Essentials of Anatomy and Physiology

387

BOX16–5 FIBER

no protective effect of fiber against colon cancer.

What we can say for sure is that fiber may not be

the only dietary or environmental factor involved.

Claims that high-fiber diets directly lower blood

levels of cholesterol and fats are not supported by

definitive clinical or experimental studies. One pos-

sible explanation may be that a person whose diet

consists largely of high-fiber foods simply eats less

of the foods high in cholesterol and fats, and this is

the reason for that person’s lower blood levels of

fats and cholesterol.

Should people try to make great changes in their

diets? Probably not, not if they are careful to limit

fat intake and to include significant quantities of

vegetables and fruits. Besides the possible benefits

of fiber, unprocessed plant foods provide important

amounts of vitamins and minerals.

Fiberis a term we use to refer to the organic mate-

rials in the cell walls of plants. These are mainly cel-

lulose and pectins. The role of dietary fiber and

possible benefits that a high-fiber diet may provide

are currently the focus of much research. It is

important to differentiate what is known from what

is, at present, merely speculation.

Many studies have shown that populations

(large groups of people, especially those of different

cultures) who consume high-fiber diets tend to

have a lower frequency of certain diseases. These

include diverticulitis, colon cancer, coronary artery

disease, diabetes, and hypertension. Such diseases

are much more common among populations

whose diets are low in vegetables, fruits, and whole

grains, and high in meat, dairy products, and

processed foods. In contrast, a 2005 study showed

Rectum

Anal canal

Anal

columns

Longitudinal muscle

Circular muscle

Rectal fold

Levator ani

muscle

Internal anal

sphincter

External anal

sphincter

Anus

A

B

Figure 16–10. (A) Internal and external anal sphincters shown in a frontal section

through the lower rectum and anal canal. (B) Position of rectum and anal canal relative to

pelvic bone.

QUESTION:The internal anal sphincter is a continuation of which part of the rectum?