After all that on the brachial plexus here is something completely different...
I happened to have the opportunity to see an MRI scan result the other day and was intrigued to learn a new word.
The scan examined the lumbar discs and nerves and in the description mentioned the thecal sac.
I had come across thecal cells in the ovary; they are the outer group of cells that surround a maturing follicle before the oocyte or egg is released at ovulation (though I do confess at that moment that their exact 'outer' position had escaped me!).
But I was curious and eventually my colleague found it in an appropriate medical reference book.
Apparently in general it means 'sheath', hence its use in the outer cells of the developing follicle, but in the lumbar spine example it is used to refer to the dural membrane. Obviously also a sheath. It is also used in description of the pupa of different insects...
See... something new every day...!