continuous with the skin where cavities open to the
surface. It is made up of two layers--a thick underlayer which contains
blood vessels, nerves, and glands, and a thin surface layer, called the
_epithelium._ The epithelium, like the cuticle, is without blood vessels,
nerves, or glands.

2. A _middle coat_, which is muscular and which forms a continuous layer
throughout the canal, except at the mouth. (Here its place is taken by the
strong muscles of mastication which are separate and distinct from each
other.) As a rule the muscles of this coat are involuntary. They surround
the canal as thin sheets and at most places form two distinct layers. In
the inner layer the fibers encircle the canal, but in the outer layer they
run longitudinally, or lengthwise, along the canal.(57)

3. An _outer_ or _serous coat_, which is limited to those portions of the
canal that occupy the abdominal cavity. This coat is not found above the
diaphragm. It is a part of the lining membrane of the cavity of the
abdomen, called

[Fig. 64]

Fig. 64--*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3.
Small intestine. 4. Pancreas.

*The Peritoneum.*--The peritoneum is to the abdominal cavity what the
pleura is to the thoracic cavity. It forms the outer covering for the
alimentary canal and other abdominal organs and supplies the inner lining
of the cavity itself. It is also the means of holding these organs in
place, some of them being suspended by it from the abdominal walls (Fig.
64). By the secretion of a small amount of liquid, it prevents friction of
the parts upon one another.

*Digestive Glands.*--The glands which provide the different fluids for
acting on the foods derive their constituents from the blood. They are
situated either in the mucous membrane or at convenient places outside of
the canal and pass their liquids into it by means of small tubes, called
ducts. In the canal the food and the digestive fluids come in direct
contact--a condition which the dissolving processes require. Each kind of
fluid is secreted by a special kind of gland and is emptied into the canal
at the place where it is needed.

*The Digestive Processes.*--Digestion is accomplished by acting upon the
food in different ways, as it is passed along the canal, with the final
result of reducing it to the form of a solution. Several distinct
processes are necessary and they occur in such