The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. The hiṅkâra is mind, the prastâva speech, the udgîtha sight, the pratihâra hearing, the nidhana
breath. That is the Gâyatra Sâman, as interwoven in the (five) prânas 1.
2. He who thus knows this Gâyatra interwoven in the prânas, keeps his senses, reaches the full life, he lives long 2, becomes great with children and cattle, great by fame. The rule of him who thus meditates on the Gâyatra is, 'Be not high-minded.'
28:3 After having explained the secret meaning of the whole Sâma-veda p. 29 ceremonial, as it is to be understood by meditation only (dhyâna),he proceeds to explain the secret meaning of the same ceremonial, giving to each its proper name in proper succession (gâyatra, rathantara, &c.), and showing the hidden purport of those names.
29:1 Cf. Kh. Up. II, 7, 1, where prâna is explained differently. The Gâyatrî itself is sometimes called prâna.
29:2 The commentator generally takes gyok in the sense of bright.