Tremor est effectus timoris quia in timore fit quaedam contractio ab exterioribus ad interiora, et propter hoc in eis accidit tremor, qui causatur ex debilitate virtutis continentis membra.
Trembling is an effect of fear because in fear there takes place a certain contraction from the outward to the inner parts of the body, the result being that trembling is occasioned in these parts, being caused by a lack of power in controlling the members.
Tullius dicit, in IV de Tusculanis quaest., quod "terrorem sequitur tremor, et pallor, et dentium crepitus".
Cicero says (De Quaest. Tusc. iv, 8) that "fear is followed by trembling, pallor and chattering of the teeth".