Passage 10.19.11
ταῦτα ἐμοὶ δοκεῖν ἐνομίσθη τοῖς Γαλάταις ἐς μίμησιν τοῦ ἐν Πέρσαις ἀριθμοῦ τῶν μυρίων, οἳ ἐκαλοῦντο Ἀθάνατοι. διάφορα δὲ ἦν, ὅτι κατελέγοντο ἀντὶ τῶν ἀποθνησκόντων ὑπὸ μὲν Περσῶν τῆς μάχης ὕστερον, Γαλάταις δὲ ὑπʼ αὐτὴν τοῦ ἔργου τὴν ἀκμὴν ὁ ἀριθμὸς ἀπεπληροῦτο τῶν ἱππέων. τοῦτο ὠνόμαζον τὸ σύνταγμα τριμαρκισίαν τῇ ἐπιχωρίῳ φωνῇ· καὶ ἵππῳ τὸ ὄνομα ἴστω τις μάρκαν ὂν τὰ ὑπὸ τῶν Κελτῶν.
It seems to me the Gauls established these practices in imitation of the Persian contingent of the Ten Thousand, who were called the Immortals. There was, however, a difference: among the Persians, vacancies caused by casualties in battle were filled afterwards, while among the Gauls the troop of cavalry was restored to its full number immediately, in the very heat of combat. They called this unit "Trimarkisia" in their native tongue; and one should understand that a horse is called "marka" among the Celts.