The Physiology of the Uterus in Labor D. N. DANFORTH, M.D., R. J. GRAHAM, B.M. and A. C. IVY, M.D.l HE purpose of this article is to uterine horns (8). This ring becomes T synthesize into a rather complete fairly evident during labor at the junc­ picture the experimental observations tion of the upper and lower segments. that have been made during the past It is a more or less definite, tapering ten years in our laboratory regarding ledge, the formation of which is due the processes concerned in the evacu­ to the greater thickness or "retraction" ation of the uterus. of the muscle fibers of the upper than the lower segment. (This r ing is the The Anatomic and Physiologic " fundal or cornal sphincter" in the Divisions of the Uterus dog.) In obstructed labors it becomes It is important first to obtain a clear a very pronounced ring or band and concept (a) of the anatomical divisions is then called Bandl's ring. The ap­ of the human uterus, which may be pearance of a Bandl's ring means --­ traced directly to those in lower forms, threatened rupture of the uterus in the and (b) of the obstetrical or physio­ lower segment ; it is a pathological re­ logical divisions. See figure 1. traction ring. This is a subject that has been greatly III. The lower uterine segment is an­ confused because our knowledge of the alogous to the isthmus uteri oj Asch­ gross and microscopic anatomy of the off in the non-pregnant uterus. In human uterus, and of the embryology, woman its upper level is generally comparative anatomy, and physiology marked by the reflection of the per­ of the uterus has developed in ~ hap­ itoneum. The musculature of the isth­ hazard manner; and the facts have not mus in pregnancy, like the upper seg­ been correlated until recently. A .corre­ ment or corpus, undergoes definite lation of the known facts practically hypertrophy, and a lower segment is establishes the homology of Aschoff's grossly and definitely evident at 3 divisions of the human uteru~ with the months (6). The placenta normally obstetrical or physiological divisions. is located above the upper limit of The Divisions: the isthmus, i.e., in the corpus or cornua, or above the physiological re­ 1. The upper uterine segment. The traction ring. " corpus uteri" are the uterine horns in the dog, cat, and cow. The embryo " The isthmic musculature shows is normally implanted in this segment. definite hypertrophy, but to a lesser II. The physiologic retraction ring. The extent than that of the corpus. In the function of this ring, or Aschoff's ana­ first months of pregnancy only the tomical internal os (A, fig. 1), on the corpus cavity forms the chamber for basis of comparative obstetrics appears the egg. Thereafter the isthmus be­ to be to restrict implantation to the comes taken up more and more , un­ upper segment or corpus uteri or the til at the end of pregnancy there exists an egg chamber of two divi­ l From the Department of Physiology and Pharo sions: an upper, the cavum corporis, macology, Northwester n University Medical School. Received for pUblication, July 23, 1940. which has the function of contraction · This research was rendered possible by a gift in honor of Dr. I. N . Danforth. and discharge of the egg during de- 21 22 QUARTERLY BULLETIN, N. U. M. S. NON-PRmNAN! PREGNANT AT TEI!M ~...... -... -.:..------WNOOS GROSS UPPER ANAT. CORPUS SIi'GMEIIT CORPUS UTERI UTERI GROSS ANAT. CERVIX __ u:n:il.._______ _ Vagina A.-Anatomic internal OB, physiological retraction ring. B.-Ob~tflt.ric internal os and location of cervical sphincter. Co-External os. A.-C. An ..t. cervix. B.-C. Obstbt. cervix. Figure 1. Fig. 1. The divisions Of the human uterus. Grossly the non-pregnant uterus appears to be divided only into two parts, the corpus and the cervix. Histo­ logically, and from the viewpoint of comparative embryology and anatomy, the uterus pregnant and nonpregnant is divided into three parts, the corpus uteri or upper segment, the isthmus uteri or lower segment, and the cervix. "A" is the junction of the upper and lower segments and "B" the junction of the lower segment with the cervix, at which site is found the cervical sphincter in the dog and monkey. livery, and the lower already dilated tance to the exit of the fetus. part, corresponding to the isthmus (Stieve). See figure l. (or LUS), which during delivery ex­ The products of conception early in periences some further dilation. Un­ pregnancy occupy the upper segment der normal circumstances there is of the uterus. no doubt that the entire isthmus As the products increase and the down to the limit of typical cervical uterus grows, the isthmus uteri en­ mucosa is taken up by the egg cham­ larges, the anatomic internal os disap­ ber." (1) pears, a definite lower segment is IV. The cervix, or the obstetrical or formed, and the products of conception physiological cervix uteri. also occupy the lower segment. This A. The cervical sphincter is located is grossly evident at about 3 months. at the level of the fornix or at the The obstetric and functional division junction of the obstetrical cervix with of the uterus into an upper and lower the lower segment, or at the upper segment and cervix is demonstrated by end of the histologic cervical canal, a study of the comparative anatomy or at the obstetrical os internum. and physiology of the uterus and is This sphincter definitely exists in dog generally recognized by obstetricians. and monkey (3, 4) , and some ob­ The upper segment is relatively stetricians report its presence in more active and "stronger" than the woman. Its presence in woman is lower segment. Such a statement does not definitely established, nor uni­ not imply that the musculature of the versally recognized. lower segment is inactive, because it B. The external os, which becomes manifests resistance to stretch (tone), quite soft and patulous in pregnancy, and it contracts very actively under could not normally offer much resis- certain conditions. When labor is ob- DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 23 structed as in the case of a small or the length of the muscle fiber is rela­ deformed pelvis, or malposition of the tively fixed, and at which length fetus, the lower segment, if not relieved it contracts and relaxes. by the obstetrician, ruptures. In the The stomach, when one eats, in­ various stages of labor the lower seg­ creases the length of its muscle fib­ ment is only relatively passive when ers without a change in intra gastric compared to the upper segment. That tension. As the stomach empties, the is, most of the "expulsive work" of the length of the muscle fibers decreases uterus is performed by the upper seg­ without a change in intra gastric ten­ ment. sion. Intragastric tension, of course, increases and decreases during con­ The Four Major Properties of the traction and relaxation. Uterine Musculature When the muscle fibers increase or The 4 major properties of the uter­ decrease in length and at the in­ ine musculature are the same as those creased or decreased length mani­ possessed by smooth muscles in gen­ fest the same tension as before, a eral: (1) contraction, (2) relaxation, metro static adjustment has occurred. (3) adjustments in length without To denote the directional change in change in intra-uterine tension, (4) co­ length, we use the e x pre s s ion ordination. "mecystatic adjustment" to indi­ I. Contraction. When the uterus con­ cate that the muscle has relaxed or tracts, the intra-uterine pressure rises become elongated and has become equally throughout, but because of the relatively fixed at an tncreased form of the uterus, pressure is exert­ len g t h; we use the expression ed on the breech of the fetus and the "brachystatic adjustment" to in­ presenting part tends to descend or dicate that the muscle has shortened, to serve as a wedge about which the or after contraction it does not re­ cervix becomes effaced and dilated. turn to its original length. (This is Actual descent does not occur until not to be confused with "tone," be­ the cervix is considerably dilated. cause a change in tone means a Actual descent is the result of efface­ change in tension, which mayor may ment and dilation of the cervix. The not be accompanied by a change in cervix is pulled up and about the pre­ length.) senting part. B. Mecystasis is a state manifested Contraction costs food energy. Un­ by a muscle fiber after it has as­ less food and adequate oxygen are sumed a relatively fixed increase in available, the uterus will become ex­ length, at which length it resists hausted. If properly nourished, the stretch, contracts and relaxes, and uterine musculature should be diffi­ manifests the same tension as before cult to fatigue; for example, the stom­ elongating. See figure 2. ach and intestine in many animals 1. If mecystasis of the uterine fibers work day and night. did not occur in the latter stage of II. Relaxation. The pressure in the pregnancy, the fibers would either uterus returns to the previous level. have to grow in length, or a grad­ During the period of relaxation and ually increasing intra-uterine ten­ rest that follows a contraction, the sion would result. This latter phe­ properly nourished muscle returns to nomenon does not normally occur, its original chemical state. and the muscle fibers cease growing III. Adjustments in length without at about midterm.