HUMAN BIOLOGY

198 Chapter 11

bolus Ball of chewed and
swallowed food.

palate Roof of the mouth.

salivary amylase Enzyme
in saliva that begins the
chemical digestion of
starch.

salivary glands Glands
that produce saliva, a mix of
water, enzymes, and other
substances.

Chewing and swallowing: Food processing Begins

n    Food processing begins the moment food enters your

mouth, where enzymes begin chemical digestion of

starches.

n    Link to Carbohydrates 2.9

the teeth tear and grind bulk food

into smaller chunks

In the oral cavity, or mouth, the food you eat begins to be

broken apart by chewing. Most adults have thirty-two teeth

(Figure 11.4A). Young children have just twenty so-called

primary teeth. A tooth’s crown (Figure 11.4B) is coated with

tooth enamel. It consists of hardened calcium deposits and

is the hardest substance in the body.

The enamel covers a living, bonelike

layer called dentin. Dentin and an

inner pulp extend into the root. The

pulp cavity contains blood vessels

and nerves.

The shape of a tooth fits its func-

tion. Chisel-shaped incisors bite off

chunks of food, and cone-shaped

canines (cuspids) tear it. Premolars

and molars, with broad crowns and

rounded cusps, grind it.

Fig u r e 11.4 The structure of a tooth, including its shape,

fits its function. (© Cengage Learning)

crown

gingiva

(gum)

root

enamel

dentin

pulp cavity

(contains

nerves and

blood vessels)

bone

periodontal

membrane

B

root canal

molars

(12)

premolars

(8)

canines (4)

incisors

(8)

A lower jaw upper jaw

Fig u r e 11.5 Salivary glands release saliva into various

regions of the mouth. (© Cengage Learning)

parotid gland

submandibular

gland

sublingual gland

11.2

enzymes in saliva begin the chemical

digestion of food

Chewing mixes food with saliva from several salivary

glands (Figure 11.5). A large parotid gland nestles just in

front of each ear. Submandibular glands lie just below the

lower jaw in the floor of the mouth, and sublingual glands

are under your tongue. The tongue itself is skeletal muscle

covered by a membrane. As described in Chapter 14, its

taste receptors respond to dissolved chemicals.

Saliva is mostly water, but it includes other substances.

An important one is the enzyme salivary amylase, which

breaks down starch; chew on a soda cracker and you can

feel it becoming mushy as salivary amylase goes to work.

A buffer, bicarbonate (HCO 32 ), keeps the pH of your mouth

between 6.5 and 11.5, a range within which salivary amy-

lase can function. Saliva also contains mucins, proteins that

help bind food bits into a lubricated ball. Once it is swal-

lowed, this ball of chewed food is called a bolus (bow-lus).

Starch digestion continues in the stomach until acids there

inactivate salivary amylase.

Behind the upper teeth is a bone-reinforced section of

the palate—the roof of the mouth. It provides a hard sur-

face against which the tongue can press food it is mixing

with saliva. Tongue muscle contractions force the bolus

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