GESTATION AND LACTATION 75 



69. Parturition. In mammalian animals at the end 

 of the period of gestation and in the normal course of 

 events, the fully developed foetus is expelled from the 

 uterus. This phenomenon is known as parturition. The 

 beginnings of parturition are accompanied by a series 

 of rhythmic contractions (labor pains) of the uterus which 

 eventually result in the birth of the offspring. These 

 contractions are at first partially controlled by the will, 

 but later are entirely involuntary. That the muscular 

 movements of the uterus are not controlled entirely by 

 the central nervous system is shown by the researches 

 of Kehrer, 1 Helme and others. These investigators found 

 that a healthy uterus may rhythmically contract when 

 separated from the body if it is maintained at body tem- 

 perature without important variations. The powerful 

 muscular contractions of the uterus of mammals are 

 characterized by the mechanical stretching of the bag of 

 membranes by severe contraction of the longitudinal 

 muscle fibers and the relaxation of the circular fibers of 

 the cervix. The contraction of the uterus and the relaxa- 

 tion of the cervix causes the bag of membranes to act as 

 a fluid wedge still further extending the neck of the 

 womb. These phenomena are followed by the head and 

 fore legs of the young animal, the rhythmic contractions 

 become more frequent and more powerfully exerted, and 

 these are supplemented by the abdominal muscles in 

 the final stages of parturition. The immediate inciting 

 causes of parturition are not well known. Various 

 explanations have been attempted. Spiegelberg 2 has 

 suggested that the foetus secretes a substance which 



1 Marshall, " The Physiology of Reproduction," p. 527. 



2 Spiegelberg, ''Die Dauer der Geburt," Lehrbuch der Geburts- 

 hiilfe, vol. II, 1891. 



