1
18
UNIT 1
Organization of the Body
part of the balloon that clings to your fist can be compared to
the visceral serosa clinging to an organ’s external surface. Te
outer wall of the balloon then represents the parietal serosa that
lines the walls of the cavity. (However, unlike the balloon, the
parietal serosa is never exposed but is always fused to the cavity
wall.) In the body, the serous membranes are separated not by
air but by a thin layer of lubricating fluid, called
serous fluid
,
which is secreted by both membranes. Although there is a po-
tential space between the two membranes, the barely present,
slitlike cavity is filled with serous fluid.
Te slippery serous fluid allows the organs to slide without
friction across the cavity walls and one another as they carry out
their routine functions. Tis freedom of movement is especially
important for mobile organs such as the pumping heart and the
churning stomach.
Te serous membranes are named for the specific cavity
and organs with which they are associated. For example, as
shown in Figure 1.10b, the
parietal pericardium
lines the peri-
cardial cavity and folds back as the
visceral pericardium
, which
covers the heart. Likewise, the
parietal pleurae
(ploo
9
re) line
the walls of the thoracic cavity, and the
visceral pleurae
cover
the lungs. Te
parietal peritoneum
(per
0
ĭ-to-ne
9
um) is as-
sociated with the walls of the abdominopelvic cavity, while
the
visceral peritoneum
covers most of the organs within that
cavity. (Te pleural and peritoneal serosae are illustrated in
Figure 4.11c on p. 141.)
these regions are not physically separated by a muscular or
membrane wall. Its superior portion, the
abdominal cavity
,
contains the stomach, intestines, spleen, liver, and other organs.
Te inferior part, the
pelvic cavity
, lies in the bony pelvis and
contains the urinary bladder, some reproductive organs, and the
rectum. Te abdominal and pelvic cavities are not aligned with
each other. Instead, the bowl-shaped pelvis tips away from the
perpendicular as shown in Figure 1.9a.
Homeostatic Imbalance
1.1
When the body is subjected to physical trauma (as in an auto-
mobile accident), the abdominopelvic organs are most vulner-
able. Why? Tis is because the walls of the abdominal cavity are
formed only by trunk muscles and are not reinforced by bone.
Te pelvic organs receive a somewhat greater degree of protec-
tion from the bony pelvis.
Membranes in the Ventral Body Cavity
Te walls of the ven-
tral body cavity and the outer surfaces of the organs it contains
are covered by a thin, double-layered membrane, the
serosa
(se-
ro
9
sah), or
serous membrane
. Te part of the membrane lining
the cavity walls is called the
parietal serosa
(pah-ri
9
ĕ-tal;
parie
5
wall). It folds in on itself to form the
visceral serosa
, covering
the organs in the cavity.
You can visualize the relationship between the serosal layers
by pushing your fist into a limp balloon
(Figure 1.10a)
. Te
Cranial cavity
(contains brain)
Dorsal body
cavity
Vertebral cavity
(contains spinal
cord)
Cranial
cavity
Superior
mediastinum
Pericardial
cavity within
the mediastinum
Pleural
cavity
Vertebral
cavity
Abdomino-
pelvic
cavity
Ventral body
cavity
(thoracic and
abdominopelvic
cavities)
Abdominal cavity
(contains digestive
viscera)
Diaphragm
Pelvic cavity
(contains urinary
bladder, reproductive
organs, and rectum)
Thoracic
cavity
(contains
heart and
lungs)
(a) Lateral view
(b) Anterior view
Dorsal body cavity
Ventral body cavity
Figure 1.9
Dorsal and ventral body cavities and their subdivisions.
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