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thick fi broadipose layer. This layer is a prominent location of fat storage in the body. In addition, it functions
as an insulative layer and is the site of distribution of the main venous drainage channels of the integument
and the cutaneous nerves that supply the skin.
The subcutaneous layer, also called the hypodermis, is a layer of variable thickness that ranges
from a thin layer of loose connective tissue to a
Subcutaneous Layer
Subcutaneous layer of integument
100x
Subcutaneous adipose tissue (left), adipose cell (callout)
Section of subcutaneous layer, 200x and 640x
Superfi cial veins and cutaneous nerves in the subcutaneous layer
Step dissection of antebracial integument, anterior view
1 Epidermis of skin
2 Subcutaneous layer
3 Fascia
4 Superficial veins
5 Cutaneous nerve
6 Tendon
7 Muscle
8 Retinaculum cutis
9 Adipose cell membrane
10 Nucleus of adipose cell
11 Fat storage vacuole of adipose cell
12 Blood vessel
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Th e skeletal system forms the internal framework
for the soft tissues of the body. Th is is not a static framework, but a highly dynamic
internal scaff olding. It is dynamic in many ways. On one hand, because of its jointed
design, it shows extreme fl 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 dynamic role of support, it also serves a protective role for many organs of
the body. Th is dynamic framework also exhibits a tremendous capacity
for growth and repair. It is a storehouse of calcium ions, ions that play a
signifi cant role in many of the body’s functions.
Th e skeleton consists of 206 separate bones, ignoring various sesamoid bones and the fact that some bones represent the fusion of multiple
bones. Th ese bones range in size from the small ear ossicles measuring
a few millimeters in length to the large femur measuring up to fi ft y centimeters. Th e skeleton is divisible into two portions, the axial skeleton and the
appendicular skeleton. Th e axial skeleton includes the cranium, vertebral column,
ribs, and sternum. Th e appendicular skeleton consists of the bones of the limbs and
their girdles. Th 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 butterfl y. Bones
can be grouped into four shape categories. Although not that meaningful, the four
categories descriptively group the bones. Th e four shape categories are: long bones,
short bones, fl at bones, and irregular bones. Long bones are unique in having a diaphysis or shaft with a medullary cavity. Th e other bone types lack
this hollow tubular region. Th e short, fl 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 fl 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 soft tissues of the body. A strong foundation of skeletal
anatomy is an important starting point in understanding anatomy.
Th 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 subdivisions of the skeleton — the axial skeleton and the appendicular skeleton.
4 Skeletal System
Find more information
about the skeletal system in
25
REAL ANATOMY
26
nected together in complex array around obvious spaces in the tissue. To the unaided eye this gives the bone a spongy appearance. 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 conBone 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 replacing 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
29
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 developSkeleton
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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