The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. Let a man meditate on the fivefold Sâman, which is greater than great, as the prânas (senses). The hiṅkâra is smell 1 (nose), the prastâva speech (tongue), the udgîtha sight (eye), the pratihâra hearing (ear), the nidhana mind. These are one greater than the other.
2. What is greater than great belongs to him, nay, he conquers the worlds which are greater than
great, who knowing this meditates on the fivefold Sâman, which is greater than great, as the prânas (senses).
25:1 Prâna is explained by ghrâna, smell; possibly ghrâna may have been the original reading. Anyhow, it cannot be the mukhya prâna here, because it is distinctly represented as the lowest sense.