side. They are felt as cylindrical, irregular bodies, slipping away from the hand, and changing their position from time to time. Having located the back and the extremities, the portion of the fetal ellipse presenting at the superior strait is next ascertained. The examiner now faces the woman's feet, and, with the outstretched hands, the fingers parallel with and the middle finger over the center of Poupart's ligament, on either side, the fingers dip down beneath the ligament into the pelvic cavity. If the head is presenting, it is felt as a hard, regular, round body, the greater mass of the occiput, the sharp point of the chin, and the groove between occiput and back being often distinguishable. At the same time, the density of the head, its compressibility, its approximate size, and its relative size to the pelvis may be learned. Fig. 224.-Abdominal palpation: locating the fetal head. By auscultation the fetal heart-sounds are located, and their rate and intensity are noted. The uterine bruit and the funic souffle are often heard. The position on the abdomen at which the fetal heart-sounds are heard with greatest intensity is of diagnostic value in confirming the find, by abdominal palpation, as to position and presentation. By vaginal examination the finger detects the varying portions of the fetal body which may present at the superior strait, as the cranium, the face, the shoulder, the buttocks, the knees, feet, and, exceptionally, the elbow or hand. The position of the fetus in utero is longitudinal in 99% per cent. of all cases. The cephalic extremity presents in about 95%1⁄2 per cent., 95 per cent. being vertex presentations. In about one-half of 1 per cent. of cases the face presents; the brow very rarely. In about 3 per cent. of all cases the breech presents, and in about one-half of 1 per cent. the fetus occupies a transverse position in utero. An explanation of the great frequency of cephalic presentations is found in a voluntary assumption of that position by the fetus, because it affords it the greatest degree of comfort and the best opportunity for growth and development, the largest room being found in the fundus uteri for the lower extremities, which are freely moved and exercised. 1 An explanation of the great frequency of presentations of the vertex is afforded by the mechanical arrangement of the connection between fetal head and body, diagram matically represented by two bars attached to each other, that representing the head joined to that representing the spinal column, not at its middle, but at a point nearer one end of the bar (Fig. 225). An equal force exerted upon both ends of the lever represented by the child's head will result in the greater flexion of the longer bar, which is that portion of the fetal skull in front of spinal column. The positions of the various presentations are named by the relationship which the most prominent anatomical feature of the presenting part bears to the acetabula or to the sacro-iliac junctions of the maternal pelvis. They are, therefore, four in number. Fig. 225.-Diagram illustrating the cause of the frequency of vertex presentations. Positions of Vertex Presentations.-1. L. O. A., left occipitoanterior, the occiput looking to the left acetabulum. 2. R. O. A., right occipito-anterior. 3. R. O. P., right occipitoposterior, the occiput looking to the right sacro-iliac joint. 4. L. O. P., left occipitoposterior. Of all vertex presentations about seventy per cent. are L. O. A., thirty per cent. R. O. P. The long axis of the fetal skull very rarely lies in the left oblique diameter of the maternal pelvis. Explanation of the Frequency of L. O. A. and R. O. P.-The position of the rectum shortens the left oblique diameter of the pelvis; therefore the long diameter of the head, seeking the direction of least resistance, adjusts itself in the right oblique It is probable that other factors often enter into the assumption of a cephalic presentation by the fetus. The fact that the cephalic extremity is the heavier, and so falls toward the pelvis as the woman stands erect, and the growth of the uterus in a perpendicular rather than a lateral direction, forcing the long axis of the fetus to coincide with the long axis of the uterus, are no doubt instrumental in determining a cephalic rather than a pelvic presentation; but if one accepts this explanation unreservedly, he could not explain a breech presentation at all, nor could he account for the return of a fetus to a breech presentation after it had been turned by external version. Sir James Y. Simpson's theory, therefore, given in the text is, on the whole, the most satisfactory. diameter of the pelvis and the projection of the lumbar spinal column, to which the fetus by choice adapts its anterior concave surface, usually results in the back being turned forward and tilted a little toward the right, because of the usual right lateral version of the pregnant uterus. Thus, the left occipito-anterior position of the vertex is the commonest position in labor. Should the child's back be directed to the right, the occiput is turned posteriorly, because the chin would be pushed forward by the sigmoid flexure and rectum, this being a stronger force in the arrangement of the head than the child's inclination to adapt its concave abdominal surface to the convex surface of the maternal lumbar spine. THE FORCES INVOLVED IN THE MECHANISM OF LABOR. There are certain forces operative in every labor irrespective of fetal presentation and position. These are the forces of expulsion contributed by the uterine muscle and the abdominal muscles, and the forces of resistance contributed by the lower uterine segment, the cervix, vagina, vulva, the pelvis, and the fetal body. The forces of expulsion are furnished by a great part of the uterine muscle (the upper uterine segment) and by the muscular action of the abdominal wall. That portion of the uterine canal which must be dilated to allow the escape of the fetus is called the lower uterine segment. Its boundaries are: above, the firm attachment of the peritoneum to the uterine wall, and, below, the internal OS. That portion of the uterine wall above the point at which the dilatation of the uterine cavity begins is called the upper uterine segment; the boundary-line between these segments, often marked by a perceptible ridge, especially in obstructed labors, is called the contraction ring, or the ring of Bandl. Fig. 226.-Diagram showing the diminution of the upper uterine segment and the expansion of the lower segment during each contraction. The manner in which the uterine muscle exerts its force upon the fetal body is by a diminution of the intra-uterine area. The uterine muscle in contraction somewhat increases the longitudinal diameter of the uterus, but decidedly diminishes the transverse and anteroposterior diameters. The contraction of the abdominal muscles likewise diminishes the area of intraabdominal space. The degree of force exerted by the combined action of uterine and abdominal walls has been estimated to be from seventeen to fifty-five pounds. The forces of resistance are furnished by that portion of the parturient tract which must be dilated,-i. e., from the contraction ring to the vulva, including the lower uterine segment, the cervix, the vagina, and the vulva. The dilatation of the cervix is effected, if the membranes are preserved, by the displacement of the most easily displaceable of the uterine contents, the liquor amnii, in о Fig. 227.-Diagram illustrating the alteration in the shape of a crosssection of a uterus during its contractions. The heavy line represents the non-contracted, the dotted line the contracted, uterus (compare Fig. 228). Fig. 228.-Diagram illustrating the alteration in the shape of a sagittal section of the uterus during its contractions. The heavy line represents the non-contracted, the dotted line the contracted, uterus. the direction of least resistance,-through the cervical canal. A pouch of the membranes insinuated in the canal subjects the surrounding ring of cervical muscle to water-pressure, equally exerted in all directions, but felt by the cervix only in a lateral or horizontal direction. If the membranes are ruptured and the presenting part impinges directly on the cervix and lower uterine segment, the former is subjected to a lateral pull from all sides at once, as the presenting part pushes from above downward. The presenting part, moreover, whatever it be, is somewhat conical in form, and subjects the cervix to a lateral push as it is wedged into the cervical canal (Fig. 229). The dilatation of the lower uterine segment and of the cervix is not, however, simply mechanical, the serous infiltration of the lymph-spaces and the |