In passing the pelvis, the fœtus not only follows the curved line of the pelvic axis, but it describes a certain series of movements which alter its relations to the pelvis. [Illustration: Fig. 44.—The child in left-occipito-anterior position. (Lenoir and Tarnier.)] There are five of these movements: _flexion, descent, internal anterior rotation, extension_, and _external restitution_. =Flexion.=—Flexion is usually present before labor begins. That is, the head is bent down until the chin touches the breast. This may be modified by various conditions, but so far as it becomes extended, the mechanism is disturbed and the labor complicated, since large and less favorable diameters are brought to delivery. [Illustration: Fig. 45.—The child in right-occipito-anterior position. Shows the flexion of the head intensified at the beginning of labor. (Eden.)] Flexion is increased by pressure against the pelvic brim as labor begins. =Descent.=—As the driving force of the contractions becomes effective, the head passes the inlet and descends to the pelvic floor. When the large diameters of the head (biparietal) have passed the inlet, the head is said to be engaged. [Illustration: Fig. 46 _A_.—The descent of the head in right-occipito-anterior position. Seen from below. (Edgar.) Fig. 46 _B_.—Side view.] [Illustration: Fig. 47.—Internal anterior rotation and extension of the head in a left-occipito-anterior position. (American Text Book.)] =Internal Rotation.=—The head most frequently enters the brim with the occiput to the left and anterior (obliquely) because it finds more room and an easier passage; but upon passing this strait and entering the roomy, true pelvis, the head must rotate so that the long diameter of the head will conform to the long diameter of the pelvic outlet, which lies in a direction just opposite to the long diameter of the inlet or brim; hence, the occiput turns forward under the pubic arch. This movement is due largely to the sloping pelvic floor and the necessity of accommodation between the head and pelvis as the child is driven forward. [Illustration: Fig. 48.—Extension. _A_, the chin leaves the chest; _B_, extension in progress. (Eden.)] [Illustration: Fig. 49.—_A_, extension completed; _B_, expulsion. (Eden.)] Rotation is much retarded or entirely stopped when the head is extended instead of flexed or when it enters the inlet with the occiput posterior instead of anterior. =Extension.=—After internal, anterior rotation, the head emerges at the vulva, the occiput coming out first, then in succession the vertex, forehead and face and chin. As the chin rolls out over the perineum, it moves away from the chest wall—it becomes extended.