IMPORTANCE OF THE FEMALE REPRODUCTIVE TRACT IN THE FORMATION OF RELAXIN1

THE remarkable loosening of the pelvic ligaments which occurs both at the symphysis pubis and iliosacral junctions in the pregnant guinea pig has been observed and described by numerous investigators. These separations become so extensive, as pregnancy progresses, that a condition is attained in which the two halves of the pelvis can be freely and independently manipulated thus making possible an increase in the diameter of the birth canal. Hisaw (1926) found that pronounced relaxation of the pelvis could be induced experimentally by injecting blood serum of pregnant rabbits or guinea pigs into normal virgin guinea pigs during estrus or into castrated female guinea pigs that had been conditioned with estrogen. The pelvic ligaments usually showed marked relaxation within 8 to 12 hours after one injection of the serum. The relaxative substance was found also in the blood of pregnant sows, dogs, cats and mares, in extracts of the placentae of rabbits (Hisaw, 1927, 1929) and corpora lutea of the sow (Hisaw, F...