Titus Livius (Livy), The History of Rome, Book 35
Evan T. Sage, PhD professor of latin and head of the department of classics in the University of Pittsburgh, Ed.

5. At about the second hour the battle began. The left squadron¹ of the allies and the irregular troops² were fighting in the front line; their commanders were two lieutenants of consular rank, Marcus Marcellus³ and Tiberius Sempronius,⁴ consul of the preceding year. [2] The new consul was now with the leading standards, now holding back the legions [p. 13]in reserve, lest in their ardour for the fight they⁵ should rush forward before the signal was given. [3] He ordered two military tribunes, Quintus and Publius Minucius, to lead the cavalry of these legions beyond the flanks of the battle-line into open ground, whence, when the signal was given, they were to attack from the open.⁶ [4] As he was thus engaged, a runner from Tiberius Sempronius Longus came to him, saying that the irregulars were not holding the Gallic attack [5??] and that many of them had been killed and that those who were left, partly as a result of their exertions, partly from fear, had lost their zest for fighting. He should send in, if he saw fit, one of the two legions before a disgraceful defeat was sustained. [6] The second legion was sent forward and the irregular troops relieved.⁷ [7] Then the battle was restored, since fresh troops, a legion with full ranks, had entered the fight; and the left squadron was withdrawn from the battle and the right took its place in the battle-line. The sun with its fierce rays scorched the bodies of the Gauls, which were little capable of enduring heat; nevertheless, in dense ranks, resting now on one another, now on their shields, they withstood the attacks of the Romans. [8] When the consul saw this, he ordered Gaius Livius Salinator,⁸ who commanded the auxiliary cavalry, to charge at the utmost speed, the legionary cavalry to be in support. [9] This storm of horsemen at first threw the battle-line of the Gauls into confusion and disorder, then scattered it, but without causing a rout. [10] The captains prevented this, striking with their staffs the backs of the terror-stricken and forcing them back into the line, but the auxiliary cavalry, riding among them, would not allow this.⁹ [11] The [p. 15]consul urged the troops to make a little more effort;¹⁰ victory, he said, was in their grasp; they should press on while they saw the enemy disordered and in terror; if they permitted the ranks to be reformed they would fight again a new and doubtful battle. He ordered the standard-bearers to advance. [12] All joined in the effort and at last turned the enemy to flight. As they were fleeing and scattering this way and that in rout, at that moment the legionary cavalry was let loose to pursue them. [13] Fourteen thousand of the Boii perished on that day; one thousand and ninety-two were captured alive, seven hundred and twenty-one cavalrymen, with three of their commanders, two hundred and twelve standards and sixty-three carts were taken. [14] But for the Romans the victory was not bloodless; more than five thousand of the soldiers, Romans and allies, fell, twenty-three centurions, four commanders of allies, and Marcus Genucius and Quintus and Marcus Marcius, military tribunes of the second legion.