Cicero says (De Quaest. Tusc. iv, 8) that "fear is
followed by trembling, pallor and chattering of the teeth."

_I answer that,_ As stated above (A. 1), in fear there takes place a
certain contraction from the outward to the inner parts of the body,
the result being that the outer parts become cold; and for this
reason trembling is occasioned in these parts, being caused by a lack
of power in controlling the members: which lack of power is due to
the want of heat, which is the instrument whereby the soul moves
those members, as stated in _De Anima_ ii, 4.

Reply Obj. 1: When the heat withdraws from the outer to the inner
parts, the inward heat increases, especially in the inferior or
nutritive parts. Consequently the humid element being spent, thirst
ensues; sometimes indeed the result is a loosening of the bowels, and
urinary or even seminal evacuation. Or else such like evacuations are
due to contraction of the abdomen and testicles, as the Philosopher
says (De Problem. xxii, 11).

This suffices for the Reply to the Second Objection.

Reply Obj. 3: In fear, heat abandons the heart, with a downward
movement: hence in those who are afraid the heart especially
trembles, as also those members which are connected with the breast
where the heart resides. Hence those who fear tremble especially in
their speech, on account of the tracheal artery being near the heart.
The lower lip, too, and the lower jaw tremble, through their
connection with the heart; which explains the chattering of the
teeth. For the same reason the arms and hands tremble. Or else
because the aforesaid members are more mobile. For which reason the
knees tremble in those who are afraid, according to Isa. 35:3:
"Strengthen ye the feeble hands, and confirm the trembling [Vulg.:
'weak'] knees."
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FOURTH ARTICLE [I-II, Q. 44, Art. 4]

Whether Fear Hinders Action?

Objection 1: It would seem that fear hinders action. For action is
hindered chiefly by a disturbance in the reason, which directs
action. But fear disturbs reason, as stated above (A. 2). Therefore
fear hinders action.

Obj. 2: Further, those who fear while doing anything, are more apt to
fail: thus a man who walks on a plank placed aloft, easily falls
through fear; whereas, if he were to walk on the same plank down
below, he would not fall, through not being afraid. Therefore fear
hinders action.

Obj. 3: Further, laziness or sloth is a kind of fear. But laziness