Detailed Analysis of the Human Skeletal System: Anatomy and Divisions

This assignment provides a comprehensive overview of the human skeletal system, detailing its organization into axial and appendicular divisions. It explores the structure and function of various bone types, including long, short, flat, irregular, and sesamoid bones, as well as sutural bones. The assignment also examines bone surface markings, such as depressions, openings, and processes, and their roles in allowing the passage of soft tissues and forming joints or attachment points for connective tissues. Furthermore, it discusses the skull, including its cranial and facial bones, and special features like sutures, paranasal sinuses, and fontanels. The content emphasizes the interrelationships between the different components of the skeletal system, providing a foundation for understanding human anatomy. This assignment covers the bones of the axial skeleton, including the skull, vertebral column, and chest, and prepares the reader for a subsequent exploration of the appendicular skeleton.

7.1 DIVISIONS OF THE SKELETAL SYSTEM
OBJECTIVE
• Describe how the skeleton is organized into axial and appendicular divisions.
The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each
pair on the right and left sides of the body. The skeletons of infants and children have more than 206 bones
because some of their bones (sacrum and coccyx) fuse later in life. Examples are the hip bones and some bones
(sacrum and coccyx) of the vertebral column (backbone).
Bones of the adult skeleton are grouped into two principal divisions: the axial skeleton and the appendicular
skeleton (appendic‐ = to hang onto). Table 7.1 presents the 80 bones of the axial skeleton and the 126 bones of
the appendicular skeleton. Figure 7.1 shows how both divisions join to form the complete skeleton (the bones
of the axial skeleton are shown in blue). You can remember the names of the divisions if you think of the axial
skeleton as consisting of the bones that lie around the longitudinal axis of the human body, an imaginary
vertical line that runs through the body's center of gravity from the head to the space between the feet: skull
bones, auditory ossicles (ear bones), hyoid bone (see Figure 7.5), ribs, sternum (breastbone), and bones of the
vertebral column. The appendicular skeleton consists of the bones of the upper and lower limbs
(extremities or appendages), plus the bones forming the girdles that connect the limbs to the axial skeleton.
Functionally, the auditory ossicles in the middle ear, which vibrate in response to sound waves that strike the
eardrum, are not part of either the axial or appendicular skeleton, but they are grouped with the axial skeleton
for convenience (see Chapter 17).
Table 7.1 The Bones of the Adult Skeletal System
DIVISION OF THE SKELETON STRUCTURE NUMBER OF BONES
Axial skeleton Skull

Cranium 8
Face 14
Hyoid 1
Auditory ossicles 6
Vertebral column 26
Thorax
Sternum 1
Ribs 24
Number of bones = 80
Appendicular skeleton Pectoral (shoulder) girdles
Clavicle 2
Scapula 2
Upper limbs
Humerus 2
Ulna 2
Radius 2
Carpals 16

Metacarpals 10
Phalanges 28
Pelvic (hip) girdle
Hip, pelvic, or coxal bone 2
Lower limbs
Femur 2
Patella 2
Fibula 2
Tibia 2
Tarsals 14
Metatarsals 10
Phalanges 28
Number of bones = 126
Total bones in an adult skeleton = 206
John Gibb
Figure 7.1 Divisions of the skeletal system.
The axial skeleton is indicated in blue. (Note the position of the hyoid bone in Figure 7.5.)
The adult human skeleton consists of 206 bones grouped into two divisions: the axial skeleton and the

appendicular skeleton.
John Gibb
Which of the following structures are part of the axial skeleton, and which are part of
the appendicular skeleton? Skull, clavicle, vertebral column, shoulder girdle, humerus, pelvic
girdle, and femur.
We will organize our study of the skeletal system around the two divisions of the skeleton, with emphasis on
how the many bones of the body are interrelated. In this chapter we focus on the axial skeleton, looking first at
the skull and then at the bones of the vertebral column and the chest. In Chapter 8 we explore the appendicular
skeleton, examining in turn the bones of the pectoral (shoulder) girdle and upper limbs, and then the pelvic
(hip) girdle and the lower limbs. Before we examine the axial skeleton, we direct your attention to some
general characteristics of bones.
7.2 TYPES OF BONES
OBJECTIVE
• Classify bones based on their shape or location.
Almost all bones of the body can be classified into five main types based on shape: long, short, flat, irregular,
and sesamoid (Figure 7.2). As you learned in Chapter 6, long bones have greater length than width, consist of
a shaft and a variable number of extremities or epiphyses (ends), and are slightly curved for strength. A curved
bone absorbs the stress of the body's weight at several different points, so that it is evenly distributed. If bones
were straight, the weight of the body would be unevenly distributed, and the bone would fracture more easily.
Long bones consist mostly of compact bone tissue in their diaphyses but have considerable amounts of spongy
bone tissue in their epiphyses. Long bones vary tremendously in size and include those in the femur (thigh
bone), tibia and fibula (leg bones), humerus (arm bone), ulna and radius (forearm bones), and phalanges
(finger and toe bones).

Figure 7.2 Types of bones based on shape.
The bones are not drawn to scale.
The shapes of bones largely determine their functions.
John Gibb
Which type of bone primarily provides protection and a large surface area for muscle
attachment?
Short bones are somewhat cube‐shaped and are nearly equal in length and width. They consist of spongy bone
tissue except at the surface, which has a thin layer of compact bone tissue. Examples of short bones are most
carpal (wrist) bones and most tarsal (ankle) bones.
Flat bones are generally thin and composed of two nearly parallel plates of compact bone tissue enclosing a
layer of spongy bone tissue. Flat bones afford considerable protection and provide extensive areas for muscle
attachment. Flat bones include the cranial bones, which protect the brain; the sternum (breastbone) and ribs,
which protect organs in the thorax; and the scapulae (shoulder blades).
Irregular bones have complex shapes and cannot be grouped into any of the previous categories. They vary in
the amount of spongy and compact bone present. Such bones include the vertebrae (backbones), hip bones,
certain facial bones, and the calcaneus.
Sesamoid bones (SES‐a‐moyd = shaped like a sesame seed) develop in certain tendons where there is
considerable friction, tension, and physical stress, such as the palms and soles. They may vary in number from
person to person, are not always completely ossified, and typically measure only a few millimeters in diameter.
Notable exceptions are the two patellae (kneecaps), large sesamoid bones located in the quadriceps femoris
tendon (see Figure 11.20a) that are normally present in everyone. Functionally, sesamoid bones protect
tendons from excessive wear and tear, and they often change the direction of pull of a tendon, which improves
the mechanical advantage at a joint.
An additional type of bone is classified by location rather than shape. Sutural bones (SOO‐chur‐al; sutur‐ =
seam) are small bones located in sutures (joints) between certain cranial bones (see Figure 7.6). Their number
varies greatly from person to person.
Recall from Chapter 6 that in adults, red bone marrow is restricted to flat bones such as the ribs, sternum
(breastbone), and skull; irregular bones such as vertebrae (backbones) and hip bones; long bones such as the
proximal epiphyses of the femur (thigh bone) and humerus (arm bone); and some short bones.
7.3 BONE SURFACE MARKINGS

OBJECTIVE
• Describe the principal surface markings on bones and the functions of each.
Bones have characteristic surface markings, structural features adapted for specific functions. Most are not
present at birth but develop in response to certain forces and are most prominent in the adult skeleton. In
response to tension on a bone surface from tendons, ligaments, aponeuroses, and fasciae, new bone is
deposited, resulting in raised or roughened areas. Conversely, compression on a bone surface results in a
depression.
There are two major types of surface markings: (1) depressions and openings, which allow the passage of soft
tissues (such as blood vessels, nerves, ligaments, and tendons) or form joints, and (2) processes, projections or
outgrowths that either help form joints or serve as attachment points for connective tissue (such as ligaments
and tendons). Table 7.2 describes the various surface markings and provides examples of each.
Table 7.2 Bone Surface Markings
MARKING DESCRIPTION EXAMPLE
DEPRESSIONS AND OPENINGS: SITES ALLOWING THE PASSAGE OF SOFT TISSUE (NERVES, BLOOD VESSELS, LIGAMENTS,
TENDONS) OR FORMATION OF JOINTS
Fissure (FISH‐ur) Narrow slit between adjacent parts of
bones through which blood vessels or
nerves pass.
Superior orbital fissure of
sphenoid bone (Figure 7.12).
Foramen (fō‐RĀ‐men = hole;
plural is foramina)
Opening through which blood vessels,
nerves, or ligaments pass.
Optic foramen of sphenoid
bone (Figure 7.12).
Fossa (FOS‐a = trench; plural
is fossae, FOS‐ē)
Shallow depression. Coronoid fossa of humerus
(Figure 8.5a).
Sulcus (SUL‐kus = groove; Furrow along bone surface that Intertubercular sulcus of

plural is sulci, SUL‐sī) accommodates blood vessel, nerve, or
tendon.
humerus (Figure 8.5a).
Meatus (mē‐Ā‐tus =
passageway; plural is meati, mē‐
Ā‐tī)
Tubelike opening. External auditory meatus of
temporal bone (Figure 7.4b).
PROCESSES: PROJECTIONS OR OUTGROWTHS ON BONE THAT FORM JOINTS OR ATTACHMENT POINTS FOR
CONNECTIVE TISSUE, SUCH AS LIGAMENTS AND TENDONS
Processes that form joints
Condyle (KON‐dīl; condylus
= knuckle)
Large, round protuberance with a
smooth articular surface at end of
bone.
Lateral condyle of femur
(Figure 8.13a).
Facet (FAS‐et or fa‐SET) Smooth, flat, slightly concave or
convex articular surface.
Superior articular facet of
vertebra (Figure 7.18d).
Head Usually rounded articular projection
supported on neck (constricted portion)
of bone.
Head of femur
(Figure 8.13a).
Processes that form attachment points for connective tissue
Crest Prominent ridge or elongated
projection.
Iliac crest of hip bone
(Figure 8.10b).
Epicondyle (epi‐ = above) Typically roughened projection above
condyle.
Medial epicondyle of femur
(Figure 8.13a).
Line (linea) Long, narrow ridge or border (less
prominent than crest).
Linea aspera of femur
(Figure 8.13b).
Spinous process Sharp, slender projection. Spinous process of vertebra
(Figure 7.17).
Trochanter (trō‐KAN‐ter) Very large projection. Greater trochanter of femur

(Figure 8.13b).
Tubercle (TOO‐ber‐kul; tube
r‐ = knob)
Variably sized rounded projection. Greater tubercle of humerus
(Figure 8.5a).
Tuberosity Variably sized projection that has a
rough, bumpy surface.
Ischial tuberosity of hip
bone (Figure 8.10b).
7.4 SKULL
OBJECTIVES
• Name the cranial and facial bones and indicate whether they are paired or single.
• Describe the following special features of the skull: sutures, paranasal sinuses, and fontanels.
The skull is the bony framework of the head. It contains 22 bones (not counting the bones of the middle ears)
and rests on the superior end of the vertebral column (backbone). The bones of the skull are grouped into two
categories: cranial bones and facial bones. The cranial bones (crani‐ = brain case) form the cranial cavity,
which encloses and protects the brain. The eight cranial bones are the frontal bone, two parietal bones, two
temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone. Fourteen facial bones form the
face: two nasal bones, two maxillae (or maxillas), two zygomatic bones, the mandible, two lacrimal bones, two
palatine bones, two inferior nasal conchae, and the vomer. Exhibits 7.A through 7.G illustrate the bones of the
skull from different views.
Examples

Anatomy Overview: The Axial Skeleton ‐ The Skull
General Features and Functions
Besides forming the large cranial cavity, the skull also forms several smaller cavities, including the nasal
cavity and orbits (eye sockets), which open to the exterior. Certain skull bones also contain cavities called
paranasal sinuses that are lined with mucous membranes and open into the nasal cavity. Also within the skull
are small middle ear cavities in the temporal bones that house the structures that are involved in hearing and
equilibrium (balance).
Other than the auditory ossicles (tiny bones involved in hearing), which are located within the temporal bones,
the mandible is the only movable bone of the skull. Joints called sutures attach most of the skull bones together
and are especially noticeable on the outer surface of the skull.
The skull has many surface markings, such as foramina (rounded passageways) and fissures (slit‐like
openings) through which blood vessels and nerves pass. You will learn the names of important skull bone
surface markings as we describe each bone.
In addition to protecting the brain, the cranial bones stabilize the positions of the brain, blood vessels,
lymphatic vessels, and nerves through the attachment of their inner surfaces to meninges (membranes). The
outer surfaces of cranial bones provide large areas of attachment for muscles that move various parts of the
head. The bones also provide attachment for some muscles that produce facial expressions such as the frown of
concentration you wear when studying this book. The facial bones form the framework of the face and provide
support for the entrances to the digestive and respiratory systems. Together, the cranial and facial bones protect
and support the delicate special sense organs for vision, taste, smell, hearing, and equilibrium (balance).
Nasal Septum
The nasal cavity is a space inside the skull that is divided into right and left sides by a vertical partition called
the nasal septum, which consists of bone and cartilage. The three components of the nasal septum are the
vomer, septal cartilage, and the perpendicular plate of the ethmoid bone (Figure 7.11). The anterior border of
the vomer articulates with the septal cartilage, which is hyaline cartilage, to form the anterior portion of the
septum. The superior border of the vomer articulates with the perpendicular plate of the ethmoid bone to form
the remainder of the nasal septum. The term “broken nose,” in most cases, refers to damage to the septal
cartilage rather than the nasal bones themselves.
Figure 7.11 Nasal septum.
The structures that form the nasal septum are the perpendicular plate of the ethmoid bone, the vomer, and
septal cartilage. John Gibb
What is the function of the nasal septum?

CLINICAL CONNECTION Deviated Nasal Septum
A deviated nasal septum is one that does not run along the midline of the nasal cavity. It deviates (bends)
to one side. A blow to the nose can easily damage, or break, this delicate septum of bone and displace and
damage the cartilage. Often, when a broken nasal septum heals, the bones and cartilage deviate to one side
or the other. This deviated septum can block airflow into the constricted side of the nose, making it difficult
to breathe through that half of the nasal cavity. The deviation usually occurs at the junction of the vomer
bone with the septal cartilage. Septal deviations may also occur due to developmental abnormality. If the
deviation is severe, it may block the nasal passageway entirely. Even a partial blockage may lead to
infection. If inflammation occurs, it may cause nasal congestion, blockage of the paranasal sinus openings,
chronic sinusitis, headache, and nosebleeds. The condition usually can be corrected or improved surgically.
Orbits
Seven bones of the skull join to form each orbit (eye socket) or orbital cavity, which contains the eyeball and
associated structures (Figure 7.12). The three cranial bones of the orbit are the frontal, sphenoid, and ethmoid;
the four facial bones are the palatine, zygomatic, lacrimal, and maxilla. Each pyramid‐shaped orbit has four
regions that converge posteriorly:
1. Parts of the frontal and sphenoid bones comprise the roof of the orbit.
2. Parts of the zygomatic and sphenoid bones form the lateral wall of the orbit.
3. Parts of the maxilla, zygomatic, and palatine bones make up the floor of the orbit.
4. Parts of the maxilla, lacrimal, ethmoid, and sphenoid bones form the medial wall of the orbit.

Figure 7.12
Details of the orbit (eye socket).
The orbit is a pyramid‐shaped structure that contains the eyeball and associated structures. John Gibb
Which seven bones form the orbit?
Associated with each orbit are five openings:
1. The optic foramen (canal) is at the junction of the roof and medial wall.
2. The superior orbital fissure is at the superior lateral angle of the apex.
3. The inferior orbital fissure is at the junction of the lateral wall and floor.
4. The supraorbital foramen is on the medial side of the supraorbital margin of the frontal bone.
5. The lacrimal fossa is in the lacrimal bone.
Foramina
We mentioned most of the foramina (openings for blood vessels, nerves, or ligaments) of the skull in the
descriptions of the cranial and facial bones that they penetrate. As preparation for studying other systems of the
body, especially the nervous and cardiovascular systems, these foramina and the structures passing through
them are listed in Table 7.3. For your convenience and for future reference, the foramina are listed
alphabetically.
Unique Features of the Skull
The skull exhibits several unique features not seen in other bones of the body. These include sutures, paranasal
sinuses, and fontanels.

Sutures
A suture (SOO‐chur = seam) is an immovable joint in most cases in an adult skull that holds most skull bones
together. Sutures in the skulls of infants and children, however, often are movable and function as important
growth centers in the developing skull. The names of many sutures reflect the bones they unite. For example,
the frontozygomatic suture is between the frontal bone and the zygomatic bone. Similarly, the sphenoparietal
suture is between the sphenoid bone and the parietal bone. In other cases, however, the names of sutures are
not so obvious. Of the many sutures found in the skull, we will identify only four prominent ones:
1. The coronal suture (KO‐rō‐nal; coron‐ = relating to the frontal or coronal plane) unites the frontal
bone and both parietal bones (see Figure 7.4b).
2. The sagittal suture (SAJ‐i‐tal; sagitt‐ = arrow) unites the two parietal bones on the superior midline of
the skull (see Figure 7.4a). The sagittal suture is so named because in the infant, before the bones of the
skull are firmly united, the suture and the fontanels (soft spots) associated with it resemble an arrow.
3. The lambdoid suture (LAM‐doyd) unites the two parietal bones to the occipital bone. This suture is so
named because of its resemblance to the capital Greek letter lambda (Λ), as can be seen in
Figure 7.6 (with the help of a little imagination). Sutural bones may occur within the sagittal and
lambdoid sutures.
4. The two squamous sutures (SKWĀ‐mus; squam‐ = flat, like the flat overlapping scales of a snake)
unite the parietal and temporal bones on the lateral aspects of the skull (see Figure 7.4b).
Examples
Anatomy Overview: Fibrous Joints and Sutures
Paranasal Sinuses
The paranasal sinuses (par′‐a‐NĀ‐zal SĪ‐nus‐ez; para‐ = beside) are cavities within certain cranial and facial
bones near the nasal cavity. They are most evident in a sagittal section of the skull (Figure 7.13). The paranasal
sinuses are lined with mucous membranes that are continuous with the lining of the nasal cavity. Secretions
produced by the mucous membranes of the paranasal sinuses drain into the lateral wall of the nasal cavity.
Paranasal sinuses are quite small or absent at birth, but increase in size during two periods of facial
enlargement—during the eruption of the teeth and at the onset of puberty. They arise as outgrowths of the
nasal mucosa that project into the surrounding bones. Skull bones containing the paranasal sinuses are the