104 SKELETON. 



The os sacrum in women is shorter, more concave, and is 

 also broader in proportion to its length. The spaces, vertically, 

 between its foramina in front are very small, forming a sort of 

 ridges, which give to the bone the appearance of having been 

 compressed in its length. 



The distance between the upper and lower straits, or in other 

 words, the depth of the small pelvis in women, is not so great 

 as in men: this arises from the comparative shortness in the 

 length of the pubes, of the ischia, and of the sacrum, as just 

 mentioned. The cartilaginous joining of the pubes is thicker 

 in women. The diameters of the inferior strait, like those of 

 the superior, are longer in females. 



Accoucheurs have attached much importance to the direction 

 and length of the diameters of the small pelvis in well formed 

 women. At an average they are as follow. The superior strait 

 presents three diameters: The first or antero-posterior extends 

 from the upper extremity of the symphysis pubis, to the mid- 

 dle of the projection of the sacrum at its superior margin, and 

 measures four inches : The second diameter, or the transverse, 

 crosses the first at right angles, and extends from the middle of 

 one side of the strait to the corresponding point on the other; 

 it measures five inches: The oblique diameter extends from 

 the sacro-iliac junction of one side to the linea ilio-pectiniaover 

 the acetabulum of the other, and measures four inches and a 

 half, sometimes more.* 



At the inferior strait, the antero-posterior diameter is from 

 the lower part of the symphysis pubis to the lower end of the 

 sacrum, and measures five inches.! As the coccyx, in child- 

 bearing women, is moveable, its projection forwards is not 

 taken into the account, because it recedes by the pressure of 

 the child's head, and does not resist its passage: in some cases, 

 however, it is unfortunately fused into the sacrum, and there- 

 fore perfectly rigid, which will diminish this diameter at least 

 an inch. The transverse diameter of the inferior strait is 

 drawn from the middle of the internal margin of the tuberosity 

 of one ischium, to the corresponding point on the other, and 

 measures four inches. 



* See Dewees' System of Midwifery, 7th edition, 1835, p. 28, 

 t Dr. Dewees says four. Loc. cit. 



