This is the external opening, or mouth of the Vagina, through which the child has to pass at the termination of delivery. The external and internal lips, with the muscular and membranous tissue surrounding it, are all capable of great distension, without injury, to allow of the passage of the child. THE PERINEUM. This is the part situated between the Vulva and the Rectum. (_p._ Plate I.) It is composed of a somewhat dense and firm substance, chiefly muscular, and, like all the other parts mentioned, is capable of great distension. It is important, in many of the manipulations during labor, to be well acquainted with it; and when the child's head is passing the perineum requires supporting, to prevent its being lacerated or broken through, an accident which often happens from want of due attention, and which leads to the most serious consequences. THE PELVIS. The Pelvis is that part of the bony structure, or skeleton, of the female, in which the generative organs are placed, and through which the process of parturition is effected. An acquaintance with its natural structure, and with the changes which may be produced in its form and size, by disease and other accidents, is indispensable to those who wish to practise or understand midwifery. In early life the Pelvis is composed of several bones, many of which, after puberty, grow together. In the adult female it is customary to speak of but _four_ bones, the sacrum, the coccygis, and the two innominata, or hip bones, (see Plates VII, VIII.) In the young female these are divided into several distinct parts. PLATE VII. _Bones of the Pelvis._ _The four principal bones, as found in Mature life._--A. A. The Ossa Ilii, or Ossa Innominata, commonly called the haunch, or hip bones.--B. The Os Sacrum, or lower part of the back bone.--C. The extreme termination of the back bone, called the Os Coccygis. _The divisions into parts, as in Early life._--The Ilium, A, on each side, is in three parts; the Ilium, properly so called, marked _a. a._; the Pubis, marked _b. b._; and the Ischium, marked _c. c._ The Sacrum is in five parts, marked 1, 2, 3, 4, 5. _d._ Is the last bone of the spine, which joins the Sacrum.--_e. e._ Are the Sockets in which the upper parts of the thigh bones fit, forming the hip joints.--_g. g._ The two rings, formed by the bones of the Pubis and Ischium, each called the Foramen Magnum. [Illustration: Plate VII. Bones of the Pelvis.] [Illustration: PLATE VII.--_a._ This represents the Male Pelvis, to show the difference in structure. The letters correspond with those in Plate VII.] [Illustration: PLATE VIII. Section of the Pelvis, to show the shape and connection of those parts not distinctly visible in the full view. The section is made down the middle of the back bone, and through the symphysis pubes, in front. The letters correspond with those in Plate VII.