Understanding the Human Uterus: Anatomical and Physiological Divisions, Schemes and Mind Maps of Physiology

Download Understanding the Human Uterus: Anatomical and Physiological Divisions and more Schemes and Mind Maps Physiology in PDF only on Docsity! The Physiology of the Uterus in Labor D. N. DANFORTH, M.D., R. J. GRAHAM, B.M. and A. C. IVY, M.D.l T HE purpose of this article is to synthesize into a rather complete picture the experimental observations that have been made during the past ten years in our laboratory regarding the processes concerned in the evacu­ ation of the uterus . The Anatomic and Physiologic Divisions of the Uterus It is important first to obtain a clear concept (a) of the anatomical divisions of the human uterus, which may be traced directly to those in lower forms , and (b) of the obstetrical or physio­ logical divisions. See figure 1. This is a subject that has been greatly confused because our knowledge of the gross and microscopic anatomy of the human uterus, and of the embryology, comparative anatomy, and physiology of the uterus has developed in ~ hap­ hazard manner; and the facts have not been correlated until recently. A .corre­ lation of the known facts practically establishes the homology of Aschoff's divisions of the human uteru~ with the obstetrical or physiological divisions. The Divisions: 1. The upper uterine segment. The " corpus uteri" are the uterine horns in the dog, cat, and cow. The embryo is normally implanted in this segment. II. The physiologic retraction ring. The function of this ring, or Aschoff's ana­ tomical internal os (A, fig. 1), on the basis of comparative obstetrics appears to be to restrict implantation to the upper segment or corpus uteri or the l From the Department of Physiology and Pharo macology, Northwester n University Medical School. Received for pUblication, July 23, 1940. · This research was rendered possible by a gift in honor of Dr. I. N . Danforth. uterine horns (8). This r ing becomes fairly evident during labor at the junc­ tion of the upper and lower segments . It is a more or less definite, taper ing ledge, the formation of which is due to the greater thickness or "retraction" of the muscle fibers of the upper than the lower segment. (This r ing is the " fundal or cornal sphincter" in the dog.) In obstructed labors it becomes a very pronounced ring or band and is then called Bandl's ring. The ap­ pearance of a Bandl's r ing means --­ threatened rupture of the uter us in the lower segment ; it is a pathological re­ traction ring. 21 III. The lower uterine segment is an­ alogous to the isthmus uteri oj Asch­ off in the non-pregnant uterus. In woman its upper level is generally marked by the reflection of the per­ itoneum. The musculature of the isth­ mus in pregnancy, like the upper seg­ ment or corpus, undergoes definite hypertrophy, and a lower segment is grossly and definitely evident at 3 months (6). The placenta normally is located above the upper limit of the isthmus, i.e., in the corpus or cornua, or above the physiological re­ traction ring. " The isthmic musculature shows definite hypertrophy, but to a lesser extent than that of the corpus. In the first months of pregnancy only the corpus cavity forms the chamber for the egg. Thereafter the isthmus be­ comes taken up more and more , un­ til at the end of pregnancy there exists an egg chamber of two divi­ sions: an upper, the cavum corporis , which has the function of contraction and discharge of the egg during de- 22 QUARTERLY BULLETIN, N. U. M. S. GROSS ANAT. CORPUS UTERI GROSS ANAT. CERVIX NON-PRmNAN! __ u:n:il.._______ _ Vagina CORPUS UTERI 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. PREGNANT AT TEI!M ~ ...... -... -.:..------WNOOS UPPER SIi'GMEIIT 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 part, corresponding to the isthmus (or LUS) , which during delivery ex­ periences some further dilation. Un­ der normal circumstances there is no doubt that the entire isthmus down to the limit of typical cervical mucosa is taken up by the egg cham­ ber. " (1) tance to the exit of the fetus. (Stieve). See figure l. The products of conception early in pregnancy occupy the upper segment of the uterus. IV. The cervix, or the obstetrical or physiological cervix uteri. As the products increase and the uterus grows, the isthmus uteri en­ larges, the anatomic internal os disap­ pears, a definite lower segment is formed, and the products of conception also occupy the lower segment. This is grossly evident at about 3 months. A. The cervical sphincter is located at the level of the fornix or at the junction of the obstetrical cervix with the lower segment, or at the upper end of the histologic cervical canal, or at the obstetrical os internum. This sphincter definitely exists in dog and monkey (3, 4) , and some ob­ stetricians report its presence in woman. Its presence in woman is not definitely established, nor uni­ versally recognized. B. The external os, which becomes quite soft and patulous in pregnancy, could not normally offer much resis- The obstetric and functional division of the uterus into an upper and lower segment and cervix is demonstrated by a study of the comparative anatomy and physiology of the uterus and is generally recognized by obstetricians. The upper segment is relatively more active and "stronger" than the lower segment. Such a statement does not imply that the musculature of the lower segment is inactive, because it manifests resistance to stretch (tone), and it contracts very actively under certain conditions. When labor is ob- DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS At start of 1st stage Brachystasi s of both segment s At end of 1st stage Brachystasis of L.U.S. only Fi re 3. At end of lot st"8e Brachyste,sis of U.U.S. only, e.cc:ording to curr8J\t vleW9. Uonkey _ as in IV but more dra.wing up of ee l""fix Fig. 3. " I " is the condition at the start of the first stage. AB represents the lower uterine segment. " II" represents what would have occurred during dila­ tions of the cervix if only the lower uterine segment retracted or if only its fibers manifested brachystasis. The shortening would involve only A LB '. "III" represents what is said to be the usual type of retraction in the human, i.e. , r etraction or brachystasis occurs only in the upper segment, the lower segment being represented by A 2B '. " IV" represents what would happen if retraction or brachystasis occurred in both segments, the lower segment being represented py A 3B '. In the monkey, retraction occurs in both segments and more pulling' up of the cervix occurs than is generally thought to occur in woman. 25 pregnancy, or about the second month, are non-propulsive in type, like those rhythmic contractions that occur in the urinary bladder for some time before a "call to urinate" is experienced. When these intermittent B-H contractions become more forceful, labor " pains" are experienced, signifying a "call to labor. " In "false labor," painful contrac­ tions occur but are not accompa­ nied by brachystasis of the upper segment and dilation of the cervix. " False labor" may occur and the contractions disappear, or it may 26 QUARTERLY BULLETIN, N . U . M. S. F" 4 A and B Showing the uterus at the start and end of the first stage in w?ma~. F" ~~. S'h . th~ uterus at the end of the first stage in the ~on~ey. P .R .R ., physwlogt- tg . . .owmg C R the contraction ring. O.I. obstetncal mternal os or C .S., the cal retractwn rmg or . ., , cervi cal sphincter. O.E ., the external os. be followed by "true labor." It is difficult to define exactly the "on­ set of true labor." Arbitrarily, true labor starts when the uterine mo­ tility is of such a character as to lead to dilation and effacement of the cervix. 2. During Labor. First Stage. Stage of Dilation and Effacement of the Cervix. The upper segment con­ tracts then the lower, and then the c:rvical sphincter. The polarity of the wave of contraction is il­ lustrated in figure 5. The head (or presenting part) is pushed against the cervix and sphincter. The sphincter on relaxing along with the remainder of the uterus as­ sumes a more dilated position than before the uterine contraction, or a mecystatic adjustment has oc­ curred. Obviously dilation of the cervix tends to create some slack in the upper and lower segments. This slack must be taken up in order to keep the head in contact with the cervix. The slack is taken up by brachystatic shortening (not by ac­ tive and sustained contraction) of the upper and possibly also the low­ er segment. In the monkey (fig. 3) longitudinal brachystasis occurs in both segments, only chiefly in the upper segment during the latter portion of the second stage of la­ bor. This gradual dilation and tak­ ing-up thins out the sphincter and obstetrical cervix until it is com­ pletely dilated and practically ef­ faced. It should be pointed out here that the two halves of the uterus must contract together, or the various portions of the uterus must con­ tract in a coordinated manner, in order to effect normal effacement and dilation of the cervix (9). If for one reason or other the uterus is not in proper alignment with the bony canal, or the uterine attach­ ments are under an uneven tension, or the forces exerted by the uterine contractions are unequally applied to the breech, the "axis of descent" of the fetus will not be in proper relation to the cervix or to the birth canal, and the cervix will not be properly effaced and dilated; and in the second stage descent will not occur normally. Thus, the lack of brachystasis, or the lack of coordi­ nation in the first stage results in no dilation or very slow and un­ equal dilation. This is why the cer­ vix is sometimes referred to as the " barometer of the uterus" during the first stage. Second Stage. This stage starts DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 27 when the cervix is completely di­ lated. It is chiefly the stage of de­ scent. The head (or presenting part) descends with each uterine con­ traction. During uterine relaxation the head rises a little, but does not return normally to its original po­ sition or level because brachystatic shortening of the upper and lower segment, chiefly the upper, accord­ ing to current views, occurs. (In the monkey the cervix retracts over the head to the level of the neck of the fetus and the brim of the pubis of the mother soon after the cervix has been completely di­ lated and before the head has de­ scended more than one or two cen­ timeters.) In woman, the station or level of the vertex when the cervix surrounds the neck is not known, but should be answered by work which Drs. Caldwell and Still­ man of Sloane Hospital, New York, have in progress (5) . See figure 4.* After the birth of the. head a tempo­ rary arrest in descent is frequently ob­ served. This means that several uterine contractions must occur, or some time must elapse to permit sufficient brachystatic shortening to occur so as to bring the uterine musculature in firm contact with the hind parts, and to again pull taut the uterovaginal at­ tachments. The extent to which the cervix retracts cephalward will depend upon the extent of fixation of the vagi­ nal wall and cervix to adjacent soft and hard parts of the pelvis. The cervical sphincter may contract to some extent, about the neck, and dilate again about the shoulders as the evident contractions start. On the oth­ er hand, this may not occur because of the marked thinning and stretch- 'By placing metal clips on the posterior and anterior cervical lips and then making X·ray plates at different stages. Drs. Caldwell and Still· man have found that, at least in somElllwomen, the cervical lips may retract almost as high as in the monkey. The extent to which the cervix is retracted cephalward will vary with the degree of fixation of the vaginal walls and cervix. ing that the sphincter has undergone during the passage of the head. See figure 3. During the second stage "bearing down" or a "defecatory type" of move­ ment of the mother occurs. This is both a voluntary and an involuntary reflex act. It occurs in women or ani­ mals with a high or a thoracic transec­ tion of the spinal cord. Third Stage . After the birth of the infant, the upper segment contracts and manifests much brachystatic short­ ening. The lower segment is flaccid . Several contractions and much brachy­ stasis, or a "tetanic contraction" may be required to separate the placenta and to close the maternal blood sin­ uses. That the latter result is not ef­ fected rapidly is shown by the occur­ rence of retroplacental hematoma. Ob­ viously the placenta separates by con­ traction of the uterine fibers, and the hematoma forms in the cavity created due to incomplete closure of the ma­ ternal sinuses. A hematoma cannot form until some placental separation occurs. The hematoma is the result and not the cause of placental separa­ tion. During this period the lower seg­ ment remains flaccid. After a varying period the placenta is expelled, the uterus as a whole contracts firmly, be­ coming almost completely a pelvic or­ gan. Then after a period the uterus re­ laxes, becoming an abdominal organ, but manifests much brachystatic short­ ening relative to the ante partum state, in which condition it manifests rhyth­ mic postpartum contractions for sev­ eral days. In all of our frozen sections in the monkey, the lower segment retracts considerably, or manifests much longi­ tudinal brachystasis, when the fetus is passing through the vaginal canal. (figs. 6, 7, and 8). But, direct observa­ tions on the uterus of the monkey in labor show that as soon as the fetus is 30 QUARTERLY BULLETIN, N. U. M. S. GROSS ANATOOCAL CORPUS UTERI GROSS .ANATO}(ICAL CERVIX Figure 7. Figure 6. CORPUS UTERI Upper ltter1ne segment. histologically confirmed. ''''no,~o'il:l cally nDed. Figure 8. Fig. 6. Median sagittal section of the uterus of a mature non-pregnant Macacus rhesus monkey (magnification 2lh times). Observe contour of cervical canal. Fig . 7. Median sagittal section of a Macacus rhesus monkey near term. (lh natural si ze.) Breech presentation. Observe mucous plug in cervical canal. Fig . 8. Median sagittal section of a Macacus rhesus monkey in labor. (lh natural si:::e). Cephalic presentation. Head on perineum. Observe height of cervical lips. DANFORTH ET AL---PHYSIOLOGY OF THE UTERUS 31 .sis (transverse myelitis) of the lower portion of the spinal cord. It is possible that nervous factors may retard dilation of the cervix and disturb the pelvic mechanism con­ cerned in birth. It is reported that in . woman a sacral anesthesia favors di­ la tion of the cervix. Since in the mon­ key "reflex" contractions of the uterus can be elicited, it is rational to assume that "reflex inhibition" may occur on the basis that motor and inhibitory nerves or effects occur together. REFERENCES 1. Aschoff, L.: Die Dreiteilung des Uterus , das untere Uterinsegment (lsthmusseg­ ment) und die Placenta Praevia, Klin. Wchnschr., 1907,44;979. 2. Danforth, D. N., Greene , R. R ., and Ivy, A. C.: Effect of Amputation of Apical Portion of Uterine Horns Upon Labor, Proc. Soc. Exper. BioI. & Med., 1936-37, 35;482. 3. Ivy, A. C., Hartman, C. G. , and Koff, A.: The Contractions of the Monkey Uterus at Term, Am. J. Obst. & Gynec. , 1931, 22;388. 4. Ivy, A. C. and Rudolph, L .: The Physi­ ology of the Uterine Musculature, Surg., Gynec. & Obst., 1938,67;188. 5. Personal communication. 6. Rudolph, L. The Phenomenon of Lighten­ ing in Pregnancy and the Lower Uterine Segment, Surg. Gynec. & Obst., 1937, 64;906. 7. Rudolph, L .: Constriction Ring Dystocia , J. Obst. & Gynaec . Brit. Emp., 1935, 42;992. 8. Rudolph, L. and Ivy, A. C.:: The Physi­ ology of the Uterus in Labor. An Experi­ mental Study in the Rabbit and Dog, Am. J . Obst. & Gynec., 1930, 19;317. 9. Rudolph, L. and Ivy, A. C.: Coordination of the Uterus in Labor, Am. J. Obst. & Gynec., 1931, 21;65.