502 
The Measurements of the Pelvis 
These numbers, however, do not give a good idea of the thickness of the tissues in 
practice. The tissues would be compressed by the parts of the foetus, and, being 
composed of loose connective tissue, would take up much less room. If an 
attempt be made to measure them in a compressed condition the measurement 
is so small as to be almost negligible, and the chances of error are great. In 
what follows 1 mm. has been considered a just estimation of the thickness of 
Jbe tissues over the surfaces of these bones under these conditions. The tissues 
will therefore diminish the diameters, as measured on the moist bony pelvis, 
by 2 mm. for the obstetric conjugate, diagonal conjugate and antero-posterior 
diameter, and will increase the pubic height by 1 mm., if measured internally. 
With regard to the transverse diameter at the brim 2 mm. will also be a good 
allowance. Just at the brim, where this diameter is measured, there is nothing 
but loose tissue. The external iliac vessels and the psoas muscle are well above 
the brim, and it seems hardly likely that they can be pushed over it so as to 
diminish the transverse diameter. 
(6) The finding of the obstetric conjugate and the transverse diameter for the 
living subject. 
All the necessary information has now been collected for finding formulae to 
determine the obstetric conjugate and the transverse diameter in the living 
subject. These formulae can be obtained from the equations previously given by 
allowing for the effects of moisture and the soft parts. The same letters are used 
to indicate the lengths of the diameters, but are printed in clarendon to denote 
the length in the living subject. Consider the obstetric conjugate in the living 
subject. It is smaller than that of the moist bony pelvis by the thickness of the 
tissues over the bones, "2 cm., hence, if this thickness be added to it, we get the 
obstetric conjugate of the moist bony pelvis. This again is larger than that of 
the dry pelvis by "01 cm., the increase due to moisture. If "01 cm. be subtracted, 
therefore, the obstetric conjugate is obtained for the dry pelvis. This is expressed 
by the equation 
C + -20 - -01 = C. 
The same argument applies for the diagonal conjugate and antero-posterior 
and transverse diameters, the equations for which are 
D + -20 - -13 = D, 
A + -20 --08 = ^, 
T + -20 - -07 = T. 
With the inter-spinous and inter-cristal diameters, the pubic height, and the 
height of the hip bone, the case is different. The thickness of the soft parts 
makes the last three greater in the living subject than on the moist bony pelvis 
and, to obtain them for the latter, this thickness must be subtracted. It does not, 
however, affect the inter-spinous diameter for which therefore no correction for 
soft parts is required. The measurements of the moist bony pelvis are, as with 
