Chapter VII: Section 23: After mastication, the food is worked by the tongue and

cheeks into a saliva-soaked "bolus" and swallowed. The passage down the oesophagus is called deglutition. In the stomach it comes under the influence of the gastric juice, formed in little glandular pits in the stomach wall-- the gastric (Figure VIII. Sheet 3) and pyloric glands. This fluid is distinctly acid, its acidity being due to about one-tenth per cent {of a hundred} of hydrochloric acid, and it therefore stops any further action of the ptyalin, which can act only on neutral or slightly alkaline fluids. The gastric juice does not act on carbo-hydrates or hydrocarbons to any very noticeable degree. Its essential property is the conversion of proteids into peptones, and the ferment by which this is effected is called pepsin. Milk contains a peculiar soluble proteid, called casein, which is precipitated by a special ferment, the rennet-ferment, and the insoluble proteid, the curd, thus obtained is then acted on by the pepsin. In the manufacture of cheese, the rennetferment obtained, from the stomach of a calf is used to curdle the milk.

Section 24. After the food has undergone digestion in the stomach it passes into the duodenum, the U-shaped loop of intestine immediately succeeding the stomach. The duodenum is separated from the stomach by a ring-like muscular valve, the pylorus; this valve belongs to the class of muscles called sphincters, which, under ordinary circumstances, are closed, but which relax to open the circular central aperture. The valve at the anus, which retains the faeces, is another instance of a sphincter.

Section 25. The food at this stage is called chyme; it is an acid and soup-like fluid-- acid through the influence of the gastric juice. The temperature of the animal's body is sufficiently high to keep most of the fat in the food melted and floating in oily drops; much of the starch, has been changed to sugar, and the solid proteids to soluble peptones, but many fragments of material still float unchanged.