NOTE: Lecture notes are intended to help the student organize their notes and facilitate assimilation of the material. They are in no way a substitute for the actual lectures; however, this material will be covered on exams!
Text Reference: CHAPTER 7
The human skeleton is strong and hard, yet it is relatively light. The skeleton accounts for about 20% of the body weight. So, the skeleton weighs about 30 pounds in a 160-pound man. It is perfectly adapted for locomotion and manipulation. An S-shaped spine is responsible for our upright posture, though the pressure placed on the lower back sometimes results in swayback posture and lower back pain. Because we stand upright, we are able to use our hands for manipulation of our environment rather than locomotion, which is reflected in the anatomy of our upper extremities.
Bones come in many sizes and shapes. Bones are classified according to shape (FIG. 7.2):
LONG BONES are much longer than they are wide. Each long bone has the anatomical features described in Chapter 6 in your text and Lecture Notes 6. The DIAPHYSIS (shaft) consists primarily of compact bone, because compact bone resists the pressure of body weight. The diaphysis of some of the long bones, like the femur, are slightly curved for strength. A curved bone can absorb the stress of body weight better than a straight bone. In curved bone, the stress of body weight is distributed at several different points so that the pressure of body weight is evenly distributed throughout the bone. Inside the diaphysis is the medullary cavity (what does this cavity contain?). The EPIPHYSES of long bones consist of compact bone laid over spongy bone tissue. The long bones include the bones of the upper arm, forearm, hand, fingers, thigh, lower leg, foot, and toes.
SHORT BONES are roughly cube-shaped. They contain mostly spongy bone tissue covered by a thin layer of compact bone. There is no medullary cavity. The bones of the wrist and ankle are short bones.
SESAMOID BONES are a special type of SHORT BONE embedded within the tendons of joints where a great deal of pressure develops. They protect tendons from wear and tear, and they alter the direction of pull of a tendon. Sesamoid bones form in tendons in response to stress on joints, so the sesamoid bones vary in size and number in different individuals. The kneecaps (patellas), however, are 2 sesamoid bones that are present in all individuals. See also Figs. 8.8 (p. 211) and 8.16, (p. 223) for other sesamoid bones that most people have.
FLAT BONES are thin, flattened, and usually curved. Flat bones have two roughly parallel compact bone surfaces, with a layer of spongy bone between them. The breastbone, ribs, shoulder blades, and the cranial bones are flat bones. Flat bones provide protection to internal organs and they provide large areas for muscle attachment. Example: Cranial bones protect the brain and provide attachment sites for neck and facial muscles. The ribs and breastbone protect the thoracic cavity. The shoulder blades provide muscle attachments for many back muscles, while protecting the thoracic cavity.
IRREGULAR BONES are the bones that do not fit into any other category. They include some of the facial bones & some cranial bones, the vertebrae, and the hip bones. All of the irregular bones consist primarily of spongy bone enclosed by a thin layer of compact bone.
The external surfaces of bones often have bulges, depressions and holes. These surface markings are adapted for certain functions. See TABLE 7.2 ("Bone Surface Markings"). Learn the bone surface markings and their functions. Keep these functions in mind when you study the structure of the bones.
The adult human skeleton (FIG. 7.1) consists of 206 bones which are divided into 2 principal divisions: the axial skeleton and the appendicular skeleton (see TABLE 7.1). The AXIAL SKELETON forms the long axis of the body and it includes the bones of the skull, vertebral column, breastbone, and the rib cage. The APPENDICULAR SKELETON consists of the bones of the upper and lower extremities (limbs), the shoulder girdle, and the hip girdle. The shoulder girdle and hip girdle attach the limbs to the axial skeleton.
THE AXIAL SKELETON (CHAPTER 7)
The AXIAL SKELETON consists of 80 bones which are found in 3 major regions: the bones of the SKULL, VERTEBRAL COLUMN (backbone), and THORAX (ribs and sternum). The axial skeleton forms the LONGITUDINAL AXIS of the human body. (The longitudinal axis is a straight line that runs through the body's center of gravity. This imaginary line extends through the head and down to the space between the feet.)
The SKULL is formed by 2 sets of bones: the 8 CRANIAL BONES and the 14 FACIAL BONES (FIGS. 7.3-7.16).
The CRANIAL BONES are also called the CRANIUM. The cranial bones are mostly flat bones, but a few are irregular bones. The cranium encloses and protects the brain. Its flat bones also provide a site for the attachment of head and neck muscles.
At birth, there are FIBROUS CONNECTIVE TISSUE MEMBRANE spaces between the flat cranial bones. The larger spaces between the cranial bones are called FONTANELS (FIG. 7.14). These "soft spots" are areas that are eventually replaced with bone. Fontanels allow the skull to be compressed as it passes through the birth canal during delivery. The fontanels also permit rapid growth of the brain during infancy. The best known of these fontanels is the anterior (frontal) fontanel. The spaces between the cranial bones eventually ossify and the fontanels close by the 24th month. The spaces become serrated joints between the skull bones and are called SUTURES.
Most of the bones of the adult skull (cranial and facial) are firmly united by immobile interlocking joints called SUTURES. The one skull bone that is not connected to other skull bones by sutures is the lower jaw bone (mandible). The mandible is connected to the rest of the skull by a freely movable joint. The sutures have a serrated appearance and look like seams between the bones of the skull. See your skeletal system hand-out for the names of the major cranial sutures.
The 8 cranial bones are the FRONTAL bone, 2 PARIETAL bones, 2 TEMPORAL bones, OCCIPITAL bone, SPHENOID bone, and ETHMOID bone. Together, these bones form a kind of bony helmet around the brain.
The FACIAL BONES (1) form the framework of the face, (2) hold the eyes in the anterior position, (3) provide openings for the passage of food, liquids and air, (4) provide cavities for the organs of taste (mouth) and smell (nasal cavities), (5) secure the teeth, and (6) anchor the facial muscles of expression & speech. The 14 facial bones are the 2 NASAL bones, 2 MAXILLAE, 2 ZYGOMATIC bones, the MANDIBLE, 2 LACRIMAL bones, 2 PALATINE bones, 2 INFERIOR NASAL CONCHAE, and the VOMER.
In the anterior and lateral views of the skull, note the EYE ORBITS, which house the eyeballs. What bones form the eye orbits? See FIG. 7.12.
Look at the CRANIAL FLOOR (FIG. 7.8 & 7.15). Note that there are prominent bony ridges in the cranial floor that divide the floor into 3 distinct basin-like depressions - the ANTERIOR, POSTERIOR & MIDDLE CRANIAL FOSSAE (fossa = shallow, basin-like depression). The brain sits snugly in these cranial fossae. When looking at actual bones (which I strongly urge that you do), notice the grooves in the underside of the cranial bones where the cranial blood vessels are located.
In the ANTERIOR CRANIAL FOSSA you can see the CRISTA GALLI of the ETHMOID sticking straight up like a "cock's comb" (FIG. 7.8). Surrounding the crista galli is the CRIBRIFORM PLATE of the ETHMOID. Note the OLFACTORY FORAMINA (holes) in the cribriform plate where sensory nerves pass through from the nasal cavities to carry olfactory (smell) information to the brain.
In the MIDDLE CRANIAL FOSSA is a small depression called the SELLA TURCICA
of the SPHENOID. The sella turcica forms a cup in which the pituitary gland
sits.
In the ridge of bone that separate the middle cranial fossa from the posterior
cranial fossa is the PETROUS PORTION of the TEMPORAL bone (one on each side).
The petrous portion contains the organs of hearing and equilibrium in the inner
and middle ear. Note the INTERNAL AUDITORY MEATUS in each petrous portion where
nerves pass from the inner ear to the brain. If you look at the lateral view of
the temporal bone you can see the EXTERNAL AUDITORY MEATUS that contains the
external ear canal.
Some of the facial bones form the walls of the NASAL CAVITIES. As you identify
the facial bones, notice which ones form the outside walls of the nasal
cavities (FIG. 7.4). The nasal cavities are separated by the NASAL SEPTUM. What
bones and other structures make up the nasal septum? See FIGS. 7.5 & 7.11.
Several facial and cranial bones of the skull contain the PARANASAL SINUSES,
which are paired, hollow cavities clustered around the nasal cavity (FIG.7.13;
see also FIGS. 7.5, 7.9, 7.11). What bones contain the paranasal
sinuses? Small openings connect the sinuses to the nasal cavity.
The paranasal sinuses lighten the skull bones and serve as resonating chambers
for sound, which gives each individual a destinct voice. The nasal cavities and
paranasal sinuses are lined with mucous membranes in living persons, and we
will learn more about them when we discuss the respiratory system.
The single HYOID bone (FIGS. 7.4 & 7.16) is a unique component of the axial
skeleton because it does not articulate with any other bone. It is suspended in
the midneck region about an inch above the voicebox. It is suspended from the
styloid process of the temporal bone by ligaments and muscles. The hyoid bone
serves as an attachment site for the tongue and several neck and throat muscles
involved in swallowing and speech.
VERTEBRAL COLUMN
The VERTEBRAL COLUMN is also called the SPINE (FIG. 7.17) In an adult male, the
vertebral column measures about 28 inches in length. In an average adult female
it measures about 24 inches in length. Some people think of the vertebral
column as a rigid supporting rod. But, the vertebral column is flexible and can
rotate and bend anteriorly, posteriorly, and laterally.
The vertebral column serves as the axial support of the body trunk. The vertebral column extends from the skull, which it supports, to its anchoring point in the pelvis, where it transmits the weight of the trunk to the lower limbs. Running through its central vertebral canal is the delicate spinal cord, which the vertebral column surrounds and protects. The vertebral column also provides attachment points for the ribs and muscles of the back.
In the fetus and infant, the vertebral column consists of 33 separate bones, or VERTEBRAE. Nine of these near the bottom of the spine eventually fuse to form 2 composite bones, the sacrum and the coccyx.
The adult vertebral column contains 26 vertebrae: 7 CERVICAL VERTEBRAE (C1-C7) (cervix = neck) in the neck region; 12 THORACIC VERTEBRAE (T1-T12) (thorax = chest) posterior to the thoracic cavity; 5 LUMBAR VERTEBRAE (L1-L5) supporting the lower back; 1 SACRUM; and 1 COCCYX (tailbone).
Between the vertebrae are the INTERVERTEBRAL DISCS (FIG. 7.17). Each intervertebral disc is a cushion-like pad of FIBROCARTILAGE that surrounds a soft, elastic, semi-fluid center. The discs act as shock absorbers during walking, jumping and running, because they can absorb compressive shock. The discs also cushion the spine while it bends and rotates.
When you view the vertebral column from the side, you can see that it has an S-shape (FIG. 7.17b & c). This S-shape increases the strength of the back, helps the vertebral column to absorb the shock of walking upright, and helps to maintain balance. A newborn baby, however, is born with a C-shaped spine. The CERVICAL CURVE develops when the baby develops her neck muscles and starts to hold up her head. The LUMBAR CURVE develops when the baby first begins to sit up, stand, and walk. The THORACIC and SACRAL CURVES are what remain of the original C-shaped curvature.
THE ANATOMY OF THE TYPICAL VERTEBRA: (FIG. 7.18a-d)
1. BODY: the weight-bearing part of the vertebra.
2. VERTEBRAL ARCH: extends posteriorly from body.
3. VERTEBRAL FORAMEN: space that lies between the body and the vertebral arch.
It contains the spinal cord.
4. INTERVERTEBRAL FORAMEN: opening between the vertebrae on each side of the
vertebral column. It permits the passage of each spinal nerve on each side.
5. 7 processes: TRANSVERSE PROCESSES (2) extend laterally, SPINOUS PROCESS
(spine) project posteriorly, SUPERIOR ARTICULAR PROCESSES (2), and INFERIOR
ARTICULAR PROCESSES (2). The 2 transverse processes and the spinous process
serve as points of muscle attachment. The articular processes form joints
(articulations) with other vertebrae. Note the FACETS, which are the
articulating surfaces of the articular processes.
The CERVICAL VERTEBRAE (C1-C7) (FIG. 7.19a-d) are the smallest of the vertebrae, with small bodies and reduced transverse processes. The SPINOUS PROCESSES of the 2nd through 6th cervical vertebrae are BIFID, which means there is a cleft. Also note the FORAMINA (holes) in the TRANSVERSE PROCESSES of the cervical vertebrae.
The first 2 cervical vertebrae are highly modified. The first cervical vertebra (C1) is called the ATLAS, named for the fact that it supports the head, like Atlas supported the world on his shoulders in Greek mythology. The atlas has no body and no spinous process. It is essentially a ring of bone. The superior articular facets articulate with the occipital condyles of the skull, which permits nodding of the head.
The second cervical vertebra (C2) is the AXIS. A peg-like process called the DENS projects up through the ring of the atlas. The dens of the axis makes a pivot on which the atlas rotates. The atlas and axis arrangement permits side-to-side rotation of the head.
C3-C6 are typical cervical vertebrae. The seventh cervical vertebra (C7) is called the VERTEBRA PROMINENS. It has a large, nonbifid (no cleft) spinous process that can be seen and felt at the base of the neck. It is used as an "anatomical landmark" for counting vertebrae in living persons.
The THORACIC VERTEBRAE (T1-T12) (FIG. 7.18 & 7.20) are "typical vertebrae" in form and they increase in size from T1 to T12. The TRANSVERSE PROCESSES and BODIES of thoracic vertebrae have FACETS that articulate with the ribs. Also notice how the SPINOUS PROCESSES slant inferiorly.
The LUMBAR VERTEBRAE (L1-L5) (FIG. 7.21a-c) are sturdier in structure than thoracic vertebrae. They are larger and stronger in order to support the lower back. The bodies are larger than those of thoracic vertebrae, and the processes are short and thick. The spinous processes project posteriorly.
TABLE 7.5 summarizes the characteristics of the cervical, thoracic, & lumbar vertebrae.
The SACRUM (FIG. 7.22) is triangular in shape and is a COMPOSITE BONE formed by 5 fused sacral vertebrae (S1-S5). Fusion begins around 16-18 years and is complete by the mid-20's. Articulating inferiorly with the sacrum is the COCCYX (FIG. 7.22). It is a COMPOSITE BONE formed by the fusion of 4 coccygeal vertebrae (Co 1-4), which fuse together between 20 and 30 years of age. The coccyx is the vestigial tailbone. Except for the slight support the coccyx gives pelvic organs, it is a nearly useless bone in the adult human body.
THE THORAX (FIG. 7.23)
The skeleton portion of the thorax or chest is formed by the STERNUM, RIBS,
COSTAL CARTILAGE, and THORACIC VERTEBRAE. These bones form a protective bony
cage around the vital organs of the thoracic cavity (lungs and heart). The
thorax also supports the shoulder girdle and upper limbs, and provides
attachment points for the muscles of the back, chest and shoulders.
The STERNUM is the breastbone. There are 2 anatomical landmarks associated with the sternum: the JUGULAR (suprasternal) NOTCH and the STERNAL ANGLE. Note how the first pair of ribs attaches to the MANUBRIUM. The 2nd pair of ribs attaches at the STERNAL ANGLE. The XIPHOID PROCESS is hyaline cartilage until it completely ossifies around the age of 40.
The 12 PAIRS OF RIBS form the sides of the thorax. All ribs attach posteriorly to the thoracic vertebrae (FIG. 7.23b and 7.24) and then curve downward and forward toward the anterior body surface. The ribs gradually increase in length from the first to the seventh pairs; from pair 8 to pair 12 they decrease in length.
See FIG. 7.23b. The upper 7 rib pairs are attached directly to the sternum by individual bars of COSTAL CARTILAGE, which consists of hyaline cartilage. These 1st 7 pairs of ribs are called the TRUE or VERTEBROSTERNAL RIBS. The remaining 5 pairs of ribs are called the FALSE RIBS because they do not attach directly to the sternum like true ribs do. Rib pairs 8-10 attach to each other via costal cartilage and then attach to the costal cartilage of the 7th rib. These false ribs are called VERTEBROCHONDRAL RIBS. Rib pairs 11 and 12 are called FLOATING (VERTEBRAL) RIBS because their anterior ends do not attach to the sternum. They are attached posteriorly to the thoracic vertebrae, like all ribs. Anteriorly, the costal cartilage of the floating ribs is embedded in the muscles of the body wall.