In the great majority of cases, deformities of the pelvis remain unknown, till the period of delivery, and all that can be then done is to combat in the best possible way the difficulties they create. It is evident that the amount of difficulty depends entirely on the disproportion between the head of the child, and the passage through which it has to be born. If the head be large and the passage small the difficulty will be greatest, but if the head be small it may pass through the pelvis though under its average size. The development of the head cannot be ascertained however, before birth, except when it is unusually large from dropsy, and it is therefore always assumed to be of an average development, and the pelvis is compared accordingly. The kind of assistance required in these cases depends chiefly on the measure of the pelvic diameters, though it may be modified somewhat by other considerations. When the smallest diameter of the pelvis measures from _three inches and a half to three inches_, it is customary to leave the expulsion of the fœtus to nature, and it is generally effected, though slowly and with difficulty. If however the patient becomes exhausted, or the head be unusually large, the forceps are generally used after waiting five or six hours. In these cases the head often becomes firmly fixed in the upper strait, so that great force is needed to dislodge it. The upper part passes through, owing to the overlapping of the bones, and the scalp then bulges out like a large tumor, from being engorged with blood and serum, but the lower being more unyielding remains behind. It is therefore impossible for the head to move either way, as it is formed like a figure 8, and held by the narrow part, as will be seen by the following plate. [Illustration: PLATE XLVI. This Plate represents the head fixed, or impacted, at the upper strait of a narrow pelvis.] When the smallest diameter is not more than from _three inches to two and a half_, the birth is sometimes effected by nature, but with extreme difficulty. The accoucheur waits four or five hours, as in the former case, and then if no progress is made he applies the forceps, using great care in doing so. If the extraction is found impossible, with reasonable force, the head must be opened and made smaller, even though the child be living, because it is more proper to sacrifice it than to risk the life of the mother. In a case like this however, no one person would like to decide, unless in a great emergency; there should always be a consultation if possible. A _dwarf_, named Lepratt, who used to perform at the theatres, was delivered with the forceps by M. Dubois, though the pelvis only measured _three inches_. She perfectly recovered, though the child was born dead: it was of fair average size.