The ischium forms a plate of bone which, in part, closes the external portion of the cavity of the pelvis. Its superior border is separate for a certain distance from the external border of the ilium; there is thus left an opening of more or less considerable size, which represents or takes the place of the great sciatic notch.
Fig. 46.—Pelvis of a Bird (the Cock): External Surface, Left Side.
1, Ilium; 2, ischium; 3, pubis; 4, inferior extremity of the pubis; 5, sciatic foramen; 6, oval foramen, homologous to the obturator; 7, coccygeal vertebræ.
The pubis, long and slender, is in connection with the inferior border of the ischium, of which it follows the general direction; and circumscribes with this latter, below the cotyloid cavity, an oval orifice, which is the homologue of the obturator foramen. Its inferior extremity reaches beyond the corresponding part of the ischium, bending towards the middle line, but without joining the pubis of the opposite side. On this account there is no symphysis pubis in birds. Nevertheless, an exception must be noted in the case of the ostrich, the pubic bones of which meet in the middle line, and are articulated in form of a symphysis.
The Thigh
A single bone, the femur, forms the skeleton of this portion of the lower limb.
The Femur.—The bone of the thigh is, in man, directed downwards and inwards; this obliquity, we may remind the reader, is due to the difference in length of the two condyles which form its inferior extremity; the internal is the more prominent, the result of which is that when the femur is held vertically, the internal condyle descends lower than the external. Now, as those two articular expansions rest on the horizontal plane formed by the upper extremity of the tibia, it follows that the superior part of the femur inclines towards the side of the shorter condyle—that is to say, outwards—and that, the leg being vertical, it and the bone of the thigh unite in forming an angle, of which the apex is directed towards the inner side of the knee.
In many mammals the two condyles are equally prominent, the result of which is that the femur inclines neither inwards nor outwards, but is contained in a plane parallel to the axis of the trunk; while the leg is included in the same plane. Nevertheless, although contained in the plane which we have just indicated, the femur is obliquely placed, and directed downwards and forwards; it accordingly forms, with the pelvis, an angle, of which the opening is directed to the anterior aspect of the body.