Organization of the Body
Check Your Understanding
What is the anatomical position? Why is it important that
learn this position?
The axillary and acromial regions are both in the general area
of the shoulder. Where speciﬁcally is each located?
What type of cut would separate the brain into anterior and
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
dorsal body cavity
, which protects the fragile nervous sys-
tem organs, has two subdivisions (
, on p. 18, gold
, in the skull, encases the brain. Te
, 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
. Te ventral body cavity houses internal
organs collectively called the
gan in a body cavity), or visceral organs.
Te superior subdivision, the
is surrounded by the ribs and muscles of the chest. Te tho-
racic cavity is further subdivided into lateral
ral), each enveloping a lung, and the medial
num). Te mediastinum contains the
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
vic) by the diaphragm,
a dome-shaped muscle important in breathing. Te abdomi-
nopelvic cavity, as its name suggests, has two parts. However,
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
a ﬂat surface called a
. Te most frequently used body
planes, which lie at
right angles to one another
. A section is named for
the plane along which it is cut. Tus, a cut along a sagittal plane
produces a sagittal section.
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
(Figure 1.8c). All other sagittal planes, oﬀset from the
, 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
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
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
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-
(Text continues on p. 18.)