The uterine fibres have been usually considered as fleshy, but they differ from the red fibres of voluntary muscles, in being of a paler colour, flatter, and remarkably interwoven with each other: nevertheless they appear to be really muscular fibres from the powerful contraction with which they expel the fœtus and placenta, and nearly obliterate the cavity of the uterus. In the unimpregnated state they resemble the fibrous coat of an artery, whereas, those of the gravid uterus are more like the fibres of muscle. Most anatomists agree in describing two sets of fibres, viz. longitudinal and transverse. The external layer of fibres appears to form the round ligaments, which seem to have the same relation with them as tendon and muscle. “The fibres arise from the round ligaments, and regularly diverging spread over the fundus until they unite and form the outmost stratum of the muscular substance of the uterus. The round ligaments of the womb have been considered as useful in directing the ascent of the uterus during gestation, so as to throw it before the floating viscera of the abdomen: but in truth it could not ascend differently; and on looking to the connexion of this cord with the fibres of the uterus, we may be led to consider it as performing rather the office of a tendon than that of a ligament.”[10] “On the outer surface and lateral part of the womb, the muscular fibres run with an appearance of irregularity among the larger blood-vessels, but they are well calculated to constringe the vessels, whenever they are excited to contraction. The substance of the gravid uterus is powerfully and distinctly muscular, but the course of the fibres is less easily described than might be imagined: this is owing to the intricate interweaving of the fibres with each other—an intermixture however which greatly increases the extent of their power in diminishing the cavity of the uterus. After making sections of the substance of the womb in different directions, we have no hesitation in stating that towards the fundus the circular fibres prevail, that towards the orifice the longitudinal fibres are most apparent, and that on the whole, the most general course of the fibres is from the fundus towards the orifice.
“This prevalence of longitudinal fibres is undoubtedly a provision for diminishing the length of the organ, or for drawing the fundus towards the orifice. At the same time these longitudinal fibres must dilate the orifice and draw the lower part of the uterus over the head of the child.
“In making sections of the uterus while it retained its natural muscular contraction, I have been much struck in observing how entirely the blood-vessels were closed and invisible, and how open and distinct the mouths of the cut blood-vessels became when the same portions of the uterus were distended or relaxed. This fact of the natural contraction of the substance of the uterus closing the smallest pore of the vessels, so that no vessels are to be seen, where we nevertheless know that they are large and numerous, demonstrates that a very principal effect of the muscular action of the womb is the constringing of the numerous vessels which supply the placenta, and which must be ruptured when the placenta is separated from the womb.”
“Upon inverting the uterus, and brushing off the decidua, the muscular structure is very distinctly seen: the inner surface of the fundus consists of two sets of fibres, running in concentric circles round the orifices of the Fallopian tubes; these circles at their circumference unite and mingle, making an intricate tissue. Ruysch, I am inclined to believe, saw the circular fibres of one side only; and not adverting to the circumstance of the Fallopian tube opening in the centre of these fibres, which would have proved their lateral position, he described the muscle as seated in the centre of the fundus uteri. This structure of the inner surface of the fundus of the uterus is still adapted to the explanation of Ruysch, which was that they produced contraction and corrugation of the surface of the uterus, which, the placenta, not partaking of, the cohesion of the surface was necessarily broken. Farther, I have observed a set of fibres on the inner surface of the uterus, which are not described: they commence at the centre of the last described muscle, and having a course in some degree vortiginous, they descend in a broad irregular band towards the orifice of the uterus: these fibres co-operating with the external muscle of the uterus, and with the general mass of fibres in the substance of it, must tend to draw down the fundus in the expulsion of the fœtus, and to draw the orifice and lower segment of the uterus over the child’s head.” (C. Bell, op. cit.)
There are other circumstances which prove the muscularity of the uterus, beyond the mere evidence of its fibres, as seen during pregnancy. “In the quadruped,” as Dr. Hunter observes, “the cat particularly and the rabbit, the muscular action or peristaltic motion of the uterus is as evidently seen as that of the intestines, when the animal is opened immediately after death.” It is also proved by the powerful contraction which it exerts during labour, and “by the thickness of the fibres corresponding with their degree of contraction.” (Ibid.)
The inner surface of the uterus is lined by a smooth or somewhat flocculent membrane of a reddish colour, which is continued superiorly into the Fallopian tubes; inferiorly it becomes the lining membrane of the vagina.
Mucous follicles are only found in the cervix, especially at its lower part: when by chance these become inflamed, the orifice closes, and the follicle becomes more or less distended by a collection of thin fluid. The mucous casts of these follicles have been known by the name of ovula Nabothi, having been mistaken by an old anatomist for Graafian vesicles, which had been detached from the ovary, and conveyed into the cavity of the uterus.
The mucous membrane which lines the cervix uteri is corrugated into a number of rugæ, between which the mucous follicles are chiefly found.
Uterus duplex.