I have read this thread with interest, and decided to do some
investigation. I am an herbalist and a
 midwife, so I have alot of references around the house.  Unfortunately
references for cabbage(brassica oleracea) are hard to find .  One page in
Penelope Ody's book and I could only find one chemical reference in the
Merck Index. In P. Ody's book she comments that "it heals tissues by
encouraging cells to proliferate ". In the Merck Index it lists the
chemical   Brasine: a water soluable substance found in the green plant
material of fresh  cabbage juice; it promotes the healing of peptic ulcers.

            Although cabbage is a Cruciferae and is related to mustards(
brassica nigra or brassica alba) I think we all know from taste  there are
some big differences.  Mustard seeds and leaves are used as
counter-irritants, they are so strong that  they can produce blisters.
Other plants used as counter-irritants are chili peppers (capsicum annum)
and wintergreen (gaultheria procumbens). As most of you have probably
experienced these are not mild on the tounge, they act on your mouth
simularly to the way they act on the rest of your skin. Now granted I have
occasionaly taken a slightly spicey bite of cabbage, it is very
infrequently, and never on the order of these other plants. If its action
is do to counter-irritation it would be extremely mild, I think there is
probably more to cabbage  than this.
             When I have some free time I'll go to the University library
and see if I can find any further  references that would help explain the
action of cabbage.  Sharon Rust