232 NORMAL PARTUEITIOX. 



Colin states that he has observed the uterus of Sheep to contract for 

 forty and fifty minutes after death. 



As we have said, the resistance which the expelling forces have to 

 overcome is constituted by the foetus and its membranes, and the 

 genital passages — the os uteri, vagina, vulva, as well as the pelvis and 

 soft parts covering and lining it. The fcetus participates in the resist- 

 ance by its volume, its form, its manner of presentation, its position, 

 the conformation and texture of the tissues which unite it to the 

 mother, etc. ; while the genital passages offer resistance from their 

 form, width, extensibility, and rigidity or softness. The faeces ac- 

 cumulated in the rectum, or urine in the bladder, sometimes increase 

 the resistance to be overcome. 



In order that birth may be possible, the expelling force must be 

 greater than the resistance, and it is upon the relation between these 

 that the manner in which the fcetus is expelled will more particularly 

 depend, as well as the difficulty attending its expulsion and the time 

 required for the act of parturition. 



We have only now to allude to the influence of the expelling force 

 on the foetal membranes. We have seen that these, which we may 

 now, with their contents, designate the " water bag," assist in dilating 

 the OS, and that the uterine contractions propel them further into the 

 vagina in the form of an elongated bladder partly filled with fluid. 

 This soon appears between the labia of the vulva as a round distended 

 tumour at the moment a pain occurs, but flaccid in the interval; and 

 not long afterwards as a somewhat voluminous pediculated tumour, to 

 which each pain adds a little more fluid, until at last the membranes 

 cannot resist the strain, and rupture ; when the allantoic, and then the 

 amniotic fluids escape from them, leaving a variable quantity in the 

 uterus ; some of this is discharged into the vagina at the termination 

 of each pain, and assists in lubrifying the mucous membrane and 

 aiding in the passage of the foetus. 



It may be remarked that numerous causes influence the period when 

 this rupture occurs. In the Mare the membranes are thicker, more 

 resisting, and much less adherent to the uterus than in the Cow ; so 

 that rupture is later in taking place, and it not unfrequently happens 

 that the Foal is born in them ; though the Calf, I believe, never is. 

 But thei'e are individual differences in this respect even, and in some 

 instances it will be found that rupture takes place at the commence- 

 ment of parturition, in others towards the end ; though when this 

 takes place late it is more favourable than when it occurs early, as the 

 amniotic fluid preserves the foetus from undue compression by the 

 uterus, while it powerfully aids in the progressive and regular dilata- 

 tion of the OS and vulva, and lubrifies the passages, thus diminishing 

 friction and protecting the maternal organs fri'in injury. When rupture 

 occurs too eai'ly, and before the foetus has been sufficiently expelled, 

 the parts become dry, and labour is always longer and more painful 

 and difficult for the mother, while it is often fatal to the foetus. 



SECTION III.— SIGNS AND COURSE OF PARTURITION. 



The physiological phenomena just alluded to, and by which the 

 foetus is born, are collectively designated as "labour." The entire 

 period of labour is, for facility of description and study, divided into a 

 certain number of stages or periods — usually three or four. These are : 



