Capsula extrema, one on hay. and one on the left. side, consists of a layer of white matter sandwiched between the outside of the claustrum and the inside of the insula bark. The nerve fibre capsule consists partly of the afferent and efferent filamentous flows of the insula and partly of continuous association pathways. Among the latter, diffusion tensor tractography has been used to identify a pathway system that connects Broca's field with Wernicke's field, i.e. a path system analogous to fasciculus arcuatus and probably of importance (as yet unclear which) for our speech/language ability.
Makris N. and Pandya D. N., The extreme capsule in humans and rethinking of the language circuitry. Brain Struct.Funct., 2009, 213:343–358.