Titus Livius (Livy), The History of Rome, Book 33
Evan T. Sage, Ph.D. Professor of Latin and Head of the Department of Classics in the University of Pittsburgh, Ed.

9. Quinctius absorbed into his ranks and among¹ the standards the men who had already been engaged and gave the signal with the trumpet. [2] They say that only rarely at any other time has such a shout been raised at the beginning of a battle, for, as it happened, both armies shouted at once, and not only those who were fighting but also the reserves and those who were just then coming up to the line. [3] On the right flank, the king prevailed easily, mainly because of his position, since he was fighting from higher ground; on the left there was panic and confusion, especially since the part of the phalanx which was in the rear was still coming up; [4] the centre, which was nearer the right flank, stood watching the battle there, as if it were a spectacle which did not directly concern them. [5] The phalanx, which had come up in column rather than in line, and in a form more fitted for the march than for battle, had barely reached the saddle. [6] While it was still in disorder, Quinctius, although he saw his men retreating on the right, first sending his elephants against the enemy, attacked, thinking that the defeat of a part would involve the rest. The issue was never in doubt; the Macedonians immediately fled, turning back in terror at the first sight of the beasts. [7] The others too followed them in their flight, and one of the tribunes [8] of the soldiers, forming a plan to fit the emergency, took the soldiers of twenty companies and, leaving the action where his men were clearly victorious and making a short detour, attacked the enemy's right from behind. [9] Any army would have been dismayed by an attack from the rear; but added to the general panic of all in such a crisis was the fact that the heavy and unwieldy Macedonian [p. 301]phalanx could not change front, nor did the soldiers² who were falling back a little while before from the front upon men who were by now terrified on their own account permit this.³ [10] They were at a disadvantage too because of their position, since the ridge from which they had been fighting, when they were pursuing the soldiers who had been driven⁴ down the hill, had been given up to the enemy which had been led around behind them. [11] For a while they were caught between the two lines and slaughtered, then most of them threw away their arms and took to flight.