goes on. The flexion of the spine is now reversed. Above the brim the trunk was concave on its left side, as seen in Figs. 38 and 39. When the breech has dipped into the pelvis, the trunk becomes concave on its right side. The breech descends first. The right ischium presents at the vulva. Then the whole breech sweeps the sacral concavity and perinæum. The trunk follows. The right arm, which has not always completely risen out of the way, comes next; then the left arm; and lastly the head, taking its rotation-movement, and its movement in a circle.

opposes delivery The pelvic canal pass freely when

The cause of the difficulty that in shoulder-presentation is obvious. is too narrow to permit the child to its long axis lies across the entry. On looking at the diagram (Fig. 40), we see the shoulder driven into the pelvis, forming the apex a of a triangle whose base B c is considerably longer than E F, the trans

verse diameter of the pelvic brim. To overcome this difficulty, Nature struggles to shorten the base в C. To a certain extent she generally succeeds, and occasionally she succeeds completely.

The uterus contracts concentrically, tending to shorten all its diameters, especially its transverse diameter. The axis formed by the trunk and head of the child, which go to make up the resisting base of the triangle, is flexible; therefore в and c admit of being brought nearer to each other. But, when the utmost approximation has been obtained in this manner, we still have the entire thickness of the head, equal to four inches, and only very slightly compressible, plus the thickness of the body, which, after all possible gain by compression is effected, is equal to at least two inches more, being an inch or more in excess of the available space in the pelvic brim. As a general rule, it may be stated that no part of the child, except a leg or an arm, can traverse the pelvis along with the head, the head alone being quite large enough to fill the pelvis.

One result of the great compression exerted by the concentric contraction of the uterus is to cause such pressure upon the chest and abdomen of the child, and so to compress the placenta and cord, that the child is asphyxiated and killed. The death of the child, leading to the loss of resiliency, will, after sufficient time, admit of a much further degree of compression, and then, possibly, the child may be so doubled up and moulded that it may enter the pelvis.

The condition, therefore, of spontaneous evolution is the death of the child. If not already dead at the com

mencement, the child will almost certainly, if of medium size or larger, be killed in the course of the process. Herein lies a great distinction between spontaneous evolution and spontaneous version. A living child is favourable to version, a dead one to evolution.

SHOWS THE POSITION OF THE CHILD AFTER THE ESCAPE OF LIQUOR AMNII.

The head is strongly flexed upon the trunk, forming together the base of a wedge too large to enter the brim. The line E F represents the line of decapitation, by which proceeding the base of the opposing wedge is decomposed. The head thus being put aside, the axis of the trunk will easily be brought into coincidence with the axis of the brim, permitting delivery.

Spontaneous evolution from the first position proceeds as follows:-At first we have the oblique position of fœtus and uterus represented in Figs. 37 and 38. Secondly, strong flexion of the head upon the trunk, and descent of the shoulder into the pelvis (see Figs. 39 and 40); the head is in one iliac fossa, the trunk and breech in the other. At this stage, commonly, the membranes burst, and the arm falls

L

into the vagina, the hand appearing externally. Thirdly, increased descent of the shoulder and protrusion of the forearm, doubling with compression of the body, so that the breech is driven into the pelvis ; as soon as this takes place a movement of rotation succeeds (see Fig. 42). The inclined planes of the ischia direct the breech backwards into the sacral hollow; this backward movement of the trunk throws the head forwards over the symphysis pubis; from

FIG. 42.

RIGHT SHOULDER; FIRST POSITION AFTER ROTATION.

transverse, as the child was above the brim, it now approaches the fore-and-aft direction; the right side of the head, near its base, is forcibly jammed against the symphysis; the side of the neck corresponding to the presenting shoulder is fixed behind the symphysis pubis, and the shoulder itself is situated under the pubic arch. Fourthly, the expulsive force continuing, can only act upon the breech and trunk, the shoulder being absolutely fixed; the trunk bends more and

more upon its side, the presenting chest-wall bulges out, and makes its appearance under the pubic arch. Then, lastly, the movement in a circle of the body round the fixed shoulder is executed. The side of the trunk and of the breech sweep the perinæum and concavity of the sacrum; the legs follow. When the whole trunk is born, the movement of restitution is effected, the back turning forwards, the belly backwards. The head escapes from its forced position

FIG. 43.

RIGHT SHOULDER, FIRST POSITION; DURING MOVEMENT IN CIRCLE AROUND SYMPHYSIS.

above the symphysis, the chin turns downwards, the occiput looks upwards to the fundus uteri, the nucha is turned to the right foramen ovale. It enters in the left oblique diameter, it takes the rotation movement in the pelvis, the occiput coming under the pubic