The contraction having overspread the uterus, a sense of pain is now first felt ; the pain, like that of cramp, being usually propor tionate to the sensible tension and hardness of the organ.
After enduring for a time the state of con traction gradually' subsides, and is replaced by one of relaxation. In subsiding, the con traction observes the same order as iu com mencing, the os and cervix yielding first, while the upper portion and fundus remain longest tense and hard. From this it results that the antagonistic force, exerted by the two ex tremities of the organ, not being throughout contemporaneously and equally employed, the excess of the fundal orer the ostial contrac tion will represent the measure of the unop posed, and consequently efficient, propelling power.
The period of action is followed by one of repose, in which the organ remains relaxed, and no pain is experienced.
After an interval of variable duration con traction returns, and continues to recur in rythmical order, but with a gradually diminish ing interval, while at the same time the con tractions, especially, at the fundus, increase in intensity and duration.
As a result of these successive contractions, the os and cervix slowly yield, and a portion of the fcetal membranes, containing some liquor ainnii, protrudes, in tbe form of a pouch. This, as the os uteri becomes still further opened, is followed by the head or some other portion of the child, which, having entered the vagina, ultimately fills up the pelvis, and distends the perineum.
At this period the abdominal and pelvic muscles are brought powerfully into play. Their cooperative action is occasioned by the parts of the child occupying the pelvis irri tating structures which are abundantly sup plied by spinal nerves. And now the chief use of spinal reflex action, in relation to labour, becomes manifest, not so much in regard to the uterus itself, whose contractions are probably still mainly dependent upon its own sympathetic nerves, as in that correlation with other parts, between which and the uterus it is essential that consentaneous action should be occasionally established.
The powerful cooperation of the abdomi nal muscles, which form as it were an addi tional sheet of contractile fibre, nearly sur rounding the uterus, being thus enlisted, the passage of the child is completed with greater rapidity and certainty ; and, after a pause, the placenta and membranes are expelled, the liquor ainnii having, either altogether or in part, escaped at some earlier period of the labour.
This general sketch of the operations of the uterus in labour will suffice as an intro duction to a more detailed and critical exami nation of the nature of the forces employed, and of the manner in which these are called forth.
Of the peristaltic action of the uterus, and its cause. — From direct observation upon many mammalia, it is known that the action of the uterus is in them peristaltic, i.e., the contrac tions commence at certain points, and pass on from segment to segment slowly, and in a ver micular manner. If a single point of an organ so composed is irritated, the action starts from the point of irritation, and spreads outsi ardly, and by irritating different points, other peri staltic centres may be obtained.
Although the human uterus does not admit of the same direct methods of observation which can be employed in animals, yet from all that is known, we may conclude that its mode of contraction does not differ in any important particular from that of other simi larly constructed hollow muscles, when en gaged in propelling or expelling their contents.
The principal circumstances bearing upon this point in regard to the human uterus are the gradual and slow contraction, followed by an equally slow return to a state of relaxation —phenomena easily observed, when the hand is placed upon the abdomen of a woman in labour—a certain tremulous motion of the os uteri, when contraction is commencing, fol lowed by a sensible gradual hardening of the uterus, before the woman is herself conscious of pain ; the longer abiding of the contrac tion at the fundus than at the cervix ; and the occasional segmental contraction of the organ after labour, commonly termed hour glass contractions, which may occur at any point intermediate between the fundus and cervix, and which resembles similar contrac tions of common occurrence in other hollow muscles, whose action is peristaltic. These several circumstances, added to the general analogies, suffice to show that the action of the human uterus is peristaltic.