To what are we to attribute these uncommon, nay, these unnatural faculties? Neither physiological experiments during life, nor anatomical investigation after death, have hitherto enabled us to form an opinion. Great as the progress of science has been, we are still doubtful as to the nature of the digestive process. All the hypotheses on the subject are liable to insuperable objections. Hippocrates and Empedocles attributed digestion to the _putrefaction_ of food. Experiments have clearly demonstrated the fallacy of this doctrine: rejected food is never in a state of putridity; on the contrary, meat in a perfect state of putrescence has been restored to sweetness and freshness on being received into the stomach. Dead snakes have been found with animal substances, part of which had been swallowed and the remainder hanging out of their mouths; when the swallowed portion was fresh, and the portion exposed to the atmosphere in a state of corruption. Galen, and after his school, Grew and Santarelli, ascribed digestion to a _concoction_, during which, food was maturated by the stomach's heat, like fruit by the solar rays. Pringle and Macbride advocated the doctrine of _fermentation_; while Borelli, Keil, and Pitcairn resolved the question by the mechanism of _trituration_, making a mill of the stomach, which ground down food, according to Pitcairn's calculations, with a pressure equal to a weight of one hundred and seventeen thousand and eighty pounds. Boerhaave endeavoured to reconcile the opinions of the _concocters_ and _grinders_, by combining the supposed theory of _concoction_ and _trituration_. Lastly, Cheselden fancied that digestion was operated by a peculiar secretion in the stomach, called _gastric juice_; and Haller, Reaumur, Spallanzani, Blumenbach and most other modern physiologists, concur with him in the same opinion, although admitting that this function is most probably assisted by various accessory circumstances.