Fifth edition, vol 1. 1766
The coccyx is moveable at its connection with the sacrum, as are also the four bones that compose it, in their articulations with one another; and this motion continues in adults as well as in those of more tender years. In old age indeed, and even in young people who have suffered bruises upon the part, attended with great pain and inflammation, we frequently find the different pieces of this bone rigidly cemented together. But this anchylosis the more seldom happens, because they undergo a gentle motion at every excretion of the faeces, which helps to preserve their mobility. See Vol 2 Collect. No. 2.