Sentence:  The brain is a mass of grey and white matter, somewhat oval in shape, with fissures and indentations dividing it into convolutions or gyri, with smaller subdivisious mapped out by "sulci" and <WSD>anfractuosities</WSD>. ("Sulci" is the plural of "sulcus", which means a furrow. An <WSD>anfractuosity</WSD> is a winding or turning.) 
Meaning of the word marked with <WSD> tags:  One of the fissures (sulci) separating the convolutions of the brain, or, by analogy, in the mind.