OSTEOLOGY AND ARTHROLOGY 



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The posterior and external angle of the ischium is rough 

 and prominent ; it is the tuberosity of the ischium. This 

 forms a projection under the skin ; it also does in man when 

 the trunk is strongly inclined forwards, while the thighs are 

 maintained in the vertical position. In marsupials — opos- 

 sum, kangaroo, and phalanger — the pelvis at its pubic 

 region is surmounted by two bones, situated one on each side 

 of the median line, and arranged in the form of a fork of two 

 prongs (Fig. 45). These, which are called marsupial hones, 

 support the pouch which, in animals of this genus, lodges 



E^.C, 



Fig. 45. — Pubic Region of the Pelvis of a Marsupial (Phalanger, 



Fox). 



I, Symphysis pubis ; 2, obturator foramen ; 3. marsupial bones. 



their young, which, at the time of birth, are incapable of 

 supporting a separate existence, their development being 

 absolutely incomplete. 



In the cetaceans — for example, the dolphin — because of 

 the absence of posterior limbs, the pelvis is represented 

 by two separate bones only, which have no connection with 

 the vertebral column. In birds, the pelvis is remarkable 

 for its elongated form (see for its form Fig. 21, and for 

 details Fig. 46). The cotyloid cavity is pierced by an 

 opening, and presents on its posterior border, which is 



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