Objection 1. It would seem that the members of the body do not obey reason as to their acts. For it is evident that the members of the body are more distant from the reason, than the powers of the vegetal soul. But the powers of the vegetal soul do not obey reason, as stated above (Article 8). Therefore much less do the members of the body obey.
Objection 2. Further, the heart is the principle of animal movement. But the movement of the heart is not subject to the command of reason: for Gregory of Nyssa [Nemesius, De Nat. Hom. xxii.] says that "the pulse is not controlled by reason." Therefore the movement of the bodily members is not subject to the command of reason.
Objection 3. Further, Augustine says (De Civ. Dei xiv, 16) that "the movement of the genital members is sometimes inopportune and not desired; sometimes when sought it fails, and whereas the heart is warm with desire, the body remains cold." Therefore the movements of the members are not obedient to reason.
On the contrary, Augustine says (Confess. viii, 9): "The mind commands a movement of the hand, and so ready is the hand to obey, that scarcely can one discern obedience from command."
I answer that, The members of the body are organs of the soul's powers. Consequently according as the powers of the soul stand in respect of obedience to reason, so do the members of the body stand in respect thereof. Since then the sensitive powers are subject to the command of reason, whereas the natural powers are not; therefore all movements of members, that are moved by the sensitive powers, are subject to the command of reason; whereas those movements of members, that arise from the natural powers, are not subject to the command of reason.
Reply to Objection 1. The members do not move themselves, but are moved through the powers of the soul; of which powers, some are in closer contact with the reason than are the powers of the vegetal soul.
Reply to Objection 2. In things pertaining to intellect and will, that which is according to nature stands first, whence all other things are derived: thus from the knowledge of principles that are naturally known, is derived knowledge of the conclusions; and from volition of the end naturally desired, is derived the choice of the means. So also in bodily movements the principle is according to nature. Now the principle of bodily movements begins with the movement of the heart. Consequently the movement of the heart is according to nature, and not according to the will: for like a proper accident, it results from life, which follows from the union of soul and body. Thus the movement of heavy and light things results from their substantial form: for which reason they are said to be moved by their generator, as the Philosopher states (Phys. viii, 4). Wherefore this movement is called "vital." For which reason Gregory of Nyssa (Nemesius, De Nat. Hom. xxii) says that, just as the movement of generation and nutrition does not obey reason, so neither does the pulse which is a vital movement. By the pulse he means the movement of the heart which is indicated by the pulse veins.
Reply to Objection 3. As Augustine says (De Civ. Dei xiv, 17,20) it is in punishment of sin that the movement of these members does not obey reason: in this sense, that the soul is punished for its rebellion against God, by the insubmission of that member whereby original sin is transmitted to posterity.
But because, as we shall state later on, the effect of the sin of our first parent was that his nature was left to itself, through the withdrawal of the supernatural gift which God had bestowed on man, we must consider the natural cause of this particular member's insubmission to reason. This is stated by Aristotle (De Causis Mot. Animal.) who says that "the movements of the heart and of the organs of generation are involuntary," and that the reason of this is as follows. These members are stirred at the occasion of some apprehension; in so far as the intellect and imagination represent such things as arouse the passions of the soul, of which passions these movements are a consequence. But they are not moved at the command of the reason or intellect, because these movements are conditioned by a certain natural change of heat and cold, which change is not subject to the command of reason. This is the case with these two organs in particular, because each is as it were a separate animal being, in so far as it is a principle of life; and the principle is virtually the whole. For the heart is the principle of the senses; and from the organ of generation proceeds the seminal virtue, which is virtually the entire animal. Consequently they have their proper movements naturally: because principles must needs be natural, as stated above (Reply to Objection 2).
Source: Summa Theologica (New Advent)