2i6 NORMAL PARTURITION. 



As the uterine contractions which lead to expulsion are usually ac- 

 companied by a painful sensation (duQ to the pressure exercised on the 

 terminations of the nerves within the muscular fibres), they are in com- 

 mon parlance designated "labor pains" {dolores ad partuni)\ while the 

 resistance they have to overcome is centred in the foetus and its envel- 

 opes, as well as in the passages these have to traverse in order to reach 

 the external world. 



Expulsion is not effected by one contraction, but by a series of con- 

 tractions ; between each of these there is an interval of apparent repose, 

 during which the organ seems to be gathering strength for a new effort. 

 As in almost all unstriped muscular fibres, the reflex action following upon 

 an irritation is slow and gradual, and, according to the degree of irritation, 

 of varying intensity and duration. ; At the commencement, corresponding 

 to the slight irritation, the contraction is feeble and short, and the time 

 required to obtain the necessary sum of the persistent irritation for a new 

 reflex action as comparatively long ; through the uterine walls are not 

 relaxed on their contents, but are maintained in a certain state of con- 

 traction by the tonicity of their muscles. 



When the pains are regular, there is also a certain gradation in each 

 individual contraction. Feeble at its commencement, it gradually increases, 

 the uterus becoming harder until the maximum of contraction has been 

 reached ; this persists for some time, and then as gradually subsides. 



As parturition progresses, and the separation between the uterus and 

 foetus increases, the irritation becomes stronger, and the intervals between 

 the contractions shorter, while these latter augment progressively in inten- 

 sity and duration. The necessity for these intermissions, particularly in 

 the early stage of parturition, is obvious. They allow the animal time to 

 recover, to some extent, from the exhaustion they occasion, and permit 

 the genital passages to become gradually prepared for the exit of the 

 foetus through them ; while the latter can also recover from the inconveni- 

 ence it may suffer from the interruption between it and the uterus during 

 the pain, and especially towards the termination of the act. 



During each regular pain the whole of the uterus contracts, though the 

 fundus does so most energetically ; and the longitudinal fibres of the organ 

 are more particularly brought into play at the initial and middle stage of 

 parturition. The cornua likewise contract, are twisted on themselves 

 anteriorly, are shortened through the action of the longitudinal fibres, and 

 are brought nearer the body of the uterus, which is also shortened ; and as 

 this shortening is always taking place in the direction of the cervix, it is 

 here that the sum total of the expelling force is centred ; and it is this 

 force, commencing to operate at the fundus of the organ, and exerted on the 

 incompressible liquor amnii, which gradually opens the os for the extrusion 

 of the fcetus. The latter, with its envelopes, first acts as a stimulus to 

 the uterus, but they soon begin to play quite a mechanical part in the di- 

 lation of the already greatly shortened cervix. The latter becomes thin- 

 ner as the contractions force the bag of waters against it; so that the os 

 is gradually widened, and the cervix 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 



