PHYSIOLOGY OF PARTURITION. 



219 



and softness. The faeces accumulated in the rectum, or urine in the blad-'' 

 der, 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 foetus 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 fcetal 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 farther 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 tumor, at 

 the moment a pain occurs, but flaccid in the interval ; and not long after- 

 wards as a somewhat voluminous pediculated tumor, to which each 

 pain adds a little more fluid, until at last the membranes cannot resist 

 ihe 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 

 fcEtus. 



It may be remarked that numerous causes influence the period when 

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

 sisting, 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 there are 

 individual differences in this respect even, and in some instances it will 

 be found that rupture takes place at the commencement of parturition, in 

 others towards the end ; though when this takes place late it is more 

 favorable 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 dilatation of the os and vulva, and lubrifies 

 the passages, thus diminishing friction and protecting the maternal organs 

 from injury. When rupture occurs too early, and before the foetus has been 

 sufficiently expelled, the parts become dry, and labor is always longer, 

 and more painful and difficult for the mother, while it is often fatal to the 

 foetus. 



SECTION III. SYMPTOMS AND COURSE OF PARTURITION. 



The physiological phenomena just alluded to, and by which the foetus 

 is born, are collectively designated as " labor." The entire period of 

 labor is, for facility of description and study, divided into a certain num- 

 ber of stages or periods — usually three or four. These are: i. Prelimi- 

 nary stage ; 2 . Dilatatio?i of the os uteri ; 3 . Expiilsioti of the fcstus ; 4. 

 Expulsion of the membranes. 



I. Preliminary Stage. — Various precursory signs announce the ap- 

 proaching termination of pregnancy and the advent of labor. These may 

 be observed some hours, sometimes even for days, before that event 

 occurs. 



One of the most important signs is the enlargement and increased sen- 

 sibility of the mammae, to which the excess of blood no longer required 

 in the uterus is directed. These glands become voluminous, hard and 



