B.— NORMAL. BIRTH— BUTOKIA. 



I. 

 THE GENITAL PASSAGE. 



1. — Pelvic Canal. 



The two halves of the pelvis enclose a space, designated 

 the pelvic canal, serving the foetus as a passage-way. This 

 canal has . an inlet, an outlet, a superior, an inferior and two 

 lateral planes. 



The inlet is formed by the sacrum, its two wings, the 

 internal faces of the iliac and the anterior border of the pelvic 

 bones. This anterior opening is shaped like an ellipse, its 

 greatest diameter lying tinder an angle of 45^, and is in 

 proportion to the smaller horizontal diameter as 4:3. The 

 ellipse is flattened laterally. 



The outlet is formed by the last sacral vertebra and first 

 three coccygeal vertebrae, posterior ligaments and the ischial 

 arch. It is of circular shape ; its plane, although smaller than 

 that on the inlet, is capable of expansion. 



The superior plane or roof of the pelvic canal is not 

 horizontal, but quite strongly concave, as the result of the 

 concave inferior face of the sacrum; therefore the anterior 

 portion of the roof, beginning at the promontory (articular 

 head of the first sacral vertebra), runs backward and upward. 

 The most posterior portion of the superior plane is formed by 

 the bodies of the third and fourth coccygeal vertebrae ; conse- 

 quently, the vertical diameter of the outlet varies. The 

 inferior plane or floor of the pelvic canal is concave ; the 

 anterior part, the anterior border of the pelvis, lies about 

 4 to 5 cm. below the posterior end of the symphysis. The 

 line connecting these two points runs from before to behind, 

 at first downward and finally upward, forming with a hori- 



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