The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
17. They (the prânas) are senses, on account of being so designated, with the exception of the best (the mukhya prâna).
We have treated of the mukhya prâna and the other
eleven prânas in due order.--Now there arises another doubt, viz. whether the other prânas are functions of the mukhya prâna or different beings.--The pûrvapakshin that they are mere functions, on account of scriptural statement. For scripture, after having spoken of the chief prâna and the other prânas in proximity, declares that those other prânas have their Self in the chief prâna, 'Well, let us all assume his form. Thereupon they all assumed his form' (Bri. Up. I, 5, 2l).--Their unity is moreover ascertained from the unity of the term applied to them, viz. prâna. Otherwise there either would result the objectionable circumstance of one word having different senses, or else the word would in some places have to be taken in its primary sense, in others in a derived sense. Hence, as prâna, apâna, &c. are the five functions of the one chief prâna, so the eleven prânas also which begin with speech are mere functions of the chief prâna.--To this we reply as follows. Speech and so on are beings different from the chief prâna, on account of the difference of designation.--Which is that difference of designation?--The eleven prânas remaining if we abstract from the best one, i.e. the chief prâna, are called the sense-organs (indriya), as we see them designated in Sruti, 'from him is born breath, mind, and all organs of sense' (Mu. Up. II, 1, 3). In this and other passages prâna and the sense-organs are mentioned separately.--But in that case the mind also would have to be excluded from the class of sense-organs, like the prâna; as we see that like the latter it is separately mentioned in the passage, 'The mind and all organs of sense.' True; but in Smriti eleven sense-organs are mentioned, and on that account the mind must, like the ear, and so on, be comprised in the sense-organs. That the prâna on the other hand is a sense-organ is known neither from Smriti nor Sruti.--Now this difference of designation is appropriate only if there is difference of being. If there were unity of being it would be contradictory that the prâna although one should sometimes be designated as sense--organ and sometimes not. Consequently the other prânas are different in being from the chief prâna.--For this conclusion the following Sûtra states an additional reason.