I really like the language in this one. It seems to flow very well to me. And Jayne-thoughts!

feeling of his own mortality kicking in and ripping out his insides, as it tends to do

I really like that.

He sinks back, feeling his eyes roll back, limbs relaxing until the only point of contact with the outside world is the little pads of contact where the Doc's fingers are touching his head, the only contact that becomes all the contact.

And I like the sort of ephemeral feeling that this gives me; I never thought I'd use that term in relation to Jayne; but it is a Jayne-ephemeral, and it works for me.