“You can learn for yourself what I’m saying is true if you consider something that often happens to people. There are times when a biting humour builds up in the area around the anus and it makes us really need to go to the bathroom, but we are forced to hold it in because we are in the middle of some public business; once we’re done with it, we can’t go to the bathroom anymore and as a result we often we also get a headache and an upset stomach.”

ὅτι δὲ ἀληθές ἐστιν ὃ λέγω, πάρεστι μαθεῖν ἑκάστῳ τῶν πολλάκις ἡμῖν συμβαινόντων ἀναμνησθέντι. δακνώδης γοῦν ἐνίοτε χυμὸς εἰς τὰ κατὰ τὴν ἕδραν χωρία παραγενόμενος, ἐρεθίζει μὲν ἡμᾶς ἐπὶ τὴν ἔκκρισιν αὐτοῦ· κατασχεῖν δὲ αὐτὸν ἀναγκασθέντες, ἐπειδὰν ἐν πολιτικαῖς ὦμεν πράξεσιν, ἀπαλλαγέντες αὐτῶν οὐκέτ' ἀποκρίνομεν, αἰσθανόμεθά τε κἀκ τούτου πολλάκις τῆς κεφαλῆς ὀδυνηρᾶς γινομένης ἀνατρεπομένης τε τῆς γαστρός.

Galen, Matters of Health 6.10, 6.433 K. = 190,1–7 Koch

“For when people really need to poo but because they are held up in the agora or some other unsuitable place they hold it in for a long time, they are either no longer able to poo or their poo is small and dry. Why does this happen? Clearly it’s because effluvia have come off from them inside of us as well, from which it is clear that what is present in our intestines is also nourishment.”

οἱ γ(ὰρ) προθυμίᾳ γ(ινόμενοι) πρὸς τὸ διαχωρῆσαι,
καταλαμβανόμενοι δὲ ἐν ἀγορᾶι ἢ
ἐν ἀνεπιτηδείοις, εἶτα συσχόν-
τες ἐπὶ πλεῖον, οὐκέτι διαχωρο(ῦσιν)
ἢ διαχωροῦσιν ἐλάχιστά τε καὶ ξηρ(ά).
τίνος αἰ(τίας) γι(νομένης); δῆλον ὅτι ἀποφορᾶς καὶ ἐ̣ν(τὸς)
ἀπ' αὐτ(ῶν) γεγενημέν(ης). ἐξ ὧν φανερόν,
ὡς τροφή (ἐστιν) καὶ ἡ ἐν ἐντέροις παρακειμένη.

Anonymus Londinensis, Fragment 1,1–9 Diels

Thanks to Peter, Ralph and David for pointing these out.