The central nervous system

Perseveration of profession

Jacks

Indifferent illness

Descriptive nomenclature

Acceptability

I used to call this category of cartoons "B-sides," in reference to some kind of archaic audio storage medium. Basically, if it was jotted down while I was searching for other, better ideas, but then later conscripted into service when I didn't have time/energy/etc. to develop something better. So. There you go.

Idiomatic invitation