Objection 1. It would seem that fear does not cause contraction. For when contraction takes place, the heat and vital spirits are withdrawn inwardly. But accumulation of heat and vital spirits in the interior parts of the body, dilates the heart unto endeavors of daring, as may be seen in those who are angered: while the contrary happens in those who are afraid. Therefore fear does not cause contraction.
Objection 2. Further, when, as a result of contraction, the vital spirits and heat are accumulated in the interior parts, man cries out, as may be seen in those who are in pain. But those who fear utter nothing: on the contrary they lose their speech. Therefore fear does not cause contraction.
Objection 3. Further, shame is a kind of fear, as stated above (Question 41, Article 4). But "those who are ashamed blush," as Cicero (De Quaest. Tusc. iv, 8), and the Philosopher (Ethic. iv, 9) observe. But blushing is an indication, not of contraction, but of the reverse. Therefore contraction is not an effect of fear.
On the contrary, Damascene says (De Fide Orth. ii, 23) that "fear is a power according to systole," i.e. contraction.
I answer that, As stated above (Question 28, Article 5), in the passions of the soul, the formal element is the movement of the appetitive power, while the bodily transmutation is the material element. Both of these are mutually proportionate; and consequently the bodily transmutation assumes a resemblance to and the very nature of the appetitive movement. Now, as to the appetitive movement of the soul, fear implies a certain contraction: the reason of which is that fear arises from the imagination of some threatening evil which is difficult to repel, as stated above (Question 41, Article 2). But that a thing be difficult to repel is due to lack of power, as stated above (Question 43, Article 2): and the weaker a power is, the fewer the things to which it extends. Wherefore from the very imagination that causes fear there ensues a certain contraction in the appetite. Thus we observe in one who is dying that nature withdraws inwardly, on account of the lack of power: and again we see the inhabitants of a city, when seized with fear, leave the outskirts, and, as far as possible, make for the inner quarters. It is in resemblance to this contraction, which pertains to the appetite of the soul, that in fear a similar contraction of heat and vital spirits towards the inner parts takes place in regard to the body.
Reply to Objection 1. As the Philosopher says (De Problem. xxvii, 3), although in those who fear, the vital spirits recede from outer to the inner parts of the body, yet the movement of vital spirits is not the same in those who are angry and those who are afraid. For in those who are angry, by reason of the heat and subtlety of the vital spirits, which result from the craving for vengeance, the inward movement has an upward direction: wherefore the vital spirits and heat concentrate around the heart: the result being that an angry man is quick and brave in attacking. But in those who are afraid, on account of the condensation caused by cold, the vital spirits have a downward movement; the said cold being due to the imagined lack of power. Consequently the heat and vital spirits abandon the heart instead of concentrating around it: the result being that a man who is afraid is not quick to attack, but is more inclined to run away.
Reply to Objection 2. To everyone that is in pain, whether man or animal, it is natural to use all possible means of repelling the harmful thing that causes pain but its presence: thus we observe that animals, when in pain, attack with their jaws or with their horns. Now the greatest help for all purposes, in animals, is heat and vital spirits: wherefore when they are in pain, their nature stores up the heat and vital spirits within them, in order to make use thereof in repelling the harmful object. Hence the Philosopher says (De Problem. xxvii, 9) when the vital spirits and heat are concentrated together within, they require to find a vent in the voice: for which reason those who are in pain can scarcely refrain from crying aloud. On the other hand, in those who are afraid, the internal heat and vital spirits move from the heart downwards, as stated above (ad 1): wherefore fear hinders speech which ensues from the emission of the vital spirits in an upward direction through the mouth: the result being that fear makes its subject speechless. For this reason, too, fear "makes its subject tremble," as the Philosopher says (De Problem. xxvii, 1,6,7).
Reply to Objection 3. Mortal perils are contrary not only to the appetite of the soul, but also to nature. Consequently in such like fear, there is contraction not only in the appetite, but also in the corporeal nature: for when an animal is moved by the imagination of death, it experiences a contraction of heat towards the inner parts of the body, as though it were threatened by a natural death. Hence it is that "those who are in fear of death turn pale" (Ethic. iv, 9). But the evil that shame fears, is contrary, not to nature, but only to the appetite of the soul. Consequently there results a contraction in this appetite, but not in the corporeal nature; in fact, the soul, as though contracted in itself, is free to set the vital spirits and heat in movement, so that they spread to the outward parts of the body: the result being that those who are ashamed blush.
Source: Summa Theologica (New Advent)