Often, to reach the promontory, it is necessary to depress the elbow as much as possible and to elevate the mites excessively.
Now the diameter we have thus obtained, is not the sacro-pubic, but the sacro-sub-pubic, that is to say, an oblique line longer than the antero posterior diameter, and we must hence deduct a trifle, according to the depth, and the obliquity of the symphysis, in order to obtain the true diameter. (See Figs. 117 and 118.) Authorities do not agree as to the amount which should be deducted, varying from .2 to .6 of an inch, accord ing to the pelvis.
From his measurements Pinard concludes that: Although the height, thickness, and direction of the symphysis are the true causes of variation in the diameters, it is especially the height which is important, and then the direction, and finally the thickness; and he shows that, whenever the symphysis measures 1.5 inches and above, we must deduct fron, of an inch to .7; and whenever it measures less, it will suffice to deduct about .4 of an inch. He agrees, therefore, with Depaul.
Whenever the finger cannot reach the promontory, the presumption is allowable that labor may take place at term, although it does not neces sarily follow that the pelvis is normal, as, for instance, in the oblique oval pelvis, where we cannot reach the promontory, and yet the pelvis is far from being normal.
As for pelvimetry at the inferior strait, Breisky prefers external men suration. We suggest, on the contrary, the following method: The woman is placed in the knee-elbow position, and MI assistant pulls aside the skin of the nates, until the vulva opens. The internal border of each tuberosity is then found, and marked in ink. The distance be tween the two points is now measured, and we possess the external trans verse diameter.
The finger is then inserted into the vagina to the internal border of the tuberosity of the ischium, and one of the blades of Depaul's pelvimeter is placed against it, and the other blade against the corresponding point on the other tuberosity. Reading off the distance on the scale, and com paring the figure with that obtained above, we have the bi-ischiatic, or internal transverse diameter.
We thus obtain, approximately it is true, but with sufficient accuracy, the dimensions of the transverse diameter.
The methods of external mensuration in use, give exact enough figures, practically, for the transverse diameters of the inferior strait, as is proved by Frankenhauser's figures. His measurements were made with Osian der's instrument, and in 9 cases the following were the results: Stocker lays great stress on the value of the measurement of the hi ischial diameter, both from the standpoint of diagnosis, and of progno.
He says: " 1. Increase in the bi-ischiatic diameter indicates either a rachitic pelvis, or else deformity due to double femoral luxation. 2. A bi-ischiatic diameter of less than 3.5 inches indicates, very nearly, a funnel shapel pelvis, and these are, usually, also diminished at the superior strait 3. Diminution of the bi-ischiatic diameter in a rachitic pelvis, means a general and regular contraction. 4. If in a pelvis we find both diminu tion in the antero-posterior, and the bis-ischiatic diameters, osteomalapia is suggested. 5. Exaggeration of the bis-ischiatic diameter suggests diminution in the transverse of the superior strait. From a therapeutics: standpoint, diminution in the inferior strait, exposes to laceration of the perineum, often renders recourse to the forceps necessary, gives the in fant little chance in case of version, and calls for the early induction of premature labor.
Diagnosis of the different Forms of Pelvic Deformity.
A. Pelvis regularly and generally Contracted.—We notice, at the out set, the slight forward projection of the sacrum. Mensuration enables us to determine a more or less regular and proportionate diminution in the external diameters, and in the general circumference of the pelvis. The external conjugate is least diminished, and the distance between the anterior iliac spines in the most, resulting in a greater difference between these and the crests than normally. Internal mensuration almost always gives a moderate diminution is the sacro-sub-pubic diameter (the diagonal conjugate.) We must usually deduct to obtain the true conjugate from .3 to .5 of an inch. The cause of this disproportion between the two conjugates is above all due to the lessened inclination of the symphysis.