130 THE BONES. 



and below, and of a quadrilateral form. It offers for study : two /aces, four 

 borders, smd/our angles. 



Faces. — The superior is smooth and nearly plane, and forms part of the floor 

 of the pelvic cavity. It has a small nutrient foramen directed outwards. The 

 inferior presents some rugosities, clustered particularly about the symphysis. 



Borders. — The anterior, thick and concave, circumscribes the oval foramen 

 posteriorly. The posterior, straight and directed obliquely forwards and inwards, 

 forms, with the analogous border of the opposite bone, a large notch named tjie 

 ischial arch. It exhibits, throughout its extent, a rugged depressed lip (the 

 spine), arising from the side of the inferior face. The external, thick and 

 concave, constitutes the lesser ischiatic notch. The internal is joined to the 

 ischium of the other side, to constitute a portion of the pelvic symphysis. 



Angles. — The antero-external, or cotyloidean, is the most voluminous of the 

 four, and affords for study : 1. An excavated diarthrodial facet, making part of 

 the cotyloid cavity. 2. The posterior extremity of the supra-cotyloidean crest, 

 limited by a small transverse fissure which separates from the external border of 

 the bone. The antero-internal angle is consolidated with the posterior angle of 

 the pubis. The jwstero-external angle forms the ischial tuberositg. This is a 

 large prismatic process which looks upwards, and is prolonged by a salient 

 ridge, elongated from before to behind, with its sharp border turned outwards 

 and downwards. The postero-internal angle forms, with that of the other 

 ischium, the summit of the triangular space which constitutes the ischial arch, 

 or pubic arch of some species. 



The Coxa in General. — This bone, the three constituent parts of which we 

 have just been studying, presents for consideration, as a whole, a middle portion 

 and two extremities. The middle, very much contracted, offers, outwards and 

 downwards, the cotyloid cavity (or acetabulum), which has not yet been described, 

 because its study does not properly pertain to either of the three regions of the 

 coxa. This cavity is intended to receive the articulating head of the femur, and 

 represents the segment of a hollow sphere ; it is circimiscribed by a very salient 

 rim, which is thin at its free margin, and widely notched on the inner side. The 

 deeper portion is occupied by the roughened and depressed surface already 

 designated as the bottom of the cotyloid cavity {fandus acetabuli), which 

 communicates, by the internal notch of the rim, with the inferior groove of the 

 pubis. The anterior extremity, flattened on both sides, and formed by the 

 ilium, rests, as has been shown, on the sacrum. The posterior extremity, 

 flattened in an inverse sense to the preceding, is constituted by the pubis and 

 the ischium, and is traversed, from above to below, by the sub-pubic (or obturator) 

 foramen — the large oval aperture which separates these two bones from one 

 another, and perforates the floor of the pelvis ; this opening is closed in the 

 fresh state by muscles. 



The two coxae, by uniting in their posterior part, form the articulation to 

 which has been given the name of ischio-pubic or pelvic symphysis ; thus united, 

 the two bones represent something like a V with the opening in front — a 

 circumstance which makes the lateral diameter of the pelvis greater in front 

 than behind. 



Structure and Development of the Coxa.— To the three centres of 

 ossification which constitute the coxa, are added two complementary centres : 

 one for the anterior spinous process and spine of the ihum, another for the 

 ischial tuberosity. 



