Panel 1: As he holds an X-ray in his hands, John does that deer-in-a-headlights thousand yard stare thing that's usually Elly's thing as he does something else Elly does: be completely lacking in awareness about his being a pain in the ass complaining about how old he isn't. That's because he delivers her patented speech about how he didn't mind being thirty, he laughed when he turned thirty-five and thirty-nine went by without a hitch but forty appears to be a different thing all together because it's a round number.
Panel 2: Forty, you see, means that he's middle aged. He sets up the sort of taking the piss he subjects Elly to all the damned time by asking Jean to look at him.
Panel 3: She sinks to the challenge by telling him not to be so self-conscious.
Panel 4: After all, his middle hasn't aged any more than the rest of him.
Summary: Once again, we're reminded of two irritating facts. The first fact that I wish wasn't one is that Lynn doesn't like the idea of holding her tongue when she can say something demeaning and insensitive because she sees having to think of other people's feelings as being muzzled. The second thing is that John isn't going to understand this is how Elly feels all the time because that would mean that other people's feelings are somehow real even if he can't feel them himself.