Titus Livius (Livy), The History of Rome, Book 30
Cyrus Evans, Ed.

34. The Carthaginian line was deprived of the cavalry on both sides, when the infantry, who were now not a match for the Romans in confidence or strength, engaged. In addition to this there was one circumstance, trifling in itself, but at the same time producing important consequences in the action. On the part of the Romans the shout was uniform, and on that account louder and more terrific; while the voices of the enemy, consisting as they did of many nations of different languages, were dissonant. [2] The Romans used the stationary kind of fight, pressing upon the enemy with their own weight and that of their arms; but on the other side there was more of skirmishing and rapid movement than force. [3] Accordingly, on the first charge, the Romans immediately drove back the line of their opponents; [4] then pushing them with their elbows and the bosses of their shields, and pressing forward into the places from which they had pushed them, they advanced a considerable space, as though there had been no one to resist them, those who formed the rear urging forward those in front when they perceived the line of the enemy giving way; which circumstance itself gave great additional force in repelling them. [5] On the side of the enemy, the second line, consisting of the Africans and Carthaginians, were so far from supporting the first line when giving ground, that, on the contrary, they even retired, lest their enemy, by slaying those who made a firm resistance, should penetrate to themselves also. [6] Accordingly, the auxiliaries suddenly turned their backs, and facing about upon their own party, fled, some of them into the second line, while others slew [7??] those who did not receive them into their ranks, since before they did not [p. 1327]support them, and now refused to receive them. [8] And now there were, in a manner, two contests going on together, the Carthaginians being compelled to fight at once with the enemy and with their own party. Not even then, however, did they receive into their line the terrified and exasperated troops; [9] but, closing their ranks, drove them out of the scene of action to the wings and the surrounding plain, lest they should mingle these soldiers, terrified with defeat and wounds, with that part of their line which was firm and fresh. But such a heap of men and arms had filled the space in which the auxiliaries a little while ago had stood, that it was almost more difficult to pass through it than through a close line of troops. [10] The spearmen, therefore, who formed the front line, pursuing the enemy as each could find a way through the heap of arms and men, and streams of blood, threw into complete disorder the battalions and companies. [11] The standards also of the principes had begun to waver when they saw the line before them driven from their ground. Scipio, perceiving this, promptly ordered the signal to be given for the spearmen to retreat, and, having taken his wounded into the rear, brought the principes and triarii to the wings, in order that the line of spearmen in the centre might be more strong and secure. [12] Thus a fresh and renewed battle commenced, inasmuch as they had penetrated to their real antagonists, men equal to them in the nature of their arms, in their experience in war, in the fame of their achievements, and the greatness of their hopes and fears. [13] But the Romans were superior both in numbers and courage, for they had now routed both the cavalry and the elephants, and having already defeated the front line, were fighting against the second.