The Skeletal System

AuthorSamuel D. Hodge, Jr./Jack E. Hubbard
ProfessionSkilled litigator, is chair of the department of legal studies at Temple University/Professor of Neurology at the University of Minnesota
The Skeletal System
I have no history but
the length of my
Robin Skelton
The spine is a complex structure consisting of 26 bones that provide this region of the
body with its structure and support. In addition, these hard structures protect the spinal
cord.1 These bones, known as vertebrae, are part of the skeletal system. There is much more
to this system of the body, however, than just giving the body it’s shape. In fact, there are
a number of facts about bones that may not be widely known. For example, bone is six
times stronger than a piece of steel of the same weight. The smallest bone in the body is
the stapes (stirrup) located in the middle ear, and it is the size of a grain of rice. On the
other end of the spectrum, the femur is the longest bone in the body, and it constitutes
one-fourth of a person’s height.2 While humans have 206 bones that take years to mature,
half are located in the feet and hands.
This chapter will provide an overview of the anatomy of the skeletal system, which
consists of the bones, their associated cartilages, and the joints.3 It will also focus on the
diseases and traumas to these structures.
The surgical specialist who treats a bone injury is an orthopedic surgeon. As noted in
Wexler v. Hecht, “orthopedics is defined as that branch which is specifically concerned with
the preservation and restoration of the function of the skeletal system, its articulations
and associated structure. Thus, the approach an orthopedic surgeon will take to a patient
will be guided by his understanding of the patient’s entire skeletal system, including the
skeletal system articulations and associated structures.”4 The medical specialty dealing
mainly with bone and joint diseases is rheumatology.
People usually associate bones with the remnants of the body after death, but bones
are living tissue that serves a number of important functions. While we start with 300
bones at birth, some of these structures fuse together, and as we age, only 206 bones
of varying sizes and shapes remain. The bones of the body, along with their associated
supporting tissues, are called the skeletal system. The skeletal system actually has three
different functions—mechanical, protective, and physiological. The mechanical function is
to provide a rigid infrastructure to anchor muscles, allowing them to move the parts of
the body across joints like levers. The best example of the protective function of the skeletal
system is the skull protecting the brain or the spine protecting the spinal cord. The rib
cage and the pelvis also protect internal organs. The physiologic function of the skeletal
system involves serving as a storage repository for minerals, such as calcium, as well as
producing blood cells within the bone marrow.
To accomplish these functions, the skeletal system consists of three different com-
ponents: cartilage, joints, and bone.
The role of cartilage is to provide a pliable support for the bony skeleton such as exemplified
during childbirth. The birthing process requires an infant’s body to be flexible because of
the narrow passage through which the baby must pass. Therefore, bone frequently starts
out as hyaline cartilage and nonfused bones. An examination of a baby’s head following
birth reveals six soft spots where the bones of the skull have not yet fused together. These
areas, called fontanelles, allow the skull to flex as the head passes thorough the small
birthing canal. The anterior fontanelle is the largest opening and can be easily palpated
at the top of the infant’s head. Figure 3-1. As growth takes place, however, the cartilages
scattered throughout the body harden into bone and fuse together in a variety of locations
to form rigid structures whose unified presence makes up the skeleton. This process
is known as ossification and continues throughout life, especially with bone remodeling
following a fracture. Bones do vary as to when their epiphyseal plates ossify with fusion of
the bone. A baby’s fontanelles, for instance, will turn solid by age two in order to provide a
solid casing for the brain.5 The vertebrae ossify when a person is around 25 years old; the
scapula (shoulder blade) ossifies at 18 to 20 years old. Once the bone ossifies, its length
does not change but its diameter can thicken through exercise.
Cartilage is a resilient and flexible type of connective tissue scattered throughout the
body that forms part of the skeletal system where motion occurs. Not only does it act as
a cushion between the bones of a joint, but it also provides support and shape to a body
part. Because cartilage lacks a blood supply, it must obtain its nutrients and oxygen by
long-range diffusion. Examples of structures that contain this tissue include the knee,
intervertebral disks, ears, and nose.6 Figure 3-2.
There are several types of cartilage including hyaline cartilage, elastic cartilage, and
fibrocartilage. Hyaline cartilage forms the majority of the cartilage in the body, and it
surrounds the ends of the bones in the joints, helping the joints move smoothly. Elastic
cartilage is more flexible because it contains elastin fibers and offers a balance of structure
and flexibility. This kind of cartilage can be found in such locations as the outer ear and
the larynx. Fibrocartilage, the strongest cartilage in the body, includes the intervertebral
disks as well as tendons and ligaments attached to bones.7 Figure 3-3.
Neonatal skull.
The fontanelles are cartilage sheets
where the infant skull bones have
not joined. The anterior fonta-
nelle, a baby’s “soft spot,” is the
A—side view
B—top view
Joints are regions where two bones come together. At a joint, one bone is said to articulate,
or come together, with another bone. The joint surface where two bones make contact,
usually covered with hyaline cartilage, is called an articular surface. The function of most
joints is to provide movement. This is demonstrated by the bones of the knee and the
toes, which are called hinged joints. The shoulder is the most flexible joint in the body
and allows the arm to be rotated and moved in about 1,600 different directions. This
amazing feat is possible because the bones of the shoulder form a ball-and-socket joint.
Other joints provide for no or very little motion, such as the skull sutures where the bones
of the skull have fused together. Figure 3-4.
The Anatomy of Bone
The perception that bone is a collection of dead tissue is incorrect. This belief, however,
is fostered by the definition of the word bone, which is derived from the Greek skeletos.
This term means “dead” or “dry bone.” In reality, bone is dynamic living tissue that serves
a number of purposes, such as:
Bone provides the structure and framework of the body. One merely has to
visualize the grotesque shape of an arm or leg following a serious fracture to
appreciate this fact.
Nasal cartilage.
Most of the structure of the nose is formed by cartilage covered by
Ligaments of the knee.
Ligaments are an example of fibrocartilage, which is a tough and
elastic material.

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