(Adapted from my answer to a specific example of a similar question)
New branches and sub-branches of philosophy aren't 'created' by a board of academic philosophers, and only after the Philosophers Conspiracy votes on it can anyone write papers about the topic and get published.
The order of events is the opposite: people were asking questions, and eventually enough material accumulated on a particular subject that it warranted getting it's own tag, for practical reasons. I philosophy's younger days, there was much less to talk about, so there wasn't a need to split it up into so many different categories: members of Plato's Academy spoke about whatever they felt like talking about, and didn't bother with the labels. As more people contributed their thoughts and questions, philosophical topics grew to the point where it became more convenient to split up categories. For example, there was a time (in theory at least) that if you took a course on ethics, you could comprehensively all the important ideas that were being discussed at the time, until lots of people had many more ideas, and applied it to more categories, etc. so that today we have metaethics, normative ethics, etc. and branches of applied ethics, one of which is bioethics, and today there's even enough material on neuroethics. Neurophilosophy is a branch of philosophy because what we've learned about the nervous system is very relevant to certain philosophical questions, making it a subject that has given philosophers much to talk about.
Therefore, the reason why there isn't anything like 'gastrointestiphilosophy' (philosophy of the gastrointestinal system) as opposed to neuroethics or the like, is merely because philosophers haven't found too many interesting things to say about the gastrointestinal system. But by all means, if you have philosophical questions about the lymphatic system, or anything else at all, go ahead and ask them - talk to philosophers, get a discussion going, and maybe the day will come where I can take a class on gastrointestiphilosophy.