As he thus made his defence (tauta autou apologoumenou). Genitive absolute again with present middle participle. Paul was still speaking when Festus interrupted him in great excitement.
With a loud voice (megalēi tēi phōnēi). Associative instrumental case showing manner (Robertson, Grammar, p. 530) and the predicate use of the adjective, “with the voice loud” (elevated).
Thou art mad (mainēi). Old verb for raving. See also Joh 10:20; Act 12:15; 1Co 14:23. The enthusiasm of Paul was too much for Festus and then he had spoken of visions and resurrection from the dead (Act 26:8). “Thou art going mad” (linear present), Festus means.
Thy much learning doth turn thee to madness (ta polla se grammata eis manian peritrepei). “Is turning thee round.” Old verb peritrepō, but only here in N.T. Festus thought that Paul’s “much learning” (=“many letters,” cf. Joh 7:15 of Jesus) of the Hebrew Scriptures to which he had referred was turning his head to madness (wheels in his head) and he was going mad right before them all. The old word mania (our mania, frenzy, cf. maniac) occurs here only in N.T. Note unusual position of se between polla and grammata (Robertson, Grammar, pp. 418, 420)