Am I alone in finding it stunning that it's 2017 and we can still discover new anatomy? How can this not have been (re)discovered?
"While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measurement shows its importance — where quantitative work is more to be desired than qualitative work. An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals."
Albert Michelson is the Michelson of the Michelson-Morley experiment that showed that the speed of light was identical from all perspectives - a discovery that was soon to completely revolutionize our understanding and notion of physics. The comment was also made shortly before quantum mechanics would really begin to be delved into, further revolutionizing physics.
We only know what we know. And that's all we know - until we know more. In the case of the brain in particular we have practically zero causal and holistic understanding. And much of what we do know has been driven by pharmacological profit motive, which doesn't necessarily yield the most objective science and certainly constraints it in any case. Ultimately, I'd be rather more surprised if there are not a vast number of fundamental discoveries yet to be made!
This is not the only 'how can we not know this' thing which happens, I read a few years back that fundamental nervous system linkages (as in, the actual nerves themselves) of the clitoris had not been mapped in the 20th century because nobody felt like it mattered enough until women anatomists started looking more carefully at the cadavers...