MECHANISM OF LABOUR. 61 



plane, so as to fall into the hollow of the sacrum. In executing this 

 rotation, or pivot motion, the head turns upon its long diameter. 



The third stage is extension, which signifies, the departure of the 

 chin from the breast. 



This commences as soon as the occiput begins to emerge, and is 

 completed when the child's head is entirely born. Instead of con- 

 tinuing to bend forwards, the head now tends to turn over back- 

 wards, so as to bring the occipito-mental diameter into correspon- 

 dence with the axis of the inferior strait. 



After the head is fairly engaged in the inferior strait, it is no 

 longer in the same relation to the diameter as before ; its great 

 diameter and occipito-bregmatic circumference, however, have not 

 undergone any change, for we find, that at the inferior strait, as 

 well as at the superior, they still represent the plane and axis of that 

 strait. The bi-parietal diameter corresponds to the transverse of the 

 inferior strait, and the occipito-bregmatic to the antero-posterior. 

 The great end of rotation is to produce this relation between the 

 diameters, to effect the exit of the child's head. 



The last stage is restitution, by which is meant, the turning of the 

 vertex again to that side of the pelvis, towards which it was directed 

 before the labour began, or before rotation took place. As soon as 

 the head is born, all restraint is taken away from it, and as it is 

 incapable of retaining the twist which brought it under the arch of 

 the pubis, it returns to its natural position upon the shoulders, which 

 had been temporarily changed. In the position now under considera- 

 tion, the vertex turns towards the left thigh of the mother, and the 

 chin to the right. A short interval of ease succeeds the birth of the 

 head, af^er which the shoulders enter the excavation. They also 

 enter obliquely, and perform the rotation as above described. The 

 right shoulder is behind the right acetabulum, and rotates from right 

 to left on the right anterior incUned plane towards the 'symphysis 

 pubis ; the left is at the left sacro-iliac junction, and rotates from left 

 to right on the left posterior inclined plane into the hollow of the 

 sacrum. The right shoulder generally appears first at the pubic 

 arch, whilst the left escapes over the pei'ineum ; the vertical axis of 

 the child's body is bent, so as to accommodate itself to the axis of 

 the pelvis, which, it will be remembered, is a curved line. After the 

 shoulders emerge, the rest of the body is expelled without anything 

 peculiar to demand description. 



In the second position of the vertex, the mechanism differs but 

 little from that of the first ; the same forces are brought to bear 

 upon the child, and the same diameters offer themselves to the prin- 

 cipal diameters of the pelvis ; the head also executes the same move- 

 ments as in the preceding. The vertex, however, in this case, is be- 

 hind the right acetabulum, and rotates from right to left on the right 

 anterior inclined plane, and the forehead on the left posterior inclined 



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