What is the exciting cause of labour?— This question carries us only one stage further in the preceding course of inquiry : and the reply to it will be nearly found in the facts already stated. For if these serve to throw light upon the causes of the rhythmic and peristaltic movements of the uterus, then the conditions which determine the first rhythm and first peristalsis, or, in other words, the beginning of labour, cannot lie very remote from these.
Many circumstances may evoke the first rhythm, which being followed by others, labour becomes established. Thus, irritation of in cident nerves in various parts and organs may so force those sympathies with the uterus which, for other uses, are established by the spinal system of nerves, as to briar, on an un natural and premature form of labOur ;—but this is not the present question.
The determining causes of natural labour can be only satisfactorily sought among that class of phenomena which causes the separa tion of the ripe fruit from the stem which bears it : in a perfecting, namely, of the fruit or product of conception, so that it becomes fitted for an independent existence, and as a step preparatory to this, in a gradual meta morphosis of those tissues which, having served for a time the purpose of connecting the two together, are now no longer required by either. This connecting medium in the human subject is the decidua, which lines the whole uterus. Its metamorphoses during preg nancy have been described. Already as early as the middle of that period, the preparation has begun for a new tissue, which, after labour, is to reconstruct the lining membrane. The old attenuating and perishing decidua, now no longer needed, except at the spot where it covers the placenta, loses by degrees the cha racter of active vitality, and its tissues are con verted into molecular fat.
Other and corresponding changes, of which an account will be hereafter given (see PLA CENTA), occur in those structures in which the fcetal blood circulates. The profusely de veloped capillaries which ramify within the villi during the early and middle periods of gestation begin to suffer retrogression as the time of separation approaches, and the fcetal blood flows in more sintple and relatively fewer -channels, while, not unfrequently, entire villi become obliterated by calcification.
While these changes are proceeding in the temporary structures that serve to connect the Rents with the uterus, structures which begin in part, at least, to become effete, even before the offices for which they are formed have been fully carried out,—the tissues which are to be employed in the process of expulsion are as yet only ripening into full strength, although they also, in turn, are about to suf fer a like retrogression, but not until the ob ject of their formation has been accomplished. The contractile fibre, which constitutes the principal portion of the uterine tissue, has gradually, during pregnancy, advanced to that more complete, form which is reached com monly about the sixth month.- From this pe riod probably no new development of muscular fibre takes place,although that which is already formed appears to increase somewhat in size and power. It constitutes now a contractile tissue, capable of exerting great expulsive force. How easily, and in how many ways, the contractile power may be evoked, has been already shown. It is probable that by the series of metamorphoses already enumerated as occurring in the parts which connect the fcetus with the uterus, the entire ovum becomes gradually placed in the position of a foreign body within that organ ; a position which may be compared to that of the food within the alimentary canal. And just as the food is propelled onwards, peristaltically, by irritation of successive portions of the containing sur faces, until, with the subsequent cooperation of muscles acting under the dominion of the spinal cord, it becomes finally ejected ; so the ovum is itself apparently the excitor of those first peristalses in the uterus which initiate labour. How these become coordinated and established, and how the rhythmic periods are probably determined, has been already con sidered, as well as the means by which, during the further advances of the child over succes sive portions of the generative track, other nerve and motor forces are added to those with which the process commenced.