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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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Th e axial skeleton, comprised of the skull, vertebral column,

ribs, and sternum, forms the central axis of the

body. Th is sturdy central core is the most primitive portion of the vertebrate skeletal system. It

evolved as the initial skeleton of the fi rst vertebrate

animals, to which the limb bones (the subject of

the next chapter) were much later additions. Th 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. Th 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.

Th 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 fi xes in space important nervous structures, such as the internal

ear and eye, both of which would not function

properly in an unstable environment. Th e cranium also plays an important role in the acquisition and processing of food, respiratory gases,

and sensory input such as sound. In addition to

protecting the spinal cord, the vertebrae form a

strong, fl exible rod. Th is strong, fl 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

33

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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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Axial skeleton

Lateral view Axial skeleton

Posterior view

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 1 Frontal bone

 2 Parietal bone

 3 Occipital bone

 4 Sphenoid bone

 5 Temporal bone

 6 Ethmoid bone

 7 Inferior nasal concha

 8 Lacrimal bone

 9 Nasal bone

10 Vomer

 11 Maxilla

12 Palatine bone

13 Zygomatic bone

14 Mandible

15 Bony nasal cavity

16 Piriform aperture

17 Inferior nasal meatus

18 Middle nasal meatu

19 Orbit

Cranium

Anterior view

functions, that include protecting the delicate brain tissue, fi xing the vestibular apparatus of the inner ear in

three-dimensional space, maintaining open air passageways for respiration, and acquiring and processing

food, to name a few. There are two main subdivisions of the cranium — the neurocranium or brain box is the

region that surrounds and encases the brain, and the viscerocranium or facial skeleton is the area contributing

to the orbits, nasal cavity, and oral cavity. This page and the facing page, and the four page spreads that follow,

depict the fi ve normas, or views, of the cranium in both articulated and disarticulated cranial images. The

bones of the skull are labeled on these views, along with key landmarks that can only be labeled on the articulated cranium. Individual landmarks of the bones are labeled on the individual pictures of the cranial bones on

the pages that follow. This spread is of the norma facialis or facial aspect of the cranium.

The cranium is the composite skeleton of the head and is composed of 29 bones.

The bones of the cranium range from simple, non-descript plates of bone to the

most intricate bones of the skeleton. The cranial bones have a range of important

Cranium

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Bones of the cranium disarticulated

Anterior view

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 1 Frontal bone

 2 Parietal bone

 3 Occipital bone

 4 Sphenoid bone

 5 Temporal bone

 6 Ethmoid bone

 7 Lacrimal bone

 8 Nasal bone

 9 Maxilla

10 Zygomatic bone

 11 Mandible

12 Zygomatic arch

13 Pterygopalatine fossa

Cranium

Cranium

Lateral view

This page spread depicts the norma lateralis, or lateral aspect of the cranium. In this view both

the brain box and facial skeleton are clearly visible and the relative proportions of the two cranial

regions are evident. In the disarticulated view, only those bones that are visible in the lateral aspect are shown.

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Bones of the cranium disarticulated

Lateral view

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Cranium

 1 Parietal bone

 2 Occipital bone

 3 Sphenoid bone

 4 Temporal bone

 5 Ethmoid bone

 6 Inferior nasal concha

 7 Vomer

 8 Maxilla

 9 Palatine bone

10 Zygomatic bone

 11 Mandible

12 Choana or posterior nasal aperture

13 Inferior orbital fissure

14 Bony nasal cavity

15 Middle nasal meatus

16 Inferior nasal meatus

17 Bony palate

18 Sutural bone

Cranium

Posterior view

This page spread depicts the norma occipitalis, or occipital aspect of the cranium. From this posterior view the internal aspects of the bones of the oral and nasal cavities are clearly visible. In the

disarticulated view only those bones that are visible in the occipital aspect of the cranium are

depicted.

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Bones of the cranium disarticulated

Posterior view

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Cranium

 1 Frontal bone

 2 Parietal bone

 3 Occipital bone

 4 Temporal bone

 5 Nasal bone

 6 Maxilla

 7 Zygomatic bone

Cranium

Superior view

This page spread depicts the norma superior, or superior aspect of the cranium. This view clearly

depicts the neurocranium or brain box, while the facial skeleton is almost completely hidden from

view. In the disarticulated view only those bones that are visible in the superior aspect of the cranium

are depicted.

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Bones of the cranium disarticulated

Superior view

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Cranium

 1 Occipital bone

 2 Sphenoid bone

 3 Temporal bone

 4 Vomer

 5 Maxilla

 6 Palatine bone

 7 Zygomatic bone

 8 Bony palate

 9 Choana or posterior nasal aperture

10 Zygomatic arch

 11 Jugular foramen

12 Foramen lacerum

13 Greater palatine foramen

14 Incisive fossa

Cranium

Inferior view

This page spread depicts the norma inferior (basalis), or inferior aspect of the cranium. The mandible has been removed to more clearly reveal the basicranium. This view clearly depicts the fl oor

of the brain box, the bony palate forming the roof of the oral cavity, and mandibular tooth row. In

the disarticulated view only those bones that are visible in the inferior aspect of the cranium are

depicted.

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Bones of the cranium disarticulated

Inferior view

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Cranium

Parasagittal section of the cranium

Medial view of the right side

This page spread depicts the cranium sectioned in a parasagittal plane through the right side of

the nasal cavity just lateral to the bony nasal septum. The section below depicts the lateral wall of

the right nasal cavity, and the section on the opposite page depicts the medial (septal) wall of the

right nasal cavity. The osseous sinuses that communicate with the nasal cavity are all visible in

these sections.

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 1 Frontal bone

 2 Parietal bone

 3 Occipital bone

 4 Sphenoid bone

 5 Temporal bone

 6 Ethmoid bone

 7 Inferior nasal concha

 8 Nasal bone

 9 Vomer

10 Maxilla

 11 Palatine bone

12 Mandible

19 Ethmoidal air cells (sinuses)

20 Maxillary sinus

21 Incisive canal

22 Bony nasal septum

23 Sphenopalatine foramen

24 Inferior nasal meatus

Parasagittal section of the cranium

Medial view of the left side

13 External table of calvaria

14 Diploë

15 Internal table of calvaria

16 Groove for sigmoid sinus

17 Sphenoidal sinus

18 Frontal sinus

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Cranium

Cranium with calvaria removed

Superior or internal view of the cranial base

This page spread depicts the cranium sectioned in a horizontal plane through the neurocranium,

or brain box, revealing the internal aspects of the fl oor and roof of the sectioned cranial cavity. On

this page the fl oor of the neurocranium is visible, while on the opposing page the roof of the neurocranium is visible. The superior portion of the cranium, depicted on the opposite page, is called

the calvaria.

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 1 Frontal bone

 2 Parietal bone

 3 Occipital bone

 4 Sphenoid bone

 5 Temporal bone

 6 Ethmoid bone

 7 Clivus

 8 Foramen caecum

 9 Foramen lacerum

10 Jugular foramen

 11 Petrosphenoidal fissure

12 Petro-occipital fissure

13 Anterior cranial fossa

14 Middle cranial fossa

15 Posterior cranial fossa

16 Granular foveolae

Removed calvaria

Inferior or internal view

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Cranial Bones – Frontal

 1 Squamous part

 2 Frontal tuber

 3 Glabella

 4 Superciliary arch

 5 Supra-orbital notch or foramen

 6 Frontal notch or foramen

 7 Temporal surface

 8 Zygomatic process

 9 Frontal crest

10 Groove for superior sagittal sinus

 11 Nasal spine

12 Orbital surface

13 Trochlear spine

14 Lacrimal fossa

15 Ethmoidal notch

16 Frontal sinus

Frontal bone

Posterior view

Frontal bone

Anterior view

vertical portion termed the squama and a horizontal plate that forms the superior walls of the orbits.

The bone has a smooth external surface, while its internal surface consists of impressions made by the

meningeal vessels and scattered foramina that transmit diploic vessels. The squamous portion of the

bone is thick. It consists of internal and external laminae of compact bone sandwiching a layer of trabecular bone called diploë. Near the anterior, inferior midline the spongy bone is absent between the

external and internal laminae and in its place are variably sized spaces — the frontal sinuses. The orbital

plate consists of a thin plate of compact bone, which is often so thin that it is translucent. The frontal

bone articulates with twelve bones.

The unpaired frontal bone has a

bowl-like shape that consists of

two parts, an internally concave

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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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