THE CYCLIC RELAXATION O F THE PELVIC LIGAMENTS I N THE GUINEA PIG W. T . POMMERENKE Department of Obstetrics and Gynecology, University of Rochester, School of Medicine and Dentistry, Rochester, N e w P o r k ONE FIGURE Various manifestations of the oestrous phenomena in the guinea pig have long been recognized, and the occurrence of a definite dioestrous cycle in this species is now fully established. The communications of Stockard and Papanicolaou ('17) have reviewed the literature so comprehensively that a similar review in this place is hardly necessary. These workers have contributed extensive descriptions of histological and physiological changes which occur cyclically in the ovary, uterus, and vagina of the guinea pig, and have demonstrated the practicability of determining at any time the exact stage of the cycle by noting the microscopical picture of the vaginal smear. The sexual cycle is thus found to repeat itself every 15 to 17 days. I n a later paper ('19) these same authors described the so-called 'vaginal closure membrane ' which occludes the vagina at all times save during oestrum and parturition. Knowledge of the presence or absence of this membrane facilitates greatly the selection of animals about to come in oestrum. Morphological changes in the pelvis in a number of species of mammals approaching sexual maturity and during pregnancy have been observed. Although in a number of animals these changes are associated with actual bone absorption, a s in the pocket gopher (Hisaw, '25), they ordinarily involve the interpubic ligaments at the symphysis and those at the sacro-iliac articulations in such a manner that increased 361 T H E ANATOMICAL RECORD, VOL. 57, N O . 4, A N D S U P P L E M E N T 362 W. T. POMMERENKE mobility of the pelvis results, thus permitting of an enlargement in diameter of the pelvic canal with consequent facilitation of labor and delivery. An extraordinary example of this phenomenon is found in the case of the pregnant guinea pig. I n this species, the passage of a relatively large and welldeveloped fetus through a pelvis in which provision is not made for expansion by stretching or relaxation of the ligaments would be well-nigh impossible as may be convincingly demonstrated with prepared specimens from unimpregnated and parturient pelves, by comparing these with the fetal skull a t term. As early as 1812, Le Gallois described the mechanism in the female guinea pig, which permits the delivery of a fetus having an average head diameter of 20 mm. through a pelvic canal with a n average diameter of 11 mm. Duncan (1854), Stirling ( 'Oa), and Bland-Sutton ( '11) have published similar observations. I n the guinea pig, the relaxation of the symphyseal and sacro-iliac ligaments is a normal accompaniment of pregnancy and becomes evident a s early as the first third of pregnancy, even before the diagnosis of pregnancy can be established with certainty by palpation of the fetuses through the abdominal wall. The relaxation can be readily demonstrated by gently moving each half of the pelvis, i.e., the right and left ischia, alternately up and down. The movement, when present, can be more readily perceived by placing a finger over the symphysis during the manoeuvre. During the interval between oestral periods in the virgin guinea pig, ordinarily very little o r no relaxation can be demonstrated. However, during pregnancy, relaxation is a gradual and progressive process and continues until parturition when the lateral halves of the pelvis can be very freely moved, as they are now separated by some 2 em., a distance easily ample f o r accommodating a finger between the pelvic bones at the symphysis. Following delivery, the pelvic bones tend to approach again to the virginal state, one in which the bones are closely approximated to one another a t the symphysis with little potential movement. CYCLIC RELAXATION O F PELVIC LIGAMENTS 363 Hisaw ('29) described these anatomical changes of the syrnphysis pubis in the guinea pig during pregnancy. He found that they are under endocrinal control and are produced by a corpus luteum hormone which he named 'Relaxin,' while the animal is under the influence of the follicular hormone. He was able to produce changes in the pelvic ligaments of virgin guinea pigs by experimental means, these changes being apparently identical with those normally occurring during pregnancy. Heretofore, it has been supposed that this relaxation as it is observed in nature occurs only as a concomitant of pregnancy. Although Hisaw ('29) made the observation that out of many non-pregnant animals examined, an occasional one was found in which relaxation could be demonstrated, he regarded such cases as exceptional since ordinarily no changes could be detected during the cycle. To explain this occurrence he stated that perhaps persistent corpora lutea, in these cases, exerted their influence after the follicles had reached a considerable size. I n this laboratory, during the course of another experiment to be subsequently published, it was deemed necessary to become quite familiar with the state of the pelves of a large number of animals with reference to the absence or degree of relaxation. Increased mobility can easily be felt by tactile examination and grossly gauged by palpation. But since, a s yet, we have no readily applicable device or method for accurately measuring the rigidity or relaxation of the pelvis, we have had to rely on estimations based on acquaintance with a large number of cases. Such experience diminishes the likelihood of error and is of obvious importance in the matter of recognizing variations not only between animals but also those which may make their appearance in a single animal from time to time, and is necessary for the cognizance of controls. In palpating the pelves of stock female guinea pigs in this manner from time t o time it was soon discovered that one would not infrequently encounter mobility in an animal of 364 W. T. POMMERENKE which it was distinctly remembered that no such mobility existed a few days previously. Moreover, it was found that the new-discovered mobility was frequently absent when the animal was re-examined after the lapse of a few days. To ascertain whether or not this apparently sporadic incidence of pelvic relaxation was a normal, fairly constant phenomenon, or one of an exceptional or pathological nature, since heretofore it had been regarded only as a normal accompaniment of pregnancy, it was decided to systematically observe a colony of animals, keeping in mind the incidence, frequency, and degree of relaxation. Twenty adult virgin female guinea pigs were selected for study. Parous animals were not used because of the knowledge that in these animals the symphysis never entirely returns to the exact virginal state. These virgin animals at the beginning of the period of observation weighed from 480 to 740 gm. They were examined one or two times daily and the amount of relaxation determined and recorded. It required but a little more than 2 weeks to learn that practically all of these test animals went through phases during which relaxation could be demonstrated. Moreover, it was observed that the vaginal closure membrane, mentioned by Bischoff ( 1852) and described by Stockard and Papanicolaou ('19), was partially or completely ruptured in many of the animals showing this relaxation. It is well known that a definite time relationship exists between the rupture of this membrane and oestrum. For certain types of study it is no longer necessary to make microscopic examinations of the vaginal contents to establish the stage of the oestrous cycle. Using this simple test, observing the presence or absence of the vaginal closure membrane, a s a criterion of oestrum, it was found that the periodic relaxation of the pelvic ligaments coincided with the oestral stage of the cycle. The figure shows the results obtained during observation periods from 60 t o 70 days. During this time the oestrous cycle repeats itself about four times, the cycle usually requiring 16 to 17 days. Guinea pig no. 44 may be used as a CYCLIC RELAXATION OF PELVIC LIGAMENTS 365 typical example. This animal had five oestrous periods during the time of observation. It will be noted that no relaxation was observed in the dioestrous period. However, when oestrum approached, relaxation became perceptible. I n some I a2* I L Fig. 1 The time, in days, from the beginning of the observation period is given along the base lines. The animal numbers are given on the left. The ordinates above the base lines represent in a general way oestrum, as evidenced signs, from by the opening of the vaginal orifice. Arbitrarily a system of 1 + to 6+ was used, 1+ representing only a slight opening of the vagina, whereas 6+ represents a complete absence of the vaginal closure membrane. Using a similar arbitrary system of units, the degree of relaxation is represented by the depth of the ordinates below the base line. + 366 W. T. POMMERENKE cases it is noted that one would occasionally encounter no relaxation 1 day even though a certain degree of relaxation was observed on the day before or following. These variations, admittedly, may have been due to the personal factor in estimating the extent of relaxation, since as yet we do not possess an accurate means of measuring the degree of separation of the pubic bones. Significant changes with reference to the exact stage and manifestation of the cycle doubtless occurred between readings on successive days. For this reason we perhaps did not ‘catch’ either the opening of the vagina or the relaxation of the pelvis which may have occurred in the interim. Thus animal no. 42 appears to have missed an oestrous period which would have been expected t o appear on about the twentieth day. Animals no. 92 and no. 33 had oestral cycles in which no relaxation was noted. Animal no. 42 showed relaxation between the sixtieth and seventieth days when a n oestrous period would have been expected, but the vagina was not noted to be open during this time. It was interesting t o occasionally observe pelvic relaxation in immature animals weighing less than 290 gm.. No systematic and repeated observations were made on these animals however. As has been mentioned, Hisaw observed an occasional nonpregnant animal which showed slight relaxation, but regarded this condition as unusual. He suggested that the condition was perhaps due to the persistence of corpora lutea which continued secreting after subsequent follicles made their appearance. But whatever the explanation, from our observations, it would appear that spontaneous relaxation of the pelvic ligaments of the virgin guinea pig is hardly a rare occurrence, but rather a not infrequent accompaniment of oestrum. CYCLIC RELAXATION O F PELVIC LIGAMENTS 367 SUMMARY Twenty adult female guinea pigs were kept under observation f o r some 70 days, during which time they went through four to five oestrus periods. Attention was directed to the degree of relaxation of the pelvic ligaments. It was found that in a large proportion of cases these animals went through cycles during which definite relaxation could be noted. These cycles appeared to correspond t o the oestrous cycles. The necessity of excluding the possibility of spontaneous relaxation in controlled experiments is emphasized. L I T E R A T U R E CITED BISCIIOFF, T. L. W. 1852 Entwicklungsgcschichte des Meerschweinchens. Giessen. BLAND-SUTTON, J. 1911 The symphyseal ligament of the parturient guinea pig. Brit. Med. J., vol. 2, pp. 976-977. DUNCAN,M. 1854 Dublin Quart. J. of Med. Sciences. Quoted b y F. H. Champneys, 1911, Brit. Med. J., vol. 2, p. 1136. I I I S A T V , F. L. 1925 The influence of the ovary on the resorption of the pubic bones of the pocket gopher. J. Exp. Zool., vol. 42, pp. 410-442. 1929 The corpus luteum hormone. Experimental relaxation of the pelvic ligaments of the guinea pig. Physiol. Zool., vol. 2, pp. 59-79. LE GALLOIS,M. J. 1812 Experiences sur la Vie. Paris. STIRLING, W. 1902 Parturition in the guinea pig. Brit. Med. J., vol. 2, p. 777. 1917 The existence of a typical oestrous STOCKARD, C. R., AND G. PAPANICOLAOU cycle in the guinea pig with a study of its histological and physiological changes. Am. J. Anat., vol. 22, pp. 225-263. 1919 The vaginal closure membrane, copulation, and vaginal plug in the guinea pig with further observations of the oestrous rhythm. Biol. Bull., vol. 37, pp. 222-245.