Objection 1. It would seem that trembling is not an effect of fear. Because trembling is occasioned by cold; thus we observe that a cold person trembles. Now fear does not seem to make one cold, but rather to cause a parching heat: a sign whereof is that those who fear are thirsty, especially if their fear be very great, as in the case of those who are being led to execution. Therefore fear does not cause trembling.
Objection 2. Further, faecal evacuation is occasioned by heat; hence laxative medicines are generally warm. But these evacuations are often caused by fear. Therefore fear apparently causes heat; and consequently does not cause trembling.
Objection 3. Further, in fear, the heat is withdrawn from the outer to the inner parts of the body. If, therefore, man trembles in his outward parts, through the heat being withdrawn thus; it seems that fear should cause this trembling in all the external members. But such is not the case. Therefore trembling of the body is not caused by fear.
On the contrary, 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 (Article 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 to Objection 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 to Objection 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 Isaiah 35:3: "Strengthen ye the feeble hands, and confirm the trembling [Vulgate: 'weak'] knees."
Source: Summa Theologica (New Advent)