At the apex of this conical mamelon is observed a transverse fissure 3m-4"' in length. This is the lower or terminal orifice of the cervical canal, the os externum uteri, fig. 425., as distinguished from the os inter num,fig. 431, i, which marks the commence ment of that canal. The os externum is bor dered in front and behind by two smooth lips, whose commissure on either side forms the lateral boundaries of the orifice. The lips constitute the terminations of the anterior and posterior cervical walls respectively. They are accordingly distinguished as the anterior and posterior lips of the os uteri. Their position and form are most conveniently shown in a vertical section of the part (fig. 426. and 433.). The anterior lip is the smaller ; it projects but slightly into the vagina, but it lies at a lower level than the posterior one, on account both of the greater length of the anterior wall of the uterus, and also from the inclination of the upper part of the organ forwards. In an antero-posterior view, the anterior serves to conceal the pos terior lip, which lies higher in the pelvis, both from the comparative shortness of the pos terior uterine wall, and also from the tilting forward of the entire organ ; nevertheless the posterior lip makes a greater projection into the vagina, because the walls of that canal are reflected off at a higher point upon the cervix posteriorly than anteriorly. This un equal form of the two lips doubtless gave origin to the term os tincm, by which the older anatomists designated the part.
In no portion of the uterus is the difference between the nulliparous and multiparous or gan so marked as in the vaginal portion of the cervix. After the birth of many children, this part becomes much enlarged, soft, flaccid, and of irregular form, with notched margins ; but in the virgin it has uniformly the smooth, even, conical figure just described, while its consistence is nearly that of soft cartilage.
External suilace.—The uterus, being a hol low or cavitary organ, possesses both an ex ternal and an internal surface. The external surface exhibits two faces, anterior and poste rior ; three borders, one superior and two lateral ; and three angles, two superior and one inferior.
The anterior face is smooth, and gently convex in the transverse direction (figs. 427— 430.), but often slightly curved from above downwards (fig. 426.). It is covered by peri toneum in all but its lower part, where this membrane is reflected off to give a covering to the bladder at a distance of not less than one fourth of the entire length of the uterus from its lower extremity. The posterior face is more decidedly convex ; and in some subjects, espe cially in niultiparm, it exhibits a marked pro minence along the median line, from which the walls proceed outwardly in two nearly level planes. These, meeting the less convex an
terior walls at the lateral border, give to a transverse section of such an organ an outline more or less triangular (fig. 428.). The pos terior face also receives an investment of peri toneum. The membrane here, after covering the entire posterior surface of the uterus, usually dips down to cap the upper extremity of the vagina. (Fig. 426. c—r.) The superior uterine border is moderately convex ; it extends from the point of entrance of one Fallopian tube to that of the other (fig. 431. ff). This border is entirely co vered by peritoneum. The two lateral bor ders extend from the point of entrance of either Fallopian tube downwards to the lower extremity of the uterine neck, as far as the margins of the os uteri. These borders are flexuous, being convex above, concave towards and below the centre of the organ, again slightly convex about the middle of the cervix, and finally terminating at the os uteri, after having their continuity interrupted by the circular attachment of the vagina near the termination of the uterine neck. The lateral borders are uninvested by peritoneum; for it is here that the two laminm of that membrane, which form the broad liga ment, meet to inclose the uterus; and by these two lateral borders, the blood-vessels and nerves supplying the organ enter it without penetrating its outer or serous coat (fig. 428.).
The two superior angles are formed at the points of entrance of the Fallopian tubes. The inferior angle is occupied by the vaginal portion of the cervix and the os uteri.
Internal surface and cavities of the body and cervix.— It has been stated that the walls of the uterine body are in nearly close apposi tion internally, leaving only a small interme diate space, termed the cavity of the uterus, which is easily displayed by cutting through the substance of the °rpm. No just concep tion, however, of the real form or capacity of this interspace can be obtained by examining it with the aid of sections made in one direc tion only.
In order to obtain a correct notion of the form and extent of this cavity, it is desirable first to make a longitudinal section through the centre of the entire organ parallel with its lateral borders. The cavity- or interspace is then indicated by a line running from below upwards, and terminating within half an inch of the fundus (fig. 426.).
The upper half of this line indicates the cavity of the uterus ; the lower half; that of the ccrrix. The latter alone exhibits a true cavity; for here the parietes of the cervix are observed to diverge slightly, so as to leave a spindle-shaped canal traversing the whole length of the uterine neck (fig. 431. c c).