CH. in DIGESTION IN THE MOUTH AND STOMACH 153 



and certain proteins only are soluble in water ; starch, coagulated 

 protein (e.g. meat and boiled egg-white), and fats are insoluble. 

 The mechanical and chemical object of digestion is to modify 

 these substances, and to render them soluble and readily diffusible. 

 Mineral constituents being soluble need undergo no change in 

 the gastro-intestinal canal to fit them for entering the blood. 



The mechanism of digestion is so bound up with its chemistry, 

 that to treat them separately seems to us no less grave an error 

 than to discuss the theory of nutrition before that of digestion. 



I. The first series of methodical experiments on Digestion were 

 those of Reaumur (1683-1757). The Accademici del Cimento, 

 disciples of Galileo, and founders of the iatro-mechanical school, 

 had previously experimented on ravens, and seen that the stomach 

 of these birds is capable with its powerful muscles of pulverising 

 the hardest bodies, which lent support to the view that digestion 

 consisted essentially in trituration (Borelli, Pitcairn, Boerhaave). 

 It was known, however, that in man and mammals, where the 

 digestive powers are very great, the stomach has such thin walls 

 that digestion can only be conceived as the effect of chemical 

 solvents (Wepfer, Viridet, Valisnieri). In order to decide between 

 the mechanical and the chemical theory, Reaumur carried out a 

 series of experiments in which ostriches were made to swallow 

 perforated metal tubes containing food. The first results were 

 negative or very doubtful, but his later work on birds of prey, 

 which have a membranous stomach, yielded conclusive results, and 

 convinced him of the chemical character of the forces that, in the 

 majority of cases, effect digestion. When, however, he attempted 

 to digest in vitro by means of gastric juice obtained from sponges 

 which his tame buzzard was made to swallow and then regurgitate, 

 after which the fluid which the sponges had imbibed was squeezed 

 out, his results were negative, and he gave up the experiments. 



Nearly half a century later the same experiments were taken 

 up again by Spallanzani (1783) with complete success, and he 

 confirmed the discovery of Reaumur, and further demonstrated 

 the possibility of artificial digestion in vitro, without the inter- 

 vention of mechanical factors. He suspected the presence in the 

 gastric juice of a ferment of neutral reaction, as discovered by 

 Schwann in 1837, a ferment on which the solvent power of the 

 gastric juice depends. Lastly he made a most important dis- 

 covery from the standpoint of medicine and hygiene, i.e. the 

 non-putrefaction of gastric juice, to which is due its sterilising 

 action 011 the foods introduced ; this being possibly, as we shall 

 see, the most important function of the acid secretion of the 

 stomach. 



The Congress convened at Paris in 1823 by the Academic des 

 Sciences with the object " de determiner par une serie d'experiences 

 chimiques et physiologiques, quels sont les phenomenes qui se 



