THE MECHANISM OF LABOR ACCORDING TO THE POSITIONS.
We have said that the mechanism is the same, whatever the variety of presentation, complete or incomplete. Let us take as a type the position S. I. L. A.
Position S. L L.A.
Diagnosis.-1. Before labor.
Palpation.—The pelvic extremity occupies the left iliac foam, and the left side of the superior strait, with or without the small foetal parts; the back is in front and to the left, the head in the right flank, sometimes easily accessible, often hidden by the liver.
Auscultation. —Heart-beats in front and to the left, above the line which divides the uterus into two equal parts.
The Touch.—Foetal part inaccessible, or barely perceptible.
2. During Labor, before the Membranes hare Ruptured.—The same signs on palpation and auscultation, bag of waters sausage-shaped, hardly anything accessible to the finger.
After the Membranes have Ruptured.—The finger reaches the left but tock, then the intergluteal fossil, the anus, and the coccyx, the point of which is directed forward, and to the right. The crest of the sacrum can be felt in front and to the left.
We will now recapitulate the six periods which occur in all deliveries. First Period.—Reduction in Size.—The foetus is loosely bent on its an terior plane, the head semi-flexed, the arms and forearms crossed over the chest, the inferior extremities flexed on the anterior surface of the foetus, its back directed forward and to the left, the left side forward and to the right, the right side backward and to the left, the anterior surface backward and to the right, the intergluteal fossa in the direction of the left oblique diameter, the bis-iliac diameter in the right oblique diameter, the coccyx points backward and to the right, the crest of the sacrum is in front and to the left.
The first period consists in a closer approximation of all the foetal parts, which exaggerates the flexion of all of these, and brings together the different elements constituting the pelvic extremity; the back of the foetus curves anteriorly, the head approaches the thorax—in one word, the fcetus tends to occupy a smaller volume, it is reduced in size.
Second Period.—Engagentent.--Thus reduced, the foetus is engaged in the excavation, but its descent is always gradual. The membranes, in fact, rupture before the dilatation of the cervix is complete, and as this part of the foetus is less resistant than the cephalic extremity, the influ ence it exerts on the cervix is less pronounced. The passive dilatation of the cervix being slower, the descent of the foetal part to the pelvic floor is retarded. While the breech passes through the cervix, we notice the
evacuation of meconium, increasing in amount in proportion as the foetal abdomen is compressed; but in this instance it is of no importance, being due to merely mechanical causes.
Third Period.—Internal Rotation of the Trunk.—The rotation, which is due to the same causes as in vertex presentations, brings the anterior hip (in this case the left) under the symphysis pubis, while the right hip points to the other extremity of the coccygo-pubic diameter. The bis-iliac diameter, therefore, corresponds with the coccygo-pubic diameter, the back is turned full to the left, the anterior plane to the right.
Fourth Period.—Disengagement of the Trunk. —The left buttock being fixed under the symphysis pubis, the right buttock and hip traverse the whole posterior portion of the excavation, and since the right buttock appears first, but remains immovable under the symphysis, we see pass over the perineum, first the right buttock, then the left buttock, next the pelvic extremities, and, if the contractions are vigorous, the arms re maining applied to the chest, the latter descends in turn, the left shoulder appearing first under the symphysis; the right shoulder and right elbow passing first over the perineal commissure.
Fifth Period.—External Rotation of the Trunk, Internal Rotation of the Head.—Our object in uniting into one step the external rotation of the trunk, and the internal rotation of the head, is to show more clearly the unity of the mechanism in labor; for generally the external rotation of the trunk is completed before the internal rotation of the head com mences. The external rotation of the trunk is usually effected together with the disengagement. In fact, in proportion as the hips become dis engaged, they revolve, the left hip approaching the right side, the right hip returning toward the left anterior portion of the pelvis, so that the back, when delivered, is very nearly in the position it occupied within the pelvis; in other words, it faces toward the left thigh of the mother. Ordinarily, the shoulders follow this movement, but in certain cases they partially escape this evolution, so that, while the trunk seems to revolve on its axis, the shoulders are disengaged as we have seen above, and then the head commences its movement of internal rotation. If the uterine contractions are vigorous, the head is flexed more and more, and at the same time the occiput turns and comes to lie behind the symphysis pubis.