OF COLYMBUS TORQUATUS. 155 



apposed the superior surface of the trochanter major of the femur. The ilium posterior 

 to the cotyloid cavity continues along the spine as a simple prolongation of bone for about 

 an inch, after which it is completely fused with the ischium for the rest of its extent. 

 The ordinary ischiatic notch of man is thus converted into a foramen. The foramen in 

 this instance is of an oval shape, three fourths of an inch long, by one third in diameter, 

 lying mesially to the obturator foramen at its commencement, just posterior to the cotyle, 

 and close to the spine. 



Ischium. The ischium is quite flat, scarcely presenting a concavity or convexity in any 

 direction. Its mesial border is completely fused with the ilium, except anteriorly where 

 it separates from that bone to form the ischiatic foramen, and runs forward as a slender 

 process, separating the ischiatic from the obturator foramen, to the acetabulum, which as 

 usual it assists in forming. Posteriorly, the bone is prolonged, forming quite a long and 

 narrow but stout process, terminating in the tuber ischii, lying close alongside of the 

 transverse processes of the two first coccygeal vertebraB, but not united with them. 

 Between this projection and the process which curves forwards and outwards to meet the 

 pubes there is a deep semilunar sulcus. The external border of the bone is long and 

 concave, the latter especially at its extremities, — this concave border forming the mesial 

 margin of the obturator foramen. 



Os pubis. This is a long and exceedingly slender bone. Its anterior extremity, which 

 contributes its share in the formation of the acetabulum, is tolerably broad and strong ; 

 but the bone immediately becomes contracted to a slender straight spiculum, which extends 

 between the acetabulum and the projecting process of the ischium, forming the external 

 border of the obturator foramen. With the tip of this projecting process, which seems to 

 represent in part at least our " ascending ramus of the pubes," the bone is movably connected 

 by ligaments. From this point the bone curves quite abruptly mesiad to meet its fellow. 

 It becomes at the same time broad and thin, the breadth increasing suddenly towards 

 the end, so that the bone terminates by a wide, rounded, club-shaped free extremity. 

 As usual in the class, there is no symphysis jmbis, though the two bones closely approach 

 each other. 



The shape and position, as well as the connections of the os pubis with the rest of the 

 pelvis, are such that the free extremities of the bone must be approximated or separated 

 chiefly by the elasticity of torsion of the slender shaft of the bone. 



Acetabulum. The acetabulum has quite lost its character as a "cotyloid cavity," and 

 become a simple ring of bone. The floor of the cavity is so entirely wanting, that the 

 internal diameter of the acetabular ring is nearly as great as the external. The outer 

 border of the ring is a well-defined, elevated osseous ridge, — most marked posteriorly and 

 externally, but sinking somewhat towards the mesial line. The globular head of the 

 femur is prevented from sinking through this ring by the abutment and articulation of 

 its trochanter major with the prominent facet already described as lying above the cotyle. 

 The smooth articular surface of the acetabulum is continued uninterruptedly on to this 

 facet, so that the articulation of the head and trochanter of the femur is really only 

 a single joint. The exact character of the motions resulting from this joint will be 

 described in speaking of the femur. 



MEMOIRS DOST. SOC. NAT. HIST. VOL. I. Pt. 2. 40 



