Objection 1. It would seem that anger does not cause taciturnity. Because taciturnity is opposed to speech. But increase in anger conduces to speech; as is evident from the degrees of anger laid down by Our Lord (Matthew 5:22): where He says: "Whosoever is angry with his brother"; and "... whosoever shall say to his brother, 'Raca'"; and "... whosoever shall say to his brother, 'Thou fool.'" Therefore anger does not cause taciturnity.
Objection 2. Further, through failing to obey reason, man sometimes breaks out into unbecoming words: hence it is written (Proverbs 25:28): "As a city that lieth open and is not compassed with walls, so is a man that cannot refrain his own spirit in speaking." But anger, above all, hinders the judgment of reason, as stated above (Article 3). Consequently above all it makes one break out into unbecoming words. Therefore it does not cause taciturnity.
Objection 3. Further, it is written (Matthew 12:34): "Out of the abundance of the heart the mouth speaketh." But anger, above all, causes a disturbance in the heart, as stated above (Article 2). Therefore above all it conduces to speech. Therefore it does not cause taciturnity.
On the contrary, Gregory says (Moral. v, 30) that "when anger does not vent itself outwardly by the lips, inwardly it burns the more fiercely."
I answer that, As stated above (3; 46, 4), anger both follows an act of reason, and hinders the reason: and in both respects it may cause taciturnity. On the part of the reason, when the judgment of reason prevails so far, that although it does not curb the appetite in its inordinate desire for vengeance, yet it curbs the tongue from unbridled speech. Wherefore Gregory says (Moral. v, 30): "Sometimes when the mind is disturbed, anger, as if in judgment, commands silence." On the part of the impediment to reason because, as stated above (Article 2), the disturbance of anger reaches to the outward members, and chiefly to those members which reflect more distinctly the emotions of the heart, such as the eyes, face and tongue; wherefore, as observed above (Article 2), "the tongue stammers, the countenance takes fire, the eyes grow fierce." Consequently anger may cause such a disturbance, that the tongue is altogether deprived of speech; and taciturnity is the result.
Reply to Objection 1. Anger sometimes goes so far as to hinder the reason from curbing the tongue: but sometimes it goes yet farther, so as to paralyze the tongue and other outward members.
And this suffices for the Reply to the Second Objection.
Reply to Objection 3. The disturbance of the heart may sometimes superabound to the extend that the movements of the outward members are hindered by the inordinate movement of the heart. Thence ensue taciturnity and immobility of the outward members; and sometimes even death. If, however, the disturbance be not so great, then "out of the abundance of the heart" thus disturbed, the mouth proceeds to speak.
Source: Summa Theologica (New Advent)