230 NORMAL PARTmiTIOX. 



contractions force the bag of waters against it ; so that the os is gradu- 

 ally widened, and the cervix really becomes a part of the uterine cavity. 

 As soon as the os is slightly opened, the bag of waters enters it and acts 

 as a mechanical dilator ; then the lower parts of the fore limbs, 

 succeeded by the head of the foetus, are introduced, and from their 

 combined shape act like a wedge, until, by the eccentric pressure, the 

 chest is passed in, and the cervix being drawn over the presenting parts, 

 the OS is of the same diameter as the vagina, which then, with the 

 uterus, constitutes one common cavity. Every part of the cervix being 

 acted on by the longitudinal fibres, the aperture of the os is perfectly 

 circular at this stage — as is observed in the Cow and Goat wheh the 

 uterus is pushed back, and its orifice is visible at the vulva. Irregu- 

 larity in the contractions, however, and particularly when they are 

 rendered so from a transverse position of the fcetus, delays the dilata- 

 tion, which is otherwise rapid according to their force and frequency. 

 At first the dilatation occurs very slowly, especially in primiparai ; and 

 when the foetus presents by the croup, or when the body of the uterus 

 inclines too much downwards, causing the cervix to bend up towards 

 the sacrum, it is also very tardy. As soon, however, as the mechanical 

 action of the water-bag and foetus comes into operation, it makes rapid 

 progress in natural parturition. 



If the uterus of animals usually uniparous contain two foetuses, the 

 two cornua are about the same size, each having a foetus in the same 

 position as if there were only one in the uterine cavity. In general, the 

 two foetuses present anteriorly, although it sometimes happens that 

 the second, or the first, or even both, present posteriorly ; not infre- 

 quently the second makes a mal-presentation. When there are twins, 

 parturition is more difficult and slower than when there is only one : 

 possibly because the great distention of the uterus diminishes its con- 

 tractile power. Another feature in twin pregnancies is that parturition 

 often occurs before the ordinary time ; and even when this has been 

 reached, one or both foetuses are smaller and weaker than when there is 

 only a single foetus. 



When twin parturition sets in, the uterine contractions commence 

 almost simultaneously in both cornua, which are much less apart than 

 in the non-pregnant state ; but as the two foetuses cannot be born 

 together, that which is most advanced is delivered first, the other, which 

 is behind it, mechanically aiding in its expulsion. In the Mare, the 

 interval between the birth of twins is rarely more than ten minutes ; 

 with the Cow it may be one or two hours ; and with the Ewe half an 

 hour. When the position of the second foetus is favourable, it is usually 

 expelled more rapidly and easily than the first ; and when they are of 

 a different size, the largest is ordinarily born before the other. When 

 the number of foetuses is greater, they are also expelled successively at 

 intervals of Pome hours. 



In cases of superfoetation, if such an occurrence can take place in 

 animals other than in those the cornua of whose uterus open into the 

 vagina, the uterine contractions must be limited to the cornu contain- 

 ing the foetus whose period for birth has arrived ; otherwise, the other 

 foetus would be expelled at the same time, and there would then be a 

 birth and an abortion. 



With the small multiparous animals, in which the foetuses are 

 expelled one after the other, it may be admitted that each fraction of 

 the uterus corresponding to a foetus contracts in its turn — at first the 



