Nielsen M. Atlas of Human Anatomy
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e skeletal system forms the internal framework
for the so tissues of the body. is is not a static framework, but a highly dynamic
internal sca olding. It is dynamic in many ways. On one hand, because of its jointed
design, it shows extreme exibility of movement when acted upon by muscles.
At another extreme, the cells of skeletal tissue are constantly monitoring and
changing the micro-structure of this amazing tissue called bone, providing
it with maximal strength, toughness, and resilience. In addition to its dy-
namic role of support, it also serves a protective role for many organs of
the body. is dynamic framework also exhibits a tremendous capacity
for growth and repair. It is a storehouse of calcium ions, ions that play a
signi cant role in many of the body’s functions.
e skeleton consists of 206 separate bones, ignoring various sesa-
moid bones and the fact that some bones represent the fusion of multiple
bones. ese bones range in size from the small ear ossicles measuring
a few millimeters in length to the large femur measuring up to y centi-
meters. e skeleton is divisible into two portions, the axial skeleton and the
appendicular skeleton. e axial skeleton includes the cranium, vertebral column,
ribs, and sternum. e appendicular skeleton consists of the bones of the limbs and
their girdles. e individual bones of the skeleton come in a variety of shapes. Some are
long and tubular, while others have the spread-winged appearance of a butter y. Bones
can be grouped into four shape categories. Although not that meaningful, the four
categories descriptively group the bones. e four shape categories are: long bones,
short bones, at bones, and irregular bones. Long bones are unique in hav-
ing a diaphysis or sha with a medullary cavity. e other bone types lack
this hollow tubular region. e short, at, and irregular bones are similar
in having outer plates of compact bone surrounding internal centers of
spongy bone. In general, long bones and short bones are found in the
appendicular skeleton, while at bones and irregular bones occur in
the axial skeleton. In the right hands, the skeleton can be a library of
information. Its markings, foramina, landmarks, and canals each tell a
story about the so tissues of the body. A strong foundation of skeletal
anatomy is an important starting point in understanding anatomy.
is chapter covers bone tissue and the general structure of bones and
the skeleton. In the two chapters that follow you will explore the two subdivi-
sions of the skeleton — the axial skeleton and the appendicular skeleton.
Skeletal System
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Find more information
about the skeletal system in
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REALANATOMY
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nected together in complex array around obvious spaces in the tissue. To the unaided eye this gives the bone a spongy appear-
ance. Bone marrow fi lls the spaces in the trabecular bone. The second type of bone tissue, compact bone, is very dense and
solid looking to the unaided eye. Compact bone forms the outer surface of all bones and can range in thickness from paper thin
to many centimeters thick. Microscopic analysis of this dense bone reveals that it has many microscopic spaces containing cells
and blood vessels in circular arrangements called osteons.
The tissue bone has two general forms — trabecular or spongy bone and compact bone.
Trabecular bone is an internal bone that always resides deep to the more dense compact
bone. Like its name implies, trabecular bone has many small beams of bone tissue con-
Bone Tissue
1 Trabecular bone
2 Compact bone
3 Trabecula
4 Bone marrow
5 Osteon
6 Central canal
7 Lacuna
8 Canaliculi
9 Lamella
10 Nucleus of osteocyte
Sectioned humerus
Anterior view, proximal half frontal section
Compact bone and callout of osteon
Transverse section, 100x and 400x
Trabecular bone
Frontal section
Trabecular bone
200x
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(intramembranous ossifi cation). In endochondral ossifi cation cartilaginous growth plates remain between developing bone
centers to allow a bone to increase in length and size. During an individual’s young life, the growth plates are evident on a
radiograph and are a clear indication that the individual is still growing.
Bone tissue forms during development by either replac-
ing cartilage tissue precursors (endochondral ossifi cation)
or by developing within mesenchymal connective tissue
Cartilage Growth Plate
1 Radial diaphysis
2 Radial epiphysis
3 Ulnar diaphysis
4 Ulnar epiphysis
5 Growth plate
6 Carpal bones
7 Metacarpal bones
8 Developing diaphysial bone
9 Zone of calcified cartilage
10 Zone of hypertrophied cartilage
11 Zone of proliferating cartilage
12 Zone of resting cartilage
Radiograph of the wrist region of a child
Posterior view
Growth plate
200x
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on their size and shape. Long bones, as their name suggests, are longer in one dimension than any other dimension. The
long bones range in size from the short phalanges of the digits to the long proximal humerus and femur of the limb skeletons.
Conversely, short bones are small, block-like bones. Like the long bones, short bones occur in the limb skeletons where they
form the bones of the wrist and ankle. Flat bones are plate-like bones and are common in the cranium. The fi nal category,
irregular bones, is a mixed group of bones that have a variety of shapes and locations within the skeleton.
The bones of the skeleton come in a variety of sizes and shapes. The form of each bone
emerges from its position and functional role in the skeletal system. In an effort to classify
the different bones of the body anatomists defi ne four general categories of bones based
Bone Types
Flat bones
Long bones
Irregular bones
Short bones
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occupies the core of the bone beneath the compact bone. Areas of compact bone covered by articular cartilage form smooth
subchondral compact bone surfaces. These subchondral bone surfaces mark the joint surfaces of bones. The photos below
illustrate the basic parts and features of a long bone.
All bones share basic features in common. Compact bone tissue
forms all the visible outer surface of the bone and can vary from a
paper-thin covering to a thick wall of bone. Trabecular bone tissue
Anatomy of a Bone
1 Epiphyses
2 Diaphysis
3 Metaphysis
4 Compact bone
5 Subchondral bone
6 Trabecular bone
7 Medullary cavity
8 Epiphysial line
Femur
Anterior view
Femur
Frontal section
Proximal end of femur
Frontal section
Distal end of femur
Frontal section
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mental process combines more than 500 bone-forming centers into the fi nal 206 bones of the skeleton. This page, the facing
page, and the page that follows depict changes in the skeleton from a newborn to an adult.
The fi rst appearance of the skeletal elements arises during the second month of embryonic life
when connective tissue and cartilage precursors to the bones arise. Slowly through fetal life,
childhood, puberty, and the teenage years the bones mature into their adult forms. This develop-
Skeleton
1 Cranial bones
2 Vertebral column
3 Ribs
4 Clavicle
5 Scapula
6 Humerus
7 Ulna
8 Radius
9 Carpals
10 Metacarpals
11 Phalanges
12 Os coxae
13 Femur
14 Patella
15 Tibia
16 Fibula
17 Tarsals
18 Metatarsals
Newborn skeleton
Posterior view
Newborn skeleton
Anterior view
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Adult skeleton
Posterior view
Adult skeleton
Anterior view
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Newborn skeleton
Lateral view
1 Cranial bones
2 Vertebral column
3 Ribs
4 Clavicle
5 Scapula
6 Humerus
7 Ulna
8 Radius
9 Carpals
10 Metacarpals
11 Phalanges
12 Os coxae
13 Femur
14 Patella
15 Tibia
16 Fibula
17 Tarsals
18 Metatarsals
Adult skeleton
Lateral view
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e axial skel-
eton, comprised of the skull, vertebral column,
ribs, and sternum, forms the central axis of the
body. is sturdy central core is the most primi-
tive portion of the vertebrate skeletal system. It
evolved as the initial skeleton of the rst vertebrate
animals, to which the limb bones (the subject of
the next chapter) were much later additions. e
majority of the axial skeleton’s bony elements, from
the bones at the base of the skull through all the
vertebrae and ribs, form as serial homologues from the
segmental embryonic somites. Because of this shared
developmental similarity each body segment, from the
base of the skull to the end of the coccyx, has the same
basic skeletal design. is is clearly evident in the
structure of the vertebrae and ribs. As you study these
skeletal elements in the photos of this chapter, notice
their similarities.
e elements of the axial skeleton have
many functional roles in the body. Both the
cranial skeleton and the vertebral column
form a strong protective case around the
delicate tissues of the central nervous system.
Additionally, the cranium xes in space impor-
tant nervous structures, such as the internal
ear and eye, both of which would not function
properly in an unstable environment. e cra-
nium also plays an important role in the acqui-
sition and processing of food, respiratory gases,
and sensory input such as sound. In addition to
protecting the spinal cord, the vertebrae form a
strong, exible rod. is strong, exible column not only
forms the central support axis of the body from which
the limbs are suspended, but is also capable of a varied
range of joint movements that are essential to our daily
functions.
Find more information
about the axial skeleton in
5 Axial Skeleton
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1 Cranium
2 Hyoid bone
3 Cervical vertebral column
4 Cervical vertebra 1 - Atlas
5 Cervical vertebra 2 - Axis
6 Cervical vertebra 7
7 Thoracic vertebral column
8 Thoracic vertebra 1
9 Thoracic vertebra 12
10 Lumbar vertebral column
11 Lumbar vertebra 1
12 Lumbar vertebra 5
13 Sacrum
14 Sternum
15 Ribs
Axial skeleton
Anterior view
The axial skeleton is clearly depicted in the photos below. Note
that this portion of the skeleton consists of three principal
skeletal regions — the cranium, the vertebral column, and the
Axial Skeleton
rib cage. There are 29 cranial bones, 26 vertebral bones, and 25 bones in the rib cage. On the pages that
follow, each of the axial skeletal regions and the respective bones will be explored in greater detail.
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