Ischio-Pubic Margin

This has been already considered : observe the hinder part, where there is an outer slope occupied by the origin of Adductor magnus, and an inner one in relation with ischio-rectal fatty tissue. This region is also epiphysial, possibly a traction epiphysis, but more probably one of the atavistic variety, representing a remnant of the hypoischium of birds and reptiles.

Back of Tuberosity of Ischium : facets, upper and outer for Semimembranosus and lower and inner for Biceps and Semitendinosus, are well marked, and the latter is closely bordered by the attachment of the great sciatic ligament, from which the Biceps obtains some origin. The ligament is considered to be a part of the flexor musculature secondarily cut off by the development of the tuberosity.

Lesser Sciatic Notch : the internal pudic vessels and nerve do not touch the bone, though they go through the foramen, but are separated by the tendon from it : the nerve to the Obturator internus runs on the bone above the tendon to reach the muscle.

Spine Of Ischium

-Observe that this is incurved (Fig. 100), and the inward projection is less in the female than in the male.

Greater Notch : the foramen gives exit to Pyriformis, to seven nerves, and to three groups of vessels.* The typical shape of the notch differs in male and female bones (Fig. 112).

Posterior Inferior Spine gives attachment to great sciatic ligament (Fig. 34) and posterior superior spine to superficial oblique band of sacro-iliac ligament. The lower spine is at level of lower end of articular surface. The bone above and in front of it is covered by ligamentous fibres, on the surface of which the Gluteus maximus arises.

The main points of distinction between the male and female pelvis--with the exception of the breadth and curve of the sacrum and the measurements of the cavity-■ can be appreciated on the Os innominatum. Thus the male bone is rougher, thicker, and heavier than the female, and is higher proportionately. The difference in widtli between the male and female pubic arches can be appreciated on the separate bone if this is held in its proper position. Also in the female the everted surface of the ischio-pubic ramus is smaller, the symphysis is less deep, the acetabulum is shallower, and the distance from the acetabulum to the symphysis is greater than the breadth of the acetabulum, but is not so in the male.

In the typical female bone the Great Sciatic Notch is broader and more rounded than in the male (Fig. 112), and the distance between the tuberosity of the Ischium and the top of the great notch is greater.

There is a shght difference in the shape of the thyroid foramen in well-marked bones, but this depends on the breadth of the ischio-pubic region ; and it may be stated here that the width of the pubic arch is the principal point of distinction, and if this is doubtful the other and slighter sexual characters will be doubtful also: this applies with special force to the shape of the foramen and the distance between the ischial tuberosities in the complete pelvis, and with less force, perhaps, to the shape of the sciatic notch and the distance between the tuberosity and the margin of the notch.

The Pre auricular Groove (Fig. 112) is as a rule only found on female bones : its raison d'etre is not very apparent.

Other sexual differences can be better appreciated when the bones are articulated : thus the ilium in the female is more vertical, so that the false pelvis is relatively narrower, especially when compared with the greater breadth and capacity of the true pelvis.

The expansion of the Ihum, with its elongated curved crest, is a human characteristic, as are also the shortness of the symphysis and the comparative size and strength of the ischial tuberosities. The characters of the ihum and the great capacity of the true pelvic cavity give the human character of relative shallowness to the pelvis as a whole.

The ilium corresponds with the body of the scapula in being the dorsal element in the girdle, but the doubtful value of the ventral elements in the shoulder girdle precludes any definite comparison with the ischium and pubis, which are the ventral bars of the hip girdle. Definite homologies have been sought between the various parts of the scapula and innominate bone, and it has been assumed that the dorsal elements of the girdles have undergone a rotation on their long axes in opposite directions, so that the iliacus represents the infraspinatus and the ilio-pectineal line the spine of the scapula, with other parts to correspond. This seems to be a pure assumption without any evidence or ground of support, and it would probably be safer merely to look on the dorsal and ventral parts as corresponding in that relation without endeavouring to establish definite homologies (see pp. 63 and 80).

* Sciatic, pudic, and gluteal vessels, N. great and small sciatic, internal pudic, N. to Obturator, Quadratus femoris, upper and lower gluteal.

It is of course a different and justifiable thing to establish homology between the human and other animal bones, and in the case of the pelvis the general correspondence is very evident. Some details, however, require a little consideration. In reptiles and a few birds, and in some mammals, the ischia meet as well as the pubis, the " hypoischium " being a prolongation of the connecting bar between them (Fig. 113). This is fibro-cartilaginous, and as the pubic arch opens out in response to the demand for space for parturition, the cartilage is stranded, so to speak, on the ischial processes as the epiphysis there. The homology between the angular epiphysis and the marsupial bone, itself the homologue of the epipubis of lower vertebrates, has already been mentioned. This must not be confounded with the prepubic process found in many reptiles (Fig. 113), which, if it has any homologous counterpart in the human bone, is represented by the spine of the pubis or possibly by the ilio-pectineal eminence.

In structure the bone consists, like other flat bones, of a layer of cancellous tissue enclosed between two planes of compact bone. Where the weight of the trunk is transmitted through it-that is, in the iliopectineal line and its immediate neighbourhood, between the auricular surface and the upper and back part of the acetabulum-the strong compact layers are very thick, with corresponding decrease in the cancellous layer. The density of the cancellous tissue is increased especially in the tuberosity of the ischium.

The iliac crest can be felt directly in its whole length, and the anterior superior spine is a very useful point for many measurements during life. Of the posterior spines, the upper can be felt easily, and a transverse line joining it with its fellow goes across the second sacral spine ; the lower is felt less readily, and gives the general level of the lower limit of the sacro-iliac joint and the upper margin of the great sciatic notch. The ischial tuberosity is an important landmark. It is sometimes difficult to place definitely in subjects with well-developed buttocks when lying on their backs, but it can be found by reaching it from below ; this is because it is covered in extension by the thick lower edge of the Gluteus maximus, and by fibro-fatty tissue round this. When the leg is flexed, the edge of the muscle is drawn up and exposes the tuberosity. The ischio-pubic ramus can be followed from the tuberosity forwards. The body of the pubis can be felt, but its parts are defined with difficulty. In women, when the bladder is empty, the bone can be examined bimanually on the surface and through the vagina. The bone in the neighbourhood of the obturator foramen can be more or less examined also through the vagina.