Thanks for that Jack, I was wondering where the cat was hiding ! (At least you knew he wasn't human !).
I presume the T should actually be S (for 31 Down) ?
No apology needed Apache - I think it's great that we can discuss these things openly here - after all these are only our opinions. Well done on your completion, I guess you were adding a K to the end, so no harm done !
The real problem with 19 down is that it doesn't mention the organ concerned - the insertion of cerebral as the penultimate word would have greatly strengthened the definition and made the clue fairer, without jeopardising the surface reading, making the clue too easy to solve or adding an extra line in the newspaper.
Of course the definition can point to several possible answers, and this is honed down to one by the word length, available letters and the wordplay. My criticism of 8 down is that there are literally hundreds of membranes to choose from in nature, and thousands of other 'barriers', so this particular definition is far too loose. Qualifying this barrier as organic may have excluded thousands of others, but it would be misleading to suggest that this particular barrier has to be organic. At the end of the day the clue should be fair and the definition helpful to the solver.