CHAPTER VIII THE MECHANISM OF NORMAL LABOR
The powers of labor are primarily the uterine contractions strongly aided by the muscles of the abdomen and diaphragm. Some assistance is given by the fixation of the legs and arms and sometimes by gravity, when a sitting or standing position is maintained.
The resistances are the bony pelvis and its relatively soft coverings of muscle and fascia.
The problem is to get the awkwardly shaped passenger through the curiously shaped passage.
In the first, and a part of the second stage, the uterine contractions do not act directly upon the body of the child, for the latter is surrounded by a wall of liquor amnii.
Pressure is transmitted by a fluid medium in all directions, hence, the weak part of the wall, which is the cervix, must give way. While the membranes remain intact, or when sufficient fluid is retained, no amount of pressure can injure the fœtus. When the membranes rupture, the force of the pains is exerted directly upon the child to drive it forward, and prolonged pressure may produce injurious effects through compression of fœtus, placenta, or cord.
The progress of labor is registered usually by watching the advance of the fœtal head.
The relation of the head to the pelvic brim is of great importance, as it travels much faster and easier in certain positions than in others. The term “presentation” is used to designate that part of the child which enters or tends to enter the pelvic inlet.
The presentation is named from the part of the child which comes into apposition with the brim. Thus, one speaks of a vertex presentation, or a breech presentation, or a shoulder presentation. The presentation is determined externally by palpation.
The vertex presents in 96 per cent of all labors. With the vertex presenting, the head may occupy any one of four positions. The term “position” is used to explain the relation which the most distinctive feature of the presenting part bears to the quadrants of the pelvic inlet. Thus, the most distinctive feature or landmark of the vertex is the occiput, which is the point of direction, and so again, the position is the relation of the point of direction to the brim of the pelvis. The _point of direction_ is the part that takes precedence in the process of delivery. Thus, in all cases where the occiput is in advance, the occiput is the point of direction and the position is called occipital. Where the chin is in advance, it is mental (_mentum_ is Latin for chin.) In breech cases, the sacrum is the point of direction.