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The cyclic relaxation of the pelvic ligaments in the guinea pig.

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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.
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