in Fig. 34. Below the acetabulum the innominate bone is pierced by a
large hole, called the _obturator_ or _thyroid foramen_ (22, Fig. 34),
bounded by the following parts: behind, by the _ischium_ (20, Fig. 34);
in front and above, by the _horizontal ramus of the pubis_ (18); below,
by a bony plate formed by a prolongation of the pubis (19) proceeding
to join a corresponding prolongation of the ischium. These united
bars of bone are called the _descending ramus of the pubis_ and the
_ascending ramus of the ischium_, and they unite to form with those
of the opposite side the _pubic arch_ of the completed pelvis. The
internal surface of the innominate bone is divided into upper and lower
parts by a diagonal line--the _ilio-pectineal line_--which separates
the iliac portion of the bone from the part formed by the ischium and
pubis, and also serves to separate in the complete pelvis the upper
abdominal part (_false pelvis_) from the cavity of the true pelvis
below. Above this line is the internal expanded surface of the ilium
known as the _iliac fossa_ (10, Fig. 35). At its posterior end is the
_auricular surface_ of the ilium, for articulation with the sacrum.
Below it are, first, a flat surface corresponding to the floor of the
acetabulum, and, lower still, the obturator foramen, bounded as we
have already described. The borders of the hip-bone are distinguished
(Fig. 34) as superior, anterior, posterior, and inferior. The _superior
border_, called the _crest of the ilium_ (1, Fig. 34), is thick and
curved like an italic ∫; it is this which marks on the living model the
outline of the hip--namely, the limit between the lateral part of the
abdomen and the lateral part of the pelvis. It ends in front in the
_anterior superior spine_ (2, Fig. 34), and behind in the _posterior
superior spine_ of the ilium. The anterior border begins at the
anterior superior iliac spine, and presents in succession, from above
downwards, a notch, then a prominence called the _anterior inferior
spine of the ilium_ (7, Fig. 34), below which is a groove giving
passage to the psoas muscle (see later) bounded on its inner side by
the _ilio-pectineal eminence_; finally, this border is continuous
with the _horizontal ramus of the pubis_, and ends internally in the
_spine_ and _crest of the pubis_ (17, Fig. 34). The posterior border of
the bone is similarly marked by projections and notches. It is limited
above by the _posterior superior spine of the ilium_ (3, Fig. 34),
and below by the _tuberosity of the ischium_; and between these are
two smaller projections--the _posterior inferior spine of the ilium_
above, and the _spine of the ischium_ below. The ischial spine serves
to divide the border into two unequal notches, of which the superior,
the larger, is called the _great sciatic notch_ (12, Fig. 34), and
the inferior, the smaller, the _lesser_ sciatic notch. Finally, the
_inferior border_ is formed by the rami of the pubis and ischium. We
will finish the description of this bone by describing its four angles:
the antero-superior (2, Fig. 34) is formed by the anterior superior
iliac spine; the antero-inferior (17, Fig. 34) by the body of the
pubis, which here presents a rough oval surface of considerable extent
which articulates with the pubis of the opposite side to form the
_symphysis pubis_; the postero-inferior angle (20, Fig. 34) is formed
by the tuberosity of the ischium; and finally, the supero-posterior,
thick and flattened, presents on its outer side the posterior iliac
spines, and on its inner side the large rough _auricular surface_, for
articulation with the sacrum.
In order to construct the pelvis, the two hip-bones are joined together
(at the _symphysis pubis_), and also join the sacrum (sacro-iliac
synarthrosis) by articulations which have nothing in common with those
which we have already studied in the limbs--for example, the shoulder
or elbow. In those articulations of the limbs the bones were in contact
by smooth surfaces, which glided one upon the other without anything
interposed between them; such articulations are characterised by their
mobility. On the other hand, the sacrum is united on each side to the
hip-bones, and the hip-bones themselves articulate in front with each
other by rough surfaces, between which are placed plates, more or
less thick, of fibro-cartilage (similar to the intervertebral discs),
which, in consequence, do not allow them to glide one on the other,
but closely join them together. These articulations, which bear the
name of _synarthroses_, or _symphyses_ (_σύν_, together; _φύομαι_, to
weld), are remarkable not for mobility, but for rigidity. They are
characterised by the possession, between the articulating bones, of
plates of fibro-cartilage, which act as tough but elastic buffers.
Behind, the two _sacro-iliac synarthroses_ support the sacrum, which is
firmly wedged in between the two bones of the hip, and is slung between
them, while the strong ligaments placed above the joint permit the
sacrum to support the weight transmitted to it by the vertebral column.
In front and below, the _symphysis pubis_ (3, Fig. 35), besides having
fibro-cartilage placed between the bones and adherent to them, is also
strengthened by fibrous bands passing superficially from one bone to
the other.
These articulations weld the component parts of the pelvis (the sacrum,
with the two hip-bones) into a complete basin; but, at the same time,
owing to the elasticity of these joints, the pelvis can resist without
injury the shocks which are transmitted to it by the vertebral column
and the lower limbs. The sacro-iliac and pubic fibro-cartilages break
and check the shock which is produced--when, for example, we jump from
a height and alight on the soles of the feet.
Besides the sacro-iliac joint and the articulation of the symphysis
pubis, the pelvis also possesses other ligaments accessory or
special to the hip-bone. The accessory ligaments are those which
strengthen the sacro-iliac joint. Above the articulation are the
_ilio-lumbar_ ligament, passing from the transverse process of the
last lumbar vertebra to the iliac crest at its posterior end, and
the _sacro-vertebral_ ligament, passing from the same transverse
process to the lateral mass of the sacrum. Below the joint are the
two _sacro-sciatic ligaments_, which, arising together from the side
of the sacrum and coccyx in the form of a broad fibrous band, proceed
outwards, and, diverging, are attached, one--the _great sacro-sciatic
ligament_--to the tuberosity of the ischium, the other--the _lesser
sacro-sciatic ligament_--to the spine of the ischium. These ligaments
convert the sacro-sciatic notches into foramina, through which
important muscles pass; and, but for this fact, these ligaments would
not need mention here, as they do not appear superficially, being
covered by the mass of the gluteal muscles. The _special ligaments_
of the hip-bone are the _triangular ligament_, which is a membrane
partly filling up the pubic arch; the _obturator membrane_, which
almost completely fills the obturator foramen; and _Poupart’s ligament_
(really derived from the aponeurosis of the external oblique muscle
of the abdomen, which stretches between the anterior superior spine of
the ilium and the spine of the pubis). It is immediately subcutaneous,
and corresponds to the fold of the groin. The subcutaneous fascia
is attached to the entire length of the ligament, and so creates a
depression extending from the spine of the ilium to the pubis. This
fold of the groin marks the limit between the skin of the abdomen
and that of the thigh. As the deep fascial envelope of the thigh is
attached to Poupart’s ligament in its whole length, it pulls on the
ligament so as to make it convex downwards in the extended position of
the lower limb.
The whole pelvis is expanded above and narrower below, and the lower
part of it is, for the most part, concealed from view in the living
model. The lower limbs are attached on each side in such a manner that
they approach each other at the lower part of the pelvis, so as to
leave between them only a narrow interval--the _perineum_, which
corresponds to the genital organs and the parts between the folds of
the buttocks. But the upper outline of the pelvis is clearly marked
throughout the whole of its extent: on each side the iliac crests,
on the superior borders of the hip-bones, form a slightly undulating
line, the middle portion of which is most elevated, while its anterior
extremity inclines abruptly downwards to terminate at the anterior
superior spine of the crest of the ilium, clearly seen in the model
when the skin is not loaded with fat; in front, the pelvis presents an
extensive curved outline with its concavity upwards, the central parts
corresponding to the symphysis pubis, and the lateral parts formed by
Poupart’s ligament on each side. This anterior outline of the pelvis
forms the lower limit of the abdominal wall. This helps to give the
anterior abdominal wall the form of a shield rounded at both upper
and lower ends, a form which the ancients seem to have exaggerated
in adopting for the epigastric pit a configuration rounded instead
of oval, which is the shape it presents in the skeleton. We have
previously (page 54) shown how, in numerous cases, the form adopted by
the ancient sculptors is sufficiently justifiable.
[Illustration:
FIG. 36. FIG. 37.
Diagrams showing that the pelvis of the male (A) represents a
long segment (_a, b, c, d_) of a short cone (_a, b, x_), while
the pelvis of the female (B) represents a short segment (_a, b,
c, d_) of a long cone (_a, b, x_).]
After having studied the pelvis with regard to its mechanism and its
influence on external form, we ought now to examine it with regard
to its proportions--namely, its transverse dimensions; but as the
prominence of the hips is formed not only by the superior border of
the iliac bones, but also by the great trochanters of the femur, this
examination cannot be completed without reference to the relation of
the bones of the thigh to the pelvis. For the moment, therefore, we
must confine ourselves to the study of the proportions of the pelvis in
the male and in the female skeleton.
[Illustration: FIG. 38.
Pelvis of the female.]
Generally speaking, the pelvis of the female is broader and shorter
than that of the male; in the male (Fig. 35) the superior transverse
diameter, which is a line that passes through the most prominent
portion of the crest of the ilium of one side to the corresponding part
of the other, measures from 10 to 12 inches (on the average 11 inches);
whilst in the female this line measures from 10¼ to 13¾ inches (on the
average 12 inches). On the contrary, the height of the pelvis in man
is about 8 inches, whilst it is not more than 7¼ inches in the female.
Again, comparing (Figs. 35 and 38) a male with a female pelvis, we see
that the first is narrow, the second comparatively broad between the
ischial tuberosities.
If we suppose a line drawn at a tangent to the sides of the pelvis, we
see that these lines must be prolonged downwards for a considerable
distance in order to meet in the female pelvis, but that in the male
pelvis they join at a shorter distance from the pelvic outlet. In
order to reduce these facts to a simple formula we may say, therefore,
that the _pelvis of a male represents a long segment of a short cone_,
while _that of the female represents a short segment of a long cone_.
The student is referred to Figs. 36 and 37, which show this arrangement
by means of a simple diagram.
The particular details of shape which distinguish the pelvis in the two
sexes are the following:--_The thickness of the walls_: in the male
the bones of the pelvis are stronger, the iliac crests are thicker,
more curved, and less expanded, and the different prominences for the
insertions of muscles are better marked. The _pubic arch_, limited
above by the symphysis, and on each side by the descending rami of the
pubis, is very broad, and has everted edges in the female, in whom it
assumes the form of an elliptical arch, while in the male (compare Fig.
35 and Fig. 38 at 3, 6, 6) this same arch is narrow and raised, and
takes the form of a pointed arch. For the same reason the _tuberosities
of the ischium_ are wider apart in the female than in the male. The
_spine of the ischium_ in the female is turned backwards, and appears
less obviously in the pelvic cavity. The _obturator foramina_ are wide
and triangular in the female, while they are narrow and oval in the
pelvis of the male. The _sacrum_ is relatively shorter and broader in
the female than in the male.