The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
27. And thus (scripture also) declares.
Scripture also, after having signified the soul's abiding in the heart and its atomic size, declares by means of such
passages as 'Up to the hairs, up to the tips of the nails' (Kau. Up. IV, 20; Bri. Up. I. 4, 7), that the soul pervades the entire body by means of intelligence which is its quality.