Intestinorum

Cibi vero in alvum recepti et cum potus humore permixti, cum iam calore percocti fuerint, eorum succus inenarrabili modo per membra diffusus irrigat universum corpus et vegetat. intestinorum quoque multiplices spirae ac longitudo in se convoluta et uno tantum substricta vinculo—quam mirificum Dei opus est! nam ubi maceratos ex se cibos alvus emiserit, paulatim per illos internorum anfractus extruduntur, ut quicquid ipsis ineset succi quo corpus alitur membris omnibus dividatur. et tamen necubi forte obhaereant ac resistant—quod fieri poterat propter ipsorum voluminum flexiones in se saepe redeuntes, et fieri sine pernicie non poterat—opplevit ea intrinsecus crassiore succo ut purgamenta illa ventris ad exitus suos facilius per lubricum niterentur. illa quoqe ratio subtilissima est quod vesica—cuius usum volucres non habent—cum sit ab intestinis separata nec ullam habeat fistulam qua ex illis urinam trahat, completur tamen et humore distenditur. id quomodo fiat non est difficile pervidere. intestinorum enim partes quae ab alvo cibum potumque suscipiunt patentiores sunt quam ceterae spirae et multo tenuiores. hae vesicam complectuntur et continent: ad quas partes cum potus et cibus mixti pervenerint, firmum quidem crassius fit et transmeat. humor autem omnis per illam teneritudinem percolatur, eumque vesica, cuius aeque tenuis subtilisque membrana est, absorbet ac colligit ut foras qua natura exitium patefecit emittat.
(Lactantius, De Opificio Dei 11)

But once the stomach receives the food, thoroughly mixed with moisture from what we drink and then digested by heat, the juice from this spreads in an indescribable manner through the limbs, watering and enlivening the whole body. Also, the many coils of the intestines, folding lengthwise upon themselves and tied together in just one cord—what an amazing work of God this is! For when the stomach ejects the softened food, it is pressed out little by little through those internal bendings, and any juice inside that nourishes the body divides among all parts. However, so that nothing gets stuck and stays back anywhere—a disastrous result that could occur because the bends in those rolls often turn back on themselves—God filled them inside with thicker juice so that what is cleaned from the stomach can push its way out more easily through something slippery. This structure is also quite intricate because the bladder—whose use birds do not have—fills up with fluid and swells even though it is separated from the intestines ad has no tube to get urine from them. How this happens is not hard to see. For the parts of the intestines that get food and drink from the stomach are more open than the other coils and much finer. These surround and contain the bladder: when the mixed food and drink gets to these parts, the excrement actually becomes thicker and passes through. But all the moisture is strained through its thin fabric, and the bladder, whose membrane is equally thin and delicate, absorbs and collects it in order to expel it outside where nature makes an opening. (tr. Brian P. Copenhaver)