If the breech is in the right sacro-iliac posterior, the mechanism is the same, only rotation occurs from right to left. If the feet appear first, or the knees, the mechanism is still the same, as in the former, breech positions. Sometimes it occurs that rotation is reversed, so as to bring the occiput into the hollow of the sacrum, in which case the face will appear under the arch of the pubis, and if the chin remain flexed upon the chest, all will go on well.
Presentations of the feet, knees or breech, are more tedious and consequently rather harder for the mother than are those of the vertex. The largest part escaping first, the smaller quite easily follows; but he smaller escaping first, there is less remaining in the uterus with which to force out the most difficult part. For instance, if all but the head escapes, it often becomes a difficult matter to expel that, for want of leverage or purchase power in the uterus itself.
The child itself is much safer in vertex presentations. This is true in the first place on account of the cord, which is more likely to become compressed and strangulated in breech presentations, an accident from which the child necessarily perishes. For after the breech and abdomen are born, up to the navel, the cord must be more or less compressed till after the delivery of the head, since the placental extremity of the cord extends of course far above the head, even to the fundus of the uterus. Another danger to which the child is exposed in breech presentations arises in this manner; the smaller part escaping first, the uterus becomes sufficiently emptied to allow the contractions to detach the after-birth before the head is expelled, and the child perishes from asphyxia. Thus it is evident that it is very much safer for the child to have the head expelled first.
Presentations of the Trunk
These are all comprised under presentations of the right lateral plane, or in those of the left lateral plane, back, anterior or posterior. The head is always taken as the point of departure, it being in the left or in the right half of the pubis, constituting the left cephalo-iliac, or the right cephalo-iliac; in each case the back is either anterior or posterior. It is quite common to find the hand, or the hand and arm in the vulva in either of these positions; but this is of no account, as it neither alters nor complicates the manner at all. This presentation occurs rather more frequently than do those of the face; many estimates have been made, but a fair average would be about one in one hundred and fifty cases. A failure on the part of nature to place the child in a more auspicious position, is the only assignable cause for so unfortunate a condition.
The shoulder is usually the first point touched in making a digital examination. The acromion process is distinctly made out, and then the clavicle, the spine of the scapulae and the axillary space, all combined, and always within reach of the finger, confirm the presentation to a certainty. All these being made out, we can tell the position of the child, for the axillary space always looks away from the head, and the clavicle will also tell where the face is, as the scapula will tell where is the dorsal region of the child.
When the elbow alone is accessible to the finger, it can be recognized by three protuberances; the olecranon process and the two condyles; and also by the transverse space in the bend of the elbow and by the vicinity of the chest and the intercostal spaces, for the arm is always found lying upon the chest. The elbow always points away from the head, and the forearm is always on the anterior plane; thus we at once know where the head and face are when we diagnose an elbow.
If the forearm is not doubted up, but lies in the vagina, by turning the palmar surface upwards and in front, the thumb will always indicate which hand it is by its being next to the corresponding thigh of the mother, and then to determine where the head is, it will be necessary to slip the finger up to the axillary space.
When the hand comes out of the vulva, its dorsal surface will always correspond with the direction of the head, and the little finger with the dorsal surface of the child.
The presentation of the trunk at the superior strait is always an indication for manual treatment. Still under certain circumstances a spontaneous delivery may take place, and this is effected either by spontaneous version, or by spontaneous evolution. Of spontaneous version there are two varieties, one cephalic, the other pelvic. In the former, where the shoulder presents, the trunk ascends under the influence of the uterine contractions, and the vertex comes into the superior strait; in the latter, the head ascends into the fundus, under the same influence, and the trunk comes into the superior strait; and in either case of course a spontaneous delivery is effected.
Spontaneous Evolution *Translated from the French of Cazeaux, “The mechanism of spontaneous evolution is much better understood; and in its description we shall find all the divisions of the mechanism of natural labor in the presentations of the vertex and face. M. Velpeau admits a spontaneous cephalic, and a spontaneous pelvic evolution. But since we can conceive of a spontaneous cephalic evolution only in abortions, or in cases where the foetus is completely putrified, we shall speak of the pelvis evolution alone.
Take, for example, the first, or left cephalo-iliac position of the right shoulder. In this variety we find the cephalic extremity is placed in the left iliac fossa; the breech in the right iliac fossa; the dorsal plane of the foetus being in front, the sternal plane behind; so that its long axis is almost exactly in the direction of the transverse diameter.
Immediately after the rupture of the membranes, the waters almost entirely escape; the uterus forcibly contracts and pressing in every direction upon the trunk of the foetus, tends to engage the presenting part in the excavation.
A. Under the the influence of the uterine contractions, the foetus in its long axis is strongly flexed upon the side opposite to that which presents; in the case proposed, the head is turned towards the left side, and the breech towards the hip of the same side. This first change in the situation of the foetus may be designated as the movement of lateral flexion.
B. Then begins a second period, which we may term the period of descent; that is to say, as the contractions are renewed the shoulder tends more and more to approach the inferior strait, and the trunk bent double engages itself deeply in the excavation. But here appears the same difficulty as in presentations of the face, (see position of the face,) that it is impossible for the shoulder, the trunk being thus placed transversely, to reach the inferior strait unless the head at the same time engages with it in the excavation; or unless the neck should be long enough to reach the whole length of the lateral wall of the excavation, which we have already seen to be impossible. The descent of the shoulder is governed, then, by the length of the neck.
C. Then follows a movement of rotation, by means of which the long axis of the child, originally transverse, assumes almost exactly an anterior-posterior direction, so that the head rests above the horizontal ramus of the pubis, near its spine; and the breech above, or rather in front of the sacro-iliac symphysis. The movement of rotation being accomplished, that of descent may now be completed; since the side of the neck is placed behind the symphysis pubis equalling its whole length. Thus the forearm and arm make their appearance at the vulva, the arm and shoulder having passed under the arch of the pubis.
D. Under the powerful efforts of the uterus, the trunk, bent double, is pressed into the excavation; but the shoulder can descend no farther, because it is arrested by the shortness of the neck. The expulsive force acts upon the pelvic extremity, forcing it more and more towards the floor of the pelvis and causing it to traverse the anterior face of the sacrum, till finally it reaches, depresses, and drives the perineum before it. Presently the vulva dilates, and, the acromion remaining fixed under the symphysis, the superior and lateral portion of the chest, the inferior part, the loins of that side, the hip, the thighs, and finally, the whole extent of the lower limbs, successively make their appearance at the posterior commissure of the vulva. And the head and left shoulder only remain in the excavation, and these parts are extracted or expelled without difficulty. This last movement may be considered the fourth stage of the labor and may be named the stage of deflexion or disengagement. This movement has for its centre, the shoulder engaged beneath the symphysis; and if from this centre we extend lines to all the points of the side of the foetus, we shall have here the radii which subtend the anterior-posterior diameter of the inferior strait.
