The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
9. (The chief prâna is) neither air nor function, on account of its being mentioned separately.
An inquiry is now started concerning the nature of that chief prâna.--The pûrvapakshin maintains that the prâna is, according to Sruti, nothing but air. For Sruti says, 'Breath is air; that air assuming five forms is prâna, apâna, vyâna, udâna, samâna.'--Or else the pûrvapaksha may be formulated according to the view of another philosophical doctrine, and prâna may be considered as the combined function of all organs. For so the followers of another doctrine (viz. the Sâṅkhyas) teach, 'The five airs, prâna, &c., are the common function of the instruments 1.'
To this we reply that the prâna is neither air nor the function of an organ; for it is mentioned separately. From air prâna is distinguished in the following passage, 'Breath indeed is the fourth foot of Brahman. That foot shines as Agni with its light and warms.' If prâna were mere air, it would not be mentioned separately from air.--Thus it is also mentioned separately from the functions of the organs; for the texts enumerate speech and the other organs and mention prâna separately from them, and the function and that to which the function belongs (the organ) are identical. If it were a mere function of an organ, it would not be mentioned separately from the organs. Other passages also in which the prâna is mentioned separately from air and the organs are here to be considered so, e.g. 'From him is born breath, mind, and all organs of sense, ether, air,' &c. (Mu. Up. II, 1, 3). Nor is it possible that all the organs together should have one function (and that that function should be the prâna); for each organ has its own special function and the aggregate of them has no active power of its own.--But--an objection may be raised--the thing may take place in the manner of the moving bird-cage. Just as eleven birds shut up in one cage may, although each makes a separate effort, move the cage by the combination of their efforts; so the eleven
prânas which abide in one body may, although each has its own special function, by the combination of these functions, produce one common function called prâna.--This objection, we reply, is without force. The birds indeed may, by means of their separate subordinate efforts, which all favour the movement of the cage, move the cage by combination; that is a matter of observation. But we have no right to assume that the different prânas with their subordinate functions such as hearing &c. can, by combination, produce the function of vital breath; for there is no means to prove this, and the vital breath is in kind absolutely different from hearing and so on.--Moreover, if the vital breath were the mere function of an organ (or the organs) it could not be glorified as the 'best,' and speech and so on could not be represented as subordinate to it. Hence the vital breath is different from air and the functions (of the organs).--How then have we to understand the scriptural passage, 'The prâna is air,' &c.?--The air, we reply, passing into the adhyâtma-state, dividing itself fivefold and thus abiding in a specialized condition is called prâna. It therefore is neither a different being nor is it mere air. Hence there is room for those passages as well which identify it with air as those which do not.--Well, let this be granted. The prâna then also must be considered to be independent in this body like the individual soul, as scripture declares it to be the 'best' and the organs such as speech, &c., to be subordinate to it. For various powers are ascribed to it in scriptural passages. It is said, for instance, that when speech and the other (organs) are asleep the prâna alone is awake; that the prâna alone is not reached by death; that the prâna is the absorber, it absorbs speech, &c.; that the prâna guards the other senses (prânas) as a mother her sons 1. Hence it follows that the prâna is independent in the same way as the individual soul.--This view is impugned in the next Sûtra.
86:1 Sâṅkhya Sû. II, 31; where, however, the reading is 'sâmânya-karanavrittih,' explained by the Comm. as sâdhâranî karanasya antahkaranatrayasya vrittih parinâmabhedâ iti. Saṅkara, on the other hand, understands by karana the eleven prânas discussed previously.
87:1 Cp. Ka. Up. II, 5, 8; Bri. Up. I, 5, 21; Kh. Up. IV, 3, 3; Pr. Up. II, 13.