GENERAL ANATOMY OF THE PELVIS AND PERINEUM
The pelvic cavity is roofed by the sacrum and first two or three caudal vertebrae; it is impossible to be more specific because an arbitrary element exists in the definition of the outlet.
The roof narrows from front to back and is slightly concave in its length. The ischial tuber and spine are both less prominent than in cattle, and the contribution of the substantial sacrosciatic ligament to the lateral wall is therefore relatively greater (Figure 22-1/7). The floor is solid because the symphysis is firmly fused in mature animals. It is more or less horizontal and flat in its length, though somewhat hollowed from side to side. The pubic region presents a median swelling or ridge in young animals, and it retains this conformation in the stallion; however, the bone thins and the upper surface becomes markedly excavated in mares, especially those that have carried several foals.The entrance to the pelvic cavity faces cranioven- trally; its slope places the pubic brim below the third, or even fourth, sacral vertebra in the mare but only the second in the stallion. Viewed from the front, the inlet to the female pelvis is wide and rounded while that of the male is more angular and cramped, particularly ventrally (Figure 22-2, B). In both sexes, the outlet from the cavity is much smaller than the inlet; it is bounded by a caudal vertebra, the free edges of the sacrosciatic ligaments, and the ischial tubers and arch.
The cavity has the approximate form of a truncated cone, and the longitudinal axis is almost straight between the entrance and the exit (Figure 22-3). The pelvis of the mare is thus more favorably formed for ease of parturition than that of the cow: the entrance is wide, the exit less confined, the cavity generally more capacious, the axis without marked deflection, and a greater part of the lateral walls composed of soft tissue.
The reader is referred to page 43 for a general account of the structure of the pelvis and to Figures 22-8 and 22-19 for an indication of the topography and peritoneal relationships of the viscera.
The most distinctive feature of the perineum is its confinement between the semimembranosus muscles, which extend ventrally from their vertebral heads of origin. These muscles cover the ischial tubers and also the ischiorectal fossae, which therefore do not contribute to the surface contour. Since the muscles bury the caudal borders of the sacrosciatic ligaments, they hamper recognition of the softening that is so useful an indication of the approach of parturition in cattle.
The thin, sparsely haired, and deeply pigmented perineal skin obtains a surface sheen from the secretion of sebaceous glands. It is raised over the caudal part of the anal canal, forming a projection whose shape and salience vary with the functional state. The unusual outline of the vulva and its variable position are the subject of a later comment (p. 570). In the male the urethra may be palpated where it bends around the ischial arch.
The deeper structures of the perineum closely resemble their bovine counterparts, to which reference may be made (p. 700); differences in detail, though numerous, are not of practical significance.