BONES OF THE PELVIS. 



17 



Fig. 4. 



the heart on a playing card. It has three diameters, an antero-pos- 



terior, called also, straight or conjugate, from the promontory of the 



sacrum to the inner edge of the symphysis pubis, which measures about 



four inches. (Fig. 4, a. p.) The transverse, across the widest part of 



the brim, and at right angles to the an- 



tero-posterior, is about five inches and 



a quarter.(^ t) The oblique, from the 



sacro'iliac junction on either side to 



the opposite side of the brim above 



the acetabulum, five inches. (o o) — 



These measurements are of course 



less in the living body in consequence 



of the presence of the soft structures. 



The circumference of the brim is 



about thirteen inches. 



The cavity or excavation is the space included between the brim 

 and the outlet. It is an inch and a half deep in front, three and a 

 half at the side, and from five to six inches from the sacral promon- 

 tory to the tip of the coccyx. The brim of the pelvis is also called 

 the superior strait. 



The outlet is of an irregularly oval shape, and its diameters are 

 the reverse of those of the superior strait; thus, the antero-posterior 

 diameter, from the arch of the pubes to the tip of the coccyx is four to 

 four and a half inches. (Fig. 5, a. p,) The transverse, from one 

 tuberosity to the other, is four inches.(^ t) The outlet is bounded by 

 the tip of the coccyx at the back, by 

 the lower edge of the under fasciculus 

 of the sacro-sciatic ligaments posteri- 

 orly and laterally, by the tubera ischii 

 at the sides, by the rami of the ischia 

 and pubes anteriorly and laterally, 

 and by the symphysis pubis in front. 

 Its circumference is about twelve 

 inches, and it is also called the infe- 

 rior strait. 



_ The planes of the straits. — If a 

 piece of card or paper be cut so as to fit within the li7iea ilio-pec- 

 tinea, it_ will represent the plane of the superior strait. Now hold 

 the pelvis in the position it occupies when the individual is either 

 sitting or standing, and it will be found that the plane of the strait 

 has an inclination of about thirty-five degrees, which may be 

 increased or diminished at will by extending or flexing the lumbar 

 vertebrse. The plane of the inferior strait may also be described in 

 the same way, viz. : by fitting a card into it. It will be found that 

 they incline towards each other anteriorly, and would meet, if pro- 

 duced, about one and a half inches in front of the pubes. The whole 

 position of the pelvis in regard to the trunk of the body is oblique, 

 so that a line drawn through the trunk in the direction of its axis 



2* 



Ficr. 5. 



