ion to see that the salt has not been changed chemically.
Now add a little powdered limestone to the water and stir as before.
Observe that the limestone does not dissolve. Then add some hydrochloric
acid and observe the result. State the part played by the acid and by the
water in dissolving the limestone. Apply to the digestion of the different
classes of foods.

CHAPTER X - ORGANS AND PROCESSES OF DIGESTION

The organs of digestion are adapted to the work of dissolving the foods by
both their structure and arrangement. Most of them consist either of tubes
or cavities and these are so connected, one with the other, as to form a
continuous passageway entirely through the body. This passageway is known
as

*The Alimentary Canal. *--The alimentary canal has a length of about thirty
feet and, while it begins at the mouth, all but about eighteen inches of
it is found in the abdominal cavity. On account of its length it lies for
the most part in coils, the two largest ones being known as the small
intestine and the large intestine. Connected with the alimentary canal are
the glands that supply the liquids for acting on the food. The divisions
of the canal and most of the glands that empty liquids into it are shown
in Fig. 63 and named in the table below:

[Table]

*Coats of the Alimentary Canal.*--The walls of the alimentary canal, except
at the mouth, are distinct from the surrounding tissues and consist in
most places of at least three layers, or coats, as follows:

[Fig. 63]

Fig. 63--*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3.
Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7.
Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12.
Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid
flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and
pancreas. 19. Liver.

Diagram does not show comparative length of the small intestine.

1. An _inner coat_, or lining, known as the mucous membrane. This membrane
is not confined to the alimentary canal, but lines, as we have seen, the
different air passages. It covers, in fact, all those internal surfaces of
the body that connect with the external surface. It derives its name from
the substance which it secretes, called _mucus_. In structure it resembles
the skin, being