This is why I'm gonna need to meet an educated female

I think you two might be talking past each other - the three sections of the intestine ghost-face-nilla described are more macro anatomical divisions of the gastrointestinal tract. Like so: (<- towards mouth) =====|======|===== (towards anus->) duodenum | jejunum | ileum

Whereas the mucosa/muscularis mucosae/submucosa division you mention is a more micro-scale division of the tissues which make up all of the GIT. That is, mucosa etc. make up all of the duodenum, jejunum and ileum, but there are histological differences between the tissues in each division which are significant enough to be able to divide the gastrointestinal tract into such distinct sections and treat them as 'separate things' in histology and medicine. Which I think is what ghost_face_nilla meant by "Yes they are all small intestine, but they are treated like separate tissue in research/medicine/etc.".

(Sorry if that cleared nothing up or was unnecessary, I'm just a baby med student and it's late here. :) )

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