he 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.

Section 26. It meets now with the bile, a greenish fluid secreted by
that large and conspicuous gland the liver. The bile is not simply a
digestive secretion, like the saliva or the gastric juice; it contains
matters destined to mix in, and after a certain amount of change to
be passed out of the body with, the faeces; among these
substances, of which some portion is doubtless excretory, are
compounds containing sulphur-- the bile salts. There is also a
colouring matter, bili verdin, which may possibly also be excretory. If
the