THE PELVIS
_Anatomical Note._--The five lumbar, the five sacral and the four
coccygeal vertebrae constitute the lumbar and sacro-coccygeal section of
the vertebral column.
[Illustration: FIG. 127.--Skeleton of Pelvis, Seen from Above.]
The _sacrum_, formed by the union of the five sacral vertebrae, appears
in the adult in the form of a bone that narrows rapidly from above
downward in a general curve whose convex side is turned inward. The
coccyx has the importance of being a real and actual caudal appendage,
reduced in man to its simplest anatomical expression. On each side
of the sacrum the two ossa innominata or hip-bones are attached,
constituting a sort of massive girdle (cintura pelvica), serving as
point of attachment for the lower limbs, while at the same time it
sustains the entire weight of the body and the abdominal viscera. These
two bones are made up of three separate parts: an upper part, very
broad and rather thin (the ilium, which constitutes the flank or hip),
one in front (the os pubis), and a third behind, quite massive, and
shaped like the letter V (the ischium). The two ossa innominata and
the os sacrum form the pelvis or pelvic basin, a broad cavity with bony
walls that are by no means complete, within which are a portion of the
digestive organs and a considerable part of the organs belonging to the
genito-urinary system. The pelvis supports the vertebral column and is
in turn supported by the lower limbs, in quite marvellous equilibrium.
The maximum sexual differences of the skeleton are in relation to the
pelvis; in woman the iliac bones form a far ampler basin; in man, the
pelvis is higher and more confined and formed of more solid bones;
but it is not broader. But where the difference is most apparent is
in the pelvic _aperture_ (see Fig. 127) which divides the pelvis into
two parts, the upper or great pelvis and the lower or small pelvis.
This aperture has distinguishing marks that differ widely between the
sexes; in woman it is rounder, in man it is more elongated from front
to back and is narrowed toward the pubis. One of the most important
points of measurement in anthropology and in obstetrics is the extreme
anterior apex of the superior border of the ilium or _crista iliaca
antero-superior_. The woman in whom this dimension (the bis-iliac)
is less than 250 millimetres cannot give birth naturally; similarly
the woman who has a prominent os pubis (due to rachitis) will owe the
attainment of maternity to the intervention of surgery, and perhaps
even of the Caesarean operation.
There are also many ethnical differences in the pelvis: brachycephalics
(the mongolian race) have a broader and shallower pelvis than the
dolichocephalics, who, on the contrary, have a deeper and narrower
pelvis (the negroes). The same thing is met with, notwithstanding its
intermixture, in our own race: blond, brachycephalic women have a wider
pelvis than brunette, dolichocephalic women.
Accordingly, cranium, thorax and pelvis correspond in one and the same
ethnic type.
The abdomen extends from the arch of the diaphragm to the lower
extremity of the pelvis. It contains all the viscera of alimentation:
the digestive system together with the glands belonging to it; the
liver and pancreas, besides the renal system and, in women, the organs
of generation (uterus and ovaries). The diaphragmatic arch, having its
convex side uppermost, enters the thoracic frame as far as the first
dorsal vertebra. The intestinal mass is more noticeable and prominent
in persons having a narrow pelvis; in children, for example, the
abdomen is very prominent.
_Growth of the Pelvis._--In the skeleton of the new-born child the
pelvis differs from that of the adult in two particulars: _height_ and
_direction_. The pelvis is low in the new-born child and higher in the
adult. The central axis is more oblique from front to back (in the
higher mammals the axis of the pelvis is almost central); in the adult,
on the contrary, this axis tends to straighten up, to the point of
becoming nearly vertical, in relation, that is, to the erect position
of man. Hence in the course of growth the pelvis not only becomes
proportionally higher, but it undergoes a rotary movement around the
cotyloid axis; this movement has the effect of elevating the pubis and
bringing the ischium forward.
[Illustration: FIG. 128.]
The vertebral column rests upon the sacrum, which is the retro-cotyloid
portion of the pelvis, and its pressure tends mechanically to
straighten the pelvis (see diagram, Fig. 128). This process of
straightening has certain limits, and is dependent upon the _form
of curvature_ of the vertebral column; if this is exaggerated, as
in lordosis, the weight is thrown further forward, almost over the
cotyles; consequently, the elevation of the pelvis is not properly
accomplished (low pelvis found in lordotics). If, on the contrary, the
lumbar curvature is wanting or reversed (kyphosis), the pressure of
the column is thrown backward and the straightening up of the pelvis
is exaggerated (high pelvis found in kyphotics). Independently of
pathological deformities, there are various forms of lumbar curvature
in the vertebral column that are normal oscillations, or oscillations
acquired through adaptation.
An exaggerated lumbar curvature or saddle-back is found in children
accustomed to carry heavy loads upon their shoulders; a diminished
curvature is found in children constrained to remain in a sitting
posture for many hours a day. The sitting posture tends to cancel the
lumbar inward curve; consequently, while children are in school they
are promoting the elevation of their pelvis.
The elevation of the pelvis proceeds rapidly at the fifteenth year,
during puberty, when the muscular masses become more solid.
A woman is not fitted for motherhood, even if physically developed,
so long as her pelvis has not rotated normally. But if the rotation
is exaggerated (due to prolonged sitting posture during years of
growth), this is very unfavourable to normal childbirth. In rickets,
associated with kyphosis, there is a form of exaggerated rotated pelvis
(pubis high). The laborious "modern" childbirth, and the dangerous
childbirth in the case of women who have devoted much time to study,
must be considered in connection with these artificial anomalies. _Free
movement_ and gymnastics have for this reason, in the case of women, an
importance that extends from the individual to the species.