The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
9. And (heat is) subtle in measure; as this is thus observed.
The elementary matter of heat and the other elements
which form the substratum for the soul when passing out of this body, must be subtle in its nature and extent. This follows from the scriptural passages, which declare that it passes out by the veins and so on. Their thinness renders them capable of passing out, and their transparency (permeability) is the cause of their not being stopped by any gross substance. For these reasons they, when passing out of the body, are not perceived by bystanders.