PART III PHYSIOLOGY OF PARTURITION CHAPTER I THE CAUSES OF NORMAL LABOUR NORMAL parturition is the expulsion of the foetus at term from the uterus through the maternal passages by natural forces. Two factors, therefore, are chiefly concerned-(1) the expelling powers, and (2) the obstacles which they meet with. The latter is caused by the ovum, the object to be expelled, as well as by the state of the parturient canal which the ovum has to traverse. The normal conditions of the parturient passages and of the ovum, which, as far as they themselves are concerned, ensure a normal labour, have already been spoken of. We have now only to consider the expelling powers, their mode of origin, and the way in which they act. I. CAUSES OF THE SETTING IN OF LABOUR The natural forces which expel the ovum chiefly consist in the contractions of the unstriped muscular fibres of the uterus. It certainly is one of the most interesting subjects of midwifery to study how those contractions are excited at a certain time, and how they are continued, gradually increasing in power until the ovum is expelled. Anatomical preparations and physiological experiments have conclusively shown that the sympathetic is the motor nerve of the uterus. Whether the sacral nerves also have any influence in causing uterine contractions is very doubtful; more probably they are inhibitory nerves. The influence of the sympathetic in the expulsion of the ovum is called forth by the irritation which the ovum exerts upon the terminations of the nerves on the inner surface of the uterus; by reflex action that irritation is transformed into motor activity. Only towards the end of the tenth month this irritation begins to be exerted, because only then, through fatty degeneration of the decidua, the ovum becomes a foreign body in the uterus. The fatty degeneration of the peripheral layers of the ovum causes a separation between the ovum and the uterus, which the resulting contractions complete. The process is essentially the following: Towards the end of pregnancy a gradually increasing fatty degeneration of the decidua takes place. The organic connection which until then existed between ovum and uterus is now solved by the degeneration of the cells between those structures. At all the places where the degeneration has reached a certain stage the terminations of the nerves are irritated. But to obtain a reflex action, namely, the contraction of the uterine muscles, a certain amount of continuous irritation is necessary. This sum once obtained, a reflex action takes place in the form of a contraction, which, however, is slight at the beginning. Then a pause follows, until the sum of the irritation is again sufficient to cause a contraction. By the increase of the intensity of the contractions the uterine wall is removed from the ovum, and this separation becomes a new source of irritation to the uterine nerve-fibres. The reflex action, in the form of labour-pains, becomes more and more powerful, until they follow at last in rapid succession and complete the expulsion of the ovum. The concurrent observations of Brown-Séquard, Obernier, and Kehrer, that the irritability of the uterine nerves progressively increases with the advance of pregnancy, would tend to explain the regular setting in of labour, as well as the not unfrequent retardation of the pains in cases where the separation of the ovum has been premature. Literature.-C. C. Th. Litzmann, Artikel "Schwangerschaft" in R. Wagner's Handwörterb. d. Phys., III, 1, p. 107.-G. Veit, Verh. der Gesellsch. für Geb. in Berlin. 7 Heft, 1853, p. 122.-F. A. Kehrer, Vergl. Phys. d. Geb. d. Mensch. u. d. Säugethiere. Giessen, 1867, p. 8. CHAPTER II THE EXPELLING POWERS THE powers which expel the ovum from the uterus are chiefly vested in the contractions of the unstriped muscular fibres of the uterus. The contractions and the elasticity of the vagina and the abdominal muscles are to be considered only as auxiliary forces. A. CONTRACTIONS OF THE UTERUS With each contraction the uterus alters its shape. Whilst towards the end of pregnancy it has a flattened ovoid form, it becomes more globular during a pain. The longitudinal and transverse diameters are somewhat diminished, whilst the antero-posterior diameter enlarges. By this and the contractions of the muscular fibres of the broad and round ligaments of the uterus the fundus is somewhat tilted forwards, and consequently the circumference of the abdomen at the umbilicus becomes proportionately greater than that between the umbilicus and the symphysis. The size of the entire abdomen becomes greatly diminished in the course of parturition, and this is due partly to the contractions of the uterus and to the escape of the liquor amnii, but chiefly to the descent of the presenting foetal part, so that its measurements become those of the eighth month. The circumference at the umbilicus is on an Average 96 cm., or 39.2 in.; at the middle point, between the umbilicus and symphysis, 96 cm., or 39 in.; and the distance between the xiphoid process and the symphysis 40 cm., or 16 in. These measurements are considerably smaller than in the seventh month. 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. already explained above, a certain sum of irritation is necessary to cause a contraction. At the commencement, corresponding to the slight irritation, the contraction is feeble and short, and the time which is required to obtain the necessary sum of the persistent irritation for a new reflex action as comparatively long. But with the increasing separation of the ovum from the uterine wall the irritation also becomes stronger, and the contractions, therefore, gain in intensity and duration, and follow each other in progressively shorter intervals. At the height of the expulsive stage the irritation is so considerable that the contractions are interrupted only by short pauses. The contractions slowly increase in strength, remain a short time at their acme, and with the exhaustion of the irritation they gradually remit. The sympathetic ganglia being the motor centres of the uterus, the contractions, therefore, are involuntary. They cannot be produced by volition, or slackened or suppressed by it. Yet mental excitement is decidedly capable of altering the force of the contractions, and it remains to be settled whether the sacral nerves are inhibitory in that respect. It is also doubtful as yet whether a normal pain has a peristaltic course, or whether it simultaneously attacks the whole organ. In experiments on bitches and rabbits it can easily be seen that the uterine contractions of those animals are clearly peristaltic, but Obernier remarks that in some cases the contractions are so rapid that the single stages of them can hardly be distinguished. The peristaltic is proved only as regards the uterus bicornis. To judge from analogy, it is probable that in woman also the pain has a peristaltic course (and then it certainly begins in the muscles of the ligaments, and proceeds from the fundus towards the cervix), but from the rapidity with which the wave of the contraction passes over the whole organ it will be practically preferable to consider it simultaneous. To explain the expulsion of the ovum, the assumption of a peristaltic contraction is, on the whole, unnecessary. The muscular fibres of the fundus are more strongly developed than those of the body, and they again more so than those of the cervix. In the cervix, again, there is a spot which is entirely devoid of muscular structure. If, therefore, a simultaneous contraction of the whole organ takes place, the place devoid of muscular fibres must always be more and more enlarged, and this process of enlargement is only very slightly impeded by the circular fibres which surround that opening. The dilatation of the cervix is effected in the following way: The presenting part of the foetus, or the bag of waters, is pushed by the contractions of the uterus against the internal os uteri, so that it is forcibly separated. By its dilatation the cervix becomes a portion of the uterine cavity, and thus the presenting part gradually opens the external os uteri. The dilatation of the internal os uteri takes place exceptionally in primiparæ towards the end of pregnancy, but here also by contractions of the uterus. The more the cervix is dilated the thinner it becomes, until it is finally drawn back over the presenting part. The uterus and vagina then form one common cavity. At that time, usually somewhat earlier, the membranes rupture, and by the partial escape of the waters, and by the consequent diminution of the bulk of the ovum, the separation of the ovuni from the uterus becomes more complete. The now more powerful contractions expel the fœtus, and immediately afterwards the membranes with the placenta. By the detachment of the placenta the large vessels which carry the maternal blood to it are opened up; but they are soon compressed by the firm contraction of the uterus, and the supply of blood to them is kept within narrow limits, so that the formation of thrombi is favoured in the former placental insertion. As the name-pains-already indicates, the contractions of the uterus are attended by pain. It has its principal seat in the loins, and thence radiates into the abdomen and the thighs. It is chiefly caused by the pressure to which the terminations of the nerves contained within the contracting muscular fibres are exposed. The pain is of varying intensity, but most considerable when the fœtal head passes through the very sensitive vulva supplied by cerebrospinal nerves. Moreover, the intensity of the pain varies in different individuals. Sometimes the woman is greatly excited and bathed in perspiration; or vomiting may set in, and at the moment of the birth of the head the woman may become quite unconscious. The influence of a pain upon the whole organism is shown by the increase of pressure in the arterial system. The frequency of the pulse increases from the beginning of a pain to its acme, and slowly diminishes with its cessation. The force which the uterus is capable of exerting also varies in different individuals. From his very exact researches Schatz has found that in the passive organ during pregnancy the pressure which the "tonus" of the uterine and abdominal muscles exerts upon the uterine contents amounts to 5 centimètres of mercury. That pressure remains the same during parturition in the intervals between the pains. The intra-uterine pressure rises in the intervals between the pains, only when the muscles of the contracting uterus begin to thicken after a part of the foetus has been expelled from the uterus. Under normal conditions the increase of pressure scarcely amounts to one half of the uterine pressure which existed at the commencement of parturition. During the pains the uterine pressure rises considerably. In the cases observed by Schatz the pressure of the pains, together with that of the abdominal muscles necessary to terminate delivery, amounted to 80-250 mm. B. THE CONTRACTIONS OF THE VAGINA The comparatively narrow vagina at first opposes by its elasticity the progress of the ovum, but the presenting part of the fœtus soon forcibly distends it. If the greatest circumference of the foetus has passed, the expulsion of the succeeding foetal portions, together with the afterbirth, is considerably facilitated by the elasticity and the contractions of the unstriped muscular fibres of the hypertrophied vaginal walls. C.-ABDOMINAL PRESSURE Ab The abdominal pressure has a considerable influence upon the progress of the child. It can be voluntarily called into play. However, at the acme of the labour-pains the muscular action of the abdominal walls is involuntarily exerted by reflex action. dominal pressure acts in the following way :-The extremities are pressed against some firm support, and thus the trunk is fixed; by deep inspiration the diaphragm is pushed downwards; the abdominal muscles now contract, and the diaphragm, which descends still deeper, partly by its own contraction, but chiefly by the powerful action of the expiratory muscles (whilst the glottis is still |