The interstitial neurons of the white matter, historically neglected for no reason, are receiving more and more attention in the research of the last half century. Given that these are actually surviving nerve cells of the earliest neurons of the key embryonic subplate zone of the fetal CNS, many would agree that the interest is reasonable. Identified approximately 150 years ago, they were first declared a normal finding in the white matter of the human brain, but the opinion changed at the beginning of the last century when some neuropathologists considered them to be a pathological phenomenon in certain neurological disorders. Today, however, we know WMIN as important morphologically heterogeneous components of the white matter of the human brain, whose average number is half a billion, in percentages: a conservative estimate of about 3% of the total number of neurons in the human cortex, a step towards proving the importance of the function. For the sake of comparison, the stated number of neurons is ten times greater than those in the thalamus and even fifty times greater than those in the amygdala. Present throughout the entire subcortical white matter, WMIN can be morphologically classified into two large groups: interstitial neurons with a pyramidal morphology, which are more often found in the crowns of the gyrus, and interstitial neurons with an interneuronal morphology, which are more often found at the bottom of the sulcus. Although their function remains relatively unexplored and unknown to date, it is assumed that the relatively high density of WMIN near the cerebral cortex enables participation in essential functions of the cortical system: regulation of the flow of information to the cerebral cortex, regulation of wakefulness and regulation of local microcirculation.