The skin on my arm

I see your writing on this birthday card
I am sad
As I hold  out the card , I see the skin on my arm
Looks decorative  like sand and shingle as the tide flows back
And surges  up again.There are patterns on my skin
I could devote my life to finding the right equation
A wave equation
Or topology? Ribbed rubber  like old sheaths
Stitched together by the wind and sun
I might put paint on them and sell my arm at auction
You’d have to buy me as well
And feed me.I am a food avoidant personality
It’s the latest, it’s  a  new disorder
I live on weetabix and cheese
Chips with salt, no fish
It’s a wonder I have a skin at all
But if I say.no
You have to  believe me