Divisions of the Skeletal System Assignment

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;‐
plural is sulci, SUL sī)‐
Furrow along bone surface that
accommodates blood vessel, nerve, or
Intertubercular sulcus of
humerus (Figure 8.5a).

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