16 



OBSTETRICS. 



attained by this arrangement is lightness of structure where strength 

 is not needed. 



Of the three bones, the ilium forms part of the brim of the pelvis 

 but none of the outlet ; the ischium part of the outlet, but none of the 

 brim ; whilst the pubis forms part of both brim and outlet. 



Os sacrum terminates the vertebral column ; in form it is triangu- 

 lar with the apex of the pyramid downwards, and rather backwards. 

 It is the lightest bone in the body when its size is considered, and is 

 spongy in structure. It has four surfaces, an external and internal, 

 and t'wo lateral, is about four and a half inches in length, four inches 

 in width, and its greatest thickness is about two and a half mches. 

 The exler7tal surface is convex and rough, and has three or four 

 processes like those of the vertebrae, which may he caWed the spinous 

 processes of the sacrum ; anterior to these we find a hollow cavity 

 for the reception of the cauda equina, with four holes on each side, 



which communicate with the cavity 

 for the transmission of nerves. The 

 internal surface (Fig. 3 «) is smooth 

 and concave to the depth of half an 

 inch, and crossed by four trans- 

 verse white lines, which mark the 

 former division of its bones by car- 

 tilage. There are also four pairs 

 of holes on this surface, which 

 transmit nervous filaments, which 

 afterwards form part of the great 

 sciatic nerve. The upper edge of 

 this bone completes the brim of the 

 pelvis, and from the projection of 

 the central part it is called the pro- 

 montory of the sacrum.if) The 

 lateral surfaces'^) are rough and 

 uneven, the irregularities corres- 

 ponding with those on the ilium 

 and forming with them the sacro4liac symphysis. 



Os coccygis, is attached to the apex of the sacrum, and is so 

 named from its resemblance to the beak of a cuckoo.(*) It is com- 

 posed of three or four pieces, which play upon each other by separate 

 joints, and is of much importance in obstetrical study. Ihe sma.l 

 sciatic ligament and the ischio-coccygeal muscles are inserted mto it 

 Dimmsions of the pelvis.— The pelvis is divided mto the true and 

 false ; the upper and the lower; and the greater and the lesser by 

 the linea ilio-pectinea, all above that line being the upper, the greater, 

 or false pelvis, all below receiving the other denominations. 1 he 

 l<mer or true pelvis is of most importance to obstetricians, and is 

 divided into hrim, cavity or excavatioyi, and outlet. 



The brim is defined by the linea ilio-pectinea, and is shaped like 



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