The character of the passage will be brought to mind by recalling what was heretofore said of the diameter of the pelvis. It will be remembered that the usual antero-posterior diameter of the brim does not exceed 4½ inches while the transverse is 5¼ inches, and that at the lower outlet the antero-posterior diameter is about 5 inches and the transverse about 4 inches. The FIRST OBSTACLE which the child meets in its progress is the cervex uteri. This being composed partly of muscular fibre which acts somewhat as a sphincter, and partly of elastic celular tissue, holds the sphincter in the tissue with a tenacity which is not easily overcome. But repeated muscular contractions of the womb force down the bag of waters, which forms a sort of wedge, and this is forced down and into the os uteri, compelling it to open. There are also muscular fibres in the uterus which have a longitudinal as well as some that have a circular course, and the action of the former tend after a time to retract the os, over the fœtal head. The _second obstacle_ is the bony brim of the pelvis into which the head of the fœtus cannot pass until its long diameter is adapted to certain diameters of the pelvis. The diameter of the bony pelvis is diminished over one-fourth of an inch by the soft parts upon it, but the oblique diameter of the pelvis will admit the long diameter of the head of the child, which does not often exceed 4½ inches. The head usually presents in this way, and passes in a somewhat spiral manner until it arrives at the outlet where the diameters are adjusted to each other. The head is, however, too large to pass, even in this way, were it not that it admits of a degree of compression to facilitate the entrance and progress through; this moulding is effected by the continued pains. The head of the child which presents at the brim with the occiput towards the left acetabulum rotates during the passage, so that the occiput at its exit is directly under the symphasis pubis; the cause of the rotation is found in the form and direction of the passage and in the shape and size of the fœtal head. This presentation and position is the most common one, though either of the following is liable to occur. By naming the position we indicate just how a presenting part lies, or is turned. We adopt the following classification, which accords with several good authors: PRESENTATIONS AND POSITIONS. PRESENTATIONS. │NO. POSITION. NAME OF POSITION. A—Vertex or head │ 1 Occiput to left Left occipito-iliac │ acetabulum. anterior. │ 2 Occiput to right Right occipito-iliac │ acetabulum. anterior. │ 3 Occiput to Symphasis Occipito pubic. │ pubis.