By the end of this section, you will be able to:
- Demonstrate the anatomical position
- Describe the human body using directional and regional terms
- Identify three planes most commonly used in the study of anatomy
- Distinguish between the posterior (dorsal) and the anterior
(ventral) body cavities, identifying their subdivisions and
representative organs found in each
- Describe serous membrane and explain its function
Anatomists and health care providers use
terminology that can be bewildering to the uninitiated. However, the
purpose of this language is not to confuse, but rather to increase
precision and reduce medical errors. For example, is a scar “above the
wrist” located on the forearm two or three inches away from the hand? Or
is it at the base of the hand? Is it on the palm-side or back-side? By
using precise anatomical terminology, we eliminate ambiguity. Anatomical
terms derive from ancient Greek and Latin words. Because these
languages are no longer used in everyday conversation, the meaning of
their words does not change.
Anatomical terms are made up of roots, prefixes,
and suffixes. The root of a term often refers to an organ, tissue, or
condition, whereas the prefix or suffix often describes the root. For
example, in the disorder hypertension, the prefix “hyper-” means “high”
or “over,” and the root word “tension” refers to pressure, so the word
“hypertension” refers to abnormally high blood pressure.
Anatomical Position
To further increase precision, anatomists
standardize the way in which they view the body. Just as maps are
normally oriented with north at the top, the standard body “map,” or anatomical position,
is that of the body standing upright, with the feet at shoulder width
and parallel, toes forward. The upper limbs are held out to each side,
and the palms of the hands face forward as illustrated in Figure 1.12.
Using this standard position reduces confusion. It does not matter how
the body being described is oriented, the terms are used as if it is in
anatomical position. For example, a scar in the “anterior (front) carpal
(wrist) region” would be present on the palm side of the wrist. The
term “anterior” would be used even if the hand were palm down on a
table.
A body that is lying down is described as either prone or supine. Prone describes a face-down orientation, and supine
describes a face up orientation. These terms are sometimes used in
describing the position of the body during specific physical
examinations or surgical procedures.
Regional Terms
The human body’s numerous regions have specific terms to help increase precision (see Figure 1.12).
Notice that the term “brachium” or “arm” is reserved for the “upper
arm” and “antebrachium” or “forearm” is used rather than “lower arm.”
Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is
reserved for the portion of the lower limb between the knee and the
ankle. You will be able to describe the body’s regions using the terms
from the figure.
Directional Terms
Certain directional anatomical terms appear throughout this and any other anatomy textbook (Figure 1.13).
These terms are essential for describing the relative locations of
different body structures. For instance, an anatomist might describe one
band of tissue as “inferior to” another or a physician might describe a
tumor as “superficial to” a deeper body structure. Commit these terms
to memory to avoid confusion when you are studying or describing the
locations of particular body parts.
- Anterior (or ventral) Describes the front or direction toward the front of the body. The toes are anterior to the foot.
- Posterior (or dorsal) Describes the back or direction toward the back of the body. The popliteus is posterior to the patella.
- Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris.
- Inferior (or caudal)
describes a position below or lower than another part of the body
proper; near or toward the tail (in humans, the coccyx, or lowest part
of the spinal column). The pelvis is inferior to the abdomen.
- Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits.
- Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe.
- Proximal
describes a position in a limb that is nearer to the point of attachment
or the trunk of the body. The brachium is proximal to the antebrachium.
- Distal
describes a position in a limb that is farther from the point of
attachment or the trunk of the body. The crus is distal to the femur.
- Superficial describes a position closer to the surface of the body. The skin is superficial to the bones.
- Deep describes a position farther from the surface of the body. The brain is deep to the skull.
Body Planes
A section
is a two-dimensional surface of a three-dimensional structure that has
been cut. Modern medical imaging devices enable clinicians to obtain
“virtual sections” of living bodies. We call these scans. Body sections
and scans can be correctly interpreted, however, only if the viewer
understands the plane along which the section was made. A plane
is an imaginary two-dimensional surface that passes through the body.
There are three planes commonly referred to in anatomy and medicine, as
illustrated in Figure 1.14.
- The sagittal plane
is the plane that divides the body or an organ vertically into right
and left sides. If this vertical plane runs directly down the middle of
the body, it is called the midsagittal or median plane. If it divides
the body into unequal right and left sides, it is called a parasagittal
plane or less commonly a longitudinal section.
- The frontal plane
is the plane that divides the body or an organ into an anterior (front)
portion and a posterior (rear) portion. The frontal plane is often
referred to as a coronal plane. (“Corona” is Latin for “crown.”)
- The transverse plane
is the plane that divides the body or organ horizontally into upper and
lower portions. Transverse planes produce images referred to as cross
sections.
Body Cavities and Serous Membranes
The body maintains its internal organization by
means of membranes, sheaths, and other structures that separate
compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure 1.15).
These cavities contain and protect delicate internal organs, and the
ventral cavity allows for significant changes in the size and shape of
the organs as they perform their functions. The lungs, heart, stomach,
and intestines, for example, can expand and contract without distorting
other tissues or disrupting the activity of nearby organs.
Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities
The posterior (dorsal) and anterior (ventral)
cavities are each subdivided into smaller cavities. In the posterior
(dorsal) cavity, the cranial cavity houses the brain, and the spinal cavity
(or vertebral cavity) encloses the spinal cord. Just as the brain and
spinal cord make up a continuous, uninterrupted structure, the cranial
and spinal cavities that house them are also continuous. The brain and
spinal cord are protected by the bones of the skull and vertebral column
and by cerebrospinal fluid, a colorless fluid produced by the brain,
which cushions the brain and spinal cord within the posterior (dorsal)
cavity.
The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (see Figure 1.15). The thoracic cavity
is the more superior subdivision of the anterior cavity, and it is
enclosed by the rib cage. The thoracic cavity contains the lungs and the
heart, which is located in the mediastinum. The diaphragm forms the
floor of the thoracic cavity and separates it from the more inferior
abdominopelvic cavity. The abdominopelvic cavity
is the largest cavity in the body. Although no membrane physically
divides the abdominopelvic cavity, it can be useful to distinguish
between the abdominal cavity, the division that houses the digestive
organs, and the pelvic cavity, the division that houses the organs of
reproduction.
Abdominal Regions and Quadrants
To promote clear communication, for instance about
the location of a patient’s abdominal pain or a suspicious mass, health
care providers typically divide up the cavity into either nine regions
or four quadrants (Figure 1.16).
The more detailed regional approach
subdivides the cavity with one horizontal line immediately inferior to
the ribs and one immediately superior to the pelvis, and two vertical
lines drawn as if dropped from the midpoint of each clavicle
(collarbone). There are nine resulting regions. The simpler quadrants
approach, which is more commonly used in medicine, subdivides the cavity
with one horizontal and one vertical line that intersect at the
patient’s umbilicus (navel).
Membranes of the Anterior (Ventral) Body Cavity
A serous membrane
(also referred to a serosa) is one of the thin membranes that cover the
walls and organs in the thoracic and abdominopelvic cavities. The
parietal layers of the membranes line the walls of the body cavity
(pariet- refers to a cavity wall). The visceral layer of the membrane
covers the organs (the viscera). Between the parietal and visceral
layers is a very thin, fluid-filled serous space, or cavity (Figure 1.17).
There are three serous cavities and their associated membranes. The pleura is the serous membrane that encloses the pleural cavity; the pleural cavity surrounds the lungs. The pericardium is the serous membrane that encloses the pericardial cavity; the pericardial cavity surrounds the heart. The peritoneum
is the serous membrane that encloses the peritoneal cavity; the
peritoneal cavity surrounds several organs in the abdominopelvic cavity.
The serous membranes form fluid-filled sacs, or cavities, that are
meant to cushion and reduce friction on internal organs when they move,
such as when the lungs inflate or the heart beats. Both the parietal and
visceral serosa secrete the thin, slippery serous fluid located within
the serous cavities. The pleural cavity reduces friction between the
lungs and the body wall. Likewise, the pericardial cavity reduces
friction between the heart and the wall of the pericardium. The
peritoneal cavity reduces friction between the abdominal and pelvic
organs and the body wall. Therefore, serous membranes provide additional
protection to the viscera they enclose by reducing friction that could
lead to inflammation of the organs.