14
UNIT 1
Organization of the Body
1
Check Your Understanding
12.
What is the anatomical position? Why is it important that
you
learn this position?
13.
The axillary and acromial regions are both in the general area
of the shoulder. Where specifically is each located?
14.
What type of cut would separate the brain into anterior and
posterior parts?
For answers, see Appendix H.
Body Cavities and Membranes
Locate and name the major body cavities and their
subdivisions and associated membranes, and list the major
organs contained within them.
Name the four quadrants or nine regions of the
abdominopelvic cavity and list the organs they contain.
Anatomy and physiology textbooks typically describe two sets
of internal body cavities called the dorsal and ventral body cavi-
ties. Tese cavities are closed to the outside and provide differ-
ent degrees of protection to the organs contained within them.
Because these two cavities differ in their mode of embryonic
development, and their lining membranes, the dorsal body cav-
ity is not recognized as such in many anatomical references.
However, the idea of two sets of internal body cavities is a useful
learning concept and we use it here.
Dorsal Body Cavity
Te
dorsal body cavity
, which protects the fragile nervous sys-
tem organs, has two subdivisions (
Figure 1.9
, on p. 18, gold
areas). Te
cranial cavity
, in the skull, encases the brain. Te
vertebral
, or
spinal
,
cavity
, which runs within the bony verte-
bral column, encloses the delicate spinal cord. Te spinal cord
is essentially a continuation of the brain, and the cranial and
spinal cavities are continuous with one another.
Ventral Body Cavity
Te more anterior and larger of the closed body cavities is the
ventral body cavity
(Figure 1.9, deep-red areas). Like the dorsal
cavity, it has two major subdivisions, the
thoracic cavity
and the
abdominopelvic cavity
. Te ventral body cavity houses internal
organs collectively called the
viscera
(vis
9
er-ah;
viscus
5
an or-
gan in a body cavity), or visceral organs.
Te superior subdivision, the
thoracic cavity
(tho-ras
9
ik),
is surrounded by the ribs and muscles of the chest. Te tho-
racic cavity is further subdivided into lateral
pleural cavities
(ploo
9
ral), each enveloping a lung, and the medial
mediasti-
num
(me
0
de-ah-sti
9
num). Te mediastinum contains the
peri-
cardial cavity
(per
0
ĭ-kar
9
de-al), which encloses the heart, and
it also surrounds the remaining thoracic organs (esophagus,
trachea, and others).
Te thoracic cavity is separated from the more inferior
ab-
dominopelvic cavity
(ab-dom
9
ĭ-no-pel
9
vic) by the diaphragm,
a dome-shaped muscle important in breathing. Te abdomi-
nopelvic cavity, as its name suggests, has two parts. However,
Anatomical Variability
Although we use common directional and regional terms to
refer to all human bodies, you know from observing the faces
and body shapes of people around you that we humans differ
in our external anatomy. Te same kind of variability holds for
internal organs as well. In some bodies, for example, a nerve or
blood vessel may be somewhat out of place, or a small muscle
may be missing. Nonetheless, well over 90% of all structures
present in any human body match the textbook descriptions.
We seldom see extreme anatomical variations because they are
incompatible with life.
Body Planes and Sections
For anatomical studies, the body is o±en cut, or
sectioned
, along
a flat surface called a
plane
. Te most frequently used body
planes are
sagittal
,
frontal
, and
transverse
planes, which lie at
right angles to one another
(Figure 1.8)
. A section is named for
the plane along which it is cut. Tus, a cut along a sagittal plane
produces a sagittal section.
A
sagittal plane
(saj
9
ĭ-tal; “arrow”) is a vertical plane that
divides the body into right and le± parts. A sagittal plane that
lies exactly in the midline is the
median plane
, or
midsagittal
plane
(Figure 1.8c). All other sagittal planes, offset from the
midline, are
parasagittal planes
(
para
5
near).
Frontal planes
, like sagittal planes, lie vertically. Frontal
planes, however, divide the body into anterior and posterior
parts (Figure 1.8a). A frontal plane is also called a
coronal plane
(kŏ-ro
9
nal; “crown”).
A
transverse
, or
horizontal
,
plane
runs horizontally from
right to le±, dividing the body into superior and inferior parts
(Figure 1.8b). Of course, many different transverse planes exist,
at every possible level from head to foot. A transverse section is
also called a
cross section
.
Oblique sections
are cuts made diagonally between the
horizontal and the vertical planes. Because oblique sections are
o±en confusing and difficult to interpret, they are seldom used.
At the bottom of Figure 1.8, you can see examples of magnetic
resonance imaging (MRI) scans that correspond to the three
different sections shown in the figure. Clinically, the ability to
interpret sections made through the body, especially transverse
sections, is important. Additionally, the new medical imaging
devices (
A Closer Look
, pp. 16–17) produce sectional images
rather than three-dimensional images.
It takes practice to determine or figure out an object’s over-
all shape from sectioned material. A cross section of a banana,
for example, looks like a circle and gives no indication of the
whole banana’s crescent shape. Likewise, sectioning the body
or an organ along different planes o±en results in very different
views. For example, a transverse section of the body trunk at
the level of the kidneys would show kidney structure in cross
section very nicely. A frontal section of the body trunk would
show a different view of kidney anatomy, and a midsagittal sec-
tion would miss the kidneys completely. With experience, you
will gradually learn to relate two-dimensional sections to three-
dimensional shapes.
(Text continues on p. 18.)
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