A Primer on Anatomy: An Overview of the Body
Author | Samuel D. Hodge, Jr./Jack E. Hubbard |
Profession | Skilled litigator, is chair of the department of legal studies at Temple University/Professor of Neurology at the University of Minnesota |
Pages | 3-66 |
A Primer on Anatomy:
An Overview of the Body
The human body is
composed of a number
of individual parts, of
diverse nature, each
one of which is in itself
extremely complex.
Baruch Spinoza
(1632–1677)
1
Introduction
The spine is a complex and detailed structure, a remarkable assortment of vertebrae,
intervertebral discs, a spinal cord, and nerves.1 The medical terms used to describe its
structure, function, and orientation, however, are part of a broader language used to
explain the human form. The elements and expressions utilized to describe them origi-
nate and have their foundation in a science known as anatomy.
Anatomy is the study of the structure of the body. Once we understood how the body
was put together, we began to understand how those parts function (physiology), how
they look when they are diseased (pathology), their chemical makeup (biochemistry),
how substances affect their function (pharmacology), and how to heal the body by re-
arranging it (surgery).
This chapter serves as a primer in preparation for the remainder of this text on the
spine. It may also be used as a quick reference guide for each of the systems of the body.
Definitions
A proper starting point of any medical analysis requires an understanding of certain
basic anatomical terms, such as cell, tissue, organ, and organ system. A cell is the smallest
living component of a body system, microscopic in size and enclosed in a membrane.
Figure 1-1. A cell has a nucleus, which contains DNA (deoxyribonucleic acid), those
inherited strands that direct protein production. In turn, the nucleus floats within the
surrounding cytoplasm, a watery fluid that fills the cell and holds the organelles, those
“little organs” that help run the metabolic machinery of the cell such as mitochondria,
ribosomes, endoplasmic reticulum, and the Golgi apparatus.
The cell is often the functional unit of a system; for example, a neuron is the functional
unit of the nervous system, and a nephron is the functional unit of the kidney. Tissue (from
the Latin texere, “to weave”) consists of different cell types, each of which performs a
specific function. There are four major tissue types: nervous tissue electrically transmits
information; connective tissue serves as a matrix for support and is a filler material;
muscle produces movement by contraction; and epithelial tissue lines surfaces. An organ
is a collection of different tissues that work together for a specific purpose: the heart
pumps blood, the brain thinks and processes information, and the kidneys produce urine.
An organ system consists of several anatomical parts working together to perform a body
function. The organ systems of the body are the skeletal, muscular, nervous, circulatory,
respiratory, digestive, urinary, endocrine, reproductive, and integumentary systems. This
text is organized in an organ systems approach.
Other ways to learn about anatomy are through regional anatomy, which focuses on each
body part, such as the head or trunk; surface anatomy, which is limited to those structures
4 ◆ CHAPTER 1
that can be seen or palpated beneath the skin; and clinical anatomy, which observes how
anatomical relationships are affected by disease. Most anatomical texts blend some of
these approaches while primarily focusing on one main emphasis.
Anatomical Position and Planes
A group of important anatomical terms are those that deal with the direction and
orientation of the body. These words are frequently found in radiology and surgical
reports. To avoid confusion and misunderstanding, anatomical terminology has been
created to describe body location and position. The starting point is the anatomical
position, which represents the body standing erect and facing forward, with the feet
together and arms to the side of the body with the palms facing frontward. Figure 1-2.
The front of the body in the anatomical position is anterior (or ventral); the back is
posterior (or dorsal). For example, the nipples are anterior to the shoulder blades. The
midline is an imaginary line extending from the top of the head to the crotch that divides
the body into two equal halves. Lateral refers to a location away from the midline; medial
refers to a location toward the midline. For example, the shoulder is lateral to the neck.
With reference to the extremities, proximal refers to a point close to the trunk; distal
refers to something away from the trunk. For instance, the elbow is proximal to the
hand. Superior (cephalic) is toward the head; inferior (caudal) is toward the feet. Logically,
the hips are inferior to the shoulders. Superficial (external) refers to something near
the surface of the body; deep (internal) is away from the surface. Therefore, the skin is
superficial to the muscles.
In addition, certain planes divide the body along specific lines. These terms are
commonly found in radiology and operative reports. For instance, the median plane divides
the body into right and left halves. Figure 1-3. Sagittal planes run parallel to the median
plane and slice the body from one side to the other in a left-to-right or right-to-left
fashion. Frontal (coronal) planes are vertical planes that run at right angles to the sagittal
planes, thereby slicing the body from the front to the back. Transverse (axial) planes run
horizontally through the body at right angles to the sagittal and coronal planes. These
planes are similar to the slicing of a loaf of bread from the top to the bottom.
FIGURE 1-1.
A generalized body cell.
While cells appear in many dif-
ferent forms, they all have similar
structures; a few are shown here.
The cell is enclosed by a cell mem-
brane. The cytoplasm is a gel-like
fluid within the cell containing all
of the machinery of cell func-
tion. A number of organelles are
vital to proper cell function. The
nucleus, enclosed by a nuclear mem-
brane, contains the inherited DNA
that makes the person unique
and directs the production of that
person’s protein. The nucleolus
manufactures the ribosomal RNA
(ribonucleic acid). Ribosomes direct
protein synthesis. Endoplasmic
reticulum is an assembly line for
protein synthesis. Golgi apparatus
processes and packages proteins
and lipids manufactured by the
cell. Mitochondria supply the
power to run the cell’s metabolic
machinery.
A PRIMER ON ANATOMY: AN OVERVIEW OF THE BODY ◆ 5
FIGURE 1-2.
The anatomical position.
Facing forward (anterior) and
back (posterior). The midline is an
imaginary line bisecting into left
and right halves. Lateral refers to
anything away from the mid-
line; medial is directed toward
the midline. Proximal is directed
toward the core of the body; distal
is directed away from the body
core. Superior is toward the head;
inferior is toward the feet.
FIGURE 1-3.
Planes of the body.
Sagittal planes are those that run
parallel to the midline or median
plane. Frontal (coronal) planes
are vertically oriented and run
perpendicular to the sagittal
planes. Transverse (axial) planes
are oriented horizontally, running
perpendicular to the frontal and
sagittal planes. Oblique planes are
similar to transverse planes but
are at more of an angle.
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