CHAPTER IV
THE PELVIS
Anatomical Note.—The five lumbar, the five sacral and the four coccygeal vertebræ constitute the lumbar and sacro-coccygeal section of the vertebral column.
Fig. 127.—Skeleton of Pelvis, Seen from Above.
The sacrum, formed by the union of the five sacral vertebræ, 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 Cæsarean 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.