Judging from Afras. data the original form should be *lis-, with suffixed *-ān.
Cf. *matn- (1) ‘sinew, tendon, nerve’ (No. 191), related with metathesis?
‘Brain’ is the only meaning to develop into ‘top of the head’, ‘head, skulľ, on the one hand, and ‘marrow’, on the other.