Objection 1. It would seem that these parts are unsuitably assigned, namely, that the eyes, nose, ears, lips, hands, and feet should be anointed. For a wise physician heals the disease in its root. Now "from the heart come forth thoughts... that defile a man" (Matthew 15:19-20). Therefore the breast ought to be anointed.
Objection 2. Further, purity of mind is not less necessary to those who are departing this life than to those who are entering therein. Now those who are entering are anointed with chrism on the head by the priest, to signify purity of mind. Therefore in this sacrament those who are departing should be anointed on the head.
Objection 3. Further, the remedy should be applied where the disease is most virulent. Now spiritual sickness is most virulent in the loins in men, and in the navel in women, according to Job 40:11: "His strength is in his loins, and his force in the navel of his belly," as Gregory expounds the passage (Moral. xxxii, 11). Therefore these parts should be anointed.
Objection 4. Further, sins are committed with other parts of the body, no less than with the feet. Therefore, as the feet are anointed, so ought other members of the body to be anointed.
I answer that, The principles of sinning are the same in us as the principles of action, for a sin is an act. Now there are in us three principles of action; the first is the directing principle, namely, the cognitive power; the second is the commanding principle, namely, the appetitive power; the third is the executive principle, namely, the motive power.
Now all our knowledge has its origin in the senses. And, since the remedy for sin should be applied where sin originates in us first, for that reason the places of the five senses are anointed. the eyes, to wit, on account of the sight, the ears on account of hearing, the nostrils on account of the smell, the mouth on account of the taste, the hands on account of the touch which is keenest in the finger tips, (in some places too the loins are anointed on account of the appetite), and the feet are anointed on account of the motive power of which they are the chief instrument. And since the cognitive power is the first principle of human activity, the anointing of the five senses is observed by all, as being essential to the sacrament. But some do not observe the other unctions--some also anoint the feet but not the loins--because the appetitive and motive powers are secondary principles.
Reply to Objection 1. No thought arises in the heart without an act of the imagination which is a movement proceeding from sensation (De Anima ii). Hence the primary root of thought is not the heart, but the sensory organs, except in so far as the heart is a principle of the whole body, albeit a remote principle.
Reply to Objection 2. Those who enter have to receive purity of the mind, whereas those who are departing have to cleanse the mind. Hence the latter need to be anointed in those parts in respect of which the mind's purity may be sullied.
Reply to Objection 3. Some are wont to anoint the loins, because they are the chief seat of the concupiscible appetite: however, as stated above, the appetitive power is not the primary root.
Reply to Objection 4. The bodily organs which are the instruments of sin, are the feet, hands, and tongue, all of which are anointed, and the organs of generation which it would be unbecoming to anoint, on account of their uncleanliness, and out of respect for the sacrament.
Source: Summa Theologica (New Advent)