570 THE PHYSIOLOGY OF REPRODUCTION 



discharged along with the placenta, while others are born 

 separately. 1 



Hartmann 2 has described the phenomena attending parturition 

 in the opossum (Didelphys virginiana). The very immature young 

 make their way unaided to the mother's pouch, and in so doing 

 crawl three inches from the vagina over an entanglement of hair ; 

 amid a forest of hair the "ten-day-old foetus" finds the nipple. 

 Hartmann mentions that he found thirteen teats with eleven young 

 attached. 



In the actual process of being born the foetuses of the opossum 

 do not pass out through the lateral canals of the vagina, but break 

 through a cleft-like rupture, the "pseudo-vaginal canal," into the 

 median canal. This occurs also in Dasyurus and Perameles. 3 



The young kangaroo, like the opossum, reaches the pouch 

 unaided. 



THE NERVOUS MECHANISM OF PARTURITION 



Parturition may be considered as being normally a reflex act, the 

 centre of which is situated in the lumbar region of the spinal ' cord. 

 On the other hand, it has been shown from experiments upon animals . 

 that the transmission of impulses through the cord is not absolutely 

 essential to the occurrence of parturition. 



Simpson (Sir James) 4 removed the spinal cord from the first 

 dorsal vertebra downwards from a number of sows a few days before 

 parturition was due. Some* of the animals died as a result of the 

 operation, but in others parturition proceeded normally, excepting 

 that in each case the last foetus of the series was not born. " The 

 uterine contractions proceeding from fundus to cervix were sufficient 

 to expel the foetuses from the uterus ; and each foetus as it came into 

 the vagina was thence extruded by the force transmitted from the 

 foetus behind it; but when the last foetus came into the vagina it 

 remained there, because there was nothing to transmit the uterine 

 expulsive force, while the vaginal and abdominal muscles, being under 

 the influence of the spinal nerves, had been rendered powerless by 

 the removal of the spinal cord." 



1 Brumpt, " Parturition chez le Rat blanc," Bull. Soc. Zool. France, vol. 

 xxxii., 1907. The loosening of the placenta and other changes in Tupaia are 

 described by van Herwerden, " Die puerperalen Vorgange in der Mucosa uteri 

 von Tupaia javanica" Anat. Hefte, vol. xxxii., 1907. 



2 Hartmann, "Studies in the Development of the Opossum, and the 

 Phenomena of Parturition," Anat. Record, vol. xix., 1920. 



3 Hill, " On the Fretal Membranes, Placentation, and Parturition of the Native 

 Cat," Anat. Anz., vol. xviii., 1900. See also Proc. Linn. Soc. of N.S. Wales, 

 vols. xxiv. and xxv., 1899 and 1900. The lateral vaginal canals are described 

 as enlarging in the prooestrum, containing a lymph-like fluid during oastrus 

 when they are enormous and act as seminal reservoirs, since several days 

 elapse between copulation and ovulation and subsequently becoming reduced. 



4 Simpson, Selected Obstetric Works, edited by W. H. Black, Edinburgh, 1871. 



