Volume 1, page 5
Printed for J. Johnson, St Paulís Churchyard, London, 1794
Source: Google scan
In some subjects these bones [of the os coccygis] coalesce and form a single bone; and in others an anchylosis is formed between the sacrum and os coccygis in consequence of which the latter is shortened and turned inwards, so as to obstruct the head of the child in its passage through the pelvis. But the impediment thereby occasioned at the time of labor, may be overcome by the force with which the head of the child is propelled, and the os coccygis again separated from the sacrum with a noise loud enough to be distinctly heard, of which I have known more than one instance.