When we come to describe the form and size of the fœtal child's head, it will be found that its diameters correspond very nearly with those of the pelvic straits through which it has to pass, so that ordinarily labor presents no serious difficulty. If the head be larger than natural, from any cause, or if the Pelvis be too small, or deformed, this mutual adaptation does not exist, and delivery of course becomes difficult, or dangerous, and sometimes impossible. The only obstacle therefore, which can seriously impede the expulsion of the fœtus, or prevent it altogether, is this want of conformity, in size and shape, between its head and the bones of the Pelvis. The _soft parts_ may retard labor considerably, by being contracted or rigid, but can generally be made to give way, either by the efforts of nature or by manual assistance; and the fœtal head can be reduced in size if necessary; but insufficient size, or faulty form, in the _bones_, is irremediable. The various causes which produce deformity, or imperfect development, in the Pelvis, and unnatural growth of the child's head, will be stated in a subsequent section. For the present, we have only to do with both in the normal state. The importance of an accurate knowledge of the structure of the Pelvis, and of the changes which may be induced in it, will now be obvious; neither the theory nor the practice of Midwifery can in fact be understood without such knowledge. It is also frequently of the first importance to know, _previous to marriage_, whether the pelvis of a young person is so formed that delivery can be safely effected! Inattention to this has sacrificed the lives of many, and caused others to live for years suffering and helpless. In another place we shall give some plain rules and directions by which this important point may be determined. _The floor of the Pelvis._--The soft parts at the bottom of the basin of the Pelvis, consisting of the perineum and various muscles, are called the floor of the Pelvis--the only passage through which is by the Vulva, or mouth of the Vagina. As the head of the child descends to the bottom of the basin, it presses upon this floor, and gradually distends it, until the Vulva is sufficiently enlarged. This delay is advantageous, for if the passage was always large enough, or increased in size without any difficulty, the child would pass too suddenly, and much mischief might often result from its sudden expulsion--such as pulling down of the womb, flooding, and the falling of the child upon the ground.