From "engines" 1136

But as Parker danced, the pain was invisible. I've
always loathed one feature of ballet: the classic
dancer's smile with lips slightly parted, no
involvement of the eyes, a coldrictus signifying
complete worldly detachment. But Parker is a
superb actress. On her, that smile had warmth and
expressivity that I hadn't seen on a ballet stage. Nor
was there any sign of the ever- present pain.

5ModL < L, open mouth < pp. of ringi, to gape6
1 a sustained gaping, as of a bird‘s beak or an animal‘s mouth
2 the opening so produced
3 a fixed, gaping grin

In terminal stages, patients dying of tetanus before antitoxin became available had an abnormal
facial expression, a travesty of a smile, called "rictus".