This is anterior and the posterior shoulders, followed by trunk and lower extremities in rapid succession. An external hood-like layer which arches over the fundus and extends into the various ligaments. An internal layer consisting of sphincter-like fibers around the orifices of the tubes and the internal os. Lying between the two, a dense network of muscle fibers perforated in all directions by blood vessels. The main portion of the uterine wall is formed by this middle layer which consists of an interlacing network of muscle fibers between which extend the blood vessels.
Uterine contractions are involuntary and, for the most part, independent of extrauterine control.
It has been demonstrated that the uterus has pacemakers to produce the rhythmic coordinated contractions of labor. These pacemaker sites are found near the uterotubal junctions, although the pacemaker cells do not differ anatomically from the surrounding myocytes as they do in cardiac muscle. The interval between contractions diminishes gradually from approximately ten minutes in early labor to as little as two minutes near the end of labor. The effect of uterine contractions of this frequency and intensity is twofold on the uterine cervix. First effacement consisting of thinning of the cervix with a shortening of the endocervical canal, is produced.