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The Skeletal System

Learning Objectives

Upon completion of this chapter, the student shall be able to:

1.Rationalize the classification and histology of bones.

2.Describe the development, dynamics, and aging of the skeletal system.

3.List the main structures and functions of the axial skeleton.

4.List the main structures and functions of the appendicular skeleton.

5.Explain the skeletal systems integration with the other organ systems of the human body.

Case Study Introduction

It’s nearly dusk on a cold winter night in late November. You and your partner are trained paramedics. Your team has just been dispatched to a community residential address. Five minutes ago, a man called 911 and reported that his fiancée is in excruciating pain, and cannot move.

Once you arrive on scene, you notice a relatively young woman who appears to be in moderate distress. She is lying in bed complaining of pain throughout her back, neck, and legs. She appears to be breathing normally, with no wounds, injuries, or signs of life threats at the moment. Initially, there doesn’t seem to any cause or explanation for the patient’s pain.

The fiancé tells you that the she has been complaining of increasing back pain for the last four months, and when he came home from work today he found her in bed – she told him that she couldn’t walk anymore. The patient denies any form of trauma, including assault, falls, or any heavy lifting.

As you begin to perform a physical assessment of the patient, you tell your partner to bring a stretcher board and set up the equipment. Examination reveals a pulse of 112 beats per minute, a blood pressure of 116/78, respirations of 28 breaths a minute, with normal temperature. The skin is pink, warm, and dry. There are no obvious body deformities, injuries, wounds, or signs of swelling. A 3-lead ECG reads a normal sinus rhythm. The patient is fully alert and oriented and is able to respond to verbal commands. Your partner returns with the stretcher board and a cervical collar. You put the cervical collar on, and with the help of your partner, log-roll the patient onto the stretcher board. The crew helps load the patient into the ambulance. Once in the ambulance, you perform a more detailed assessment of the patient, and repeat the vital signs. You brainstorm possible causes of her agonizing yet obscure pain. Taking into account her young age and lack of external injury, you consider likely systemic possibilities and potential health conditions that could be a cause for her pain. You set up an IV and start the patient on two liters of oxygen via nasal cannula. You continue to obtain a full history and health report from the patient, as your partner transports with urgency…

 ■Introduction

The primary structure of the skeletal system is composed of 206 bones buried deep in muscles and other soft tissues, providing the stable framework that supports the entire human body. It should be directly noted that bones are technically regarded as organs.

However, some academic texts and professors will refer to bones as tissue. Bones are comprised of osseous tissue, or bone tissue, as well as other types of tissue: connective tissue, epithelium, and nervous tissue that collectively work together and function as a single unit, which is the very definition of an organ.

Moreover, bones are living organs – they continuously adapt and aid the body in responding to varying environments. Nevertheless, our discussion of the skeletal system will begin with an overview of its function, followed by a classification of the types of bone, based on examination of their structure.

From there, we will examine the microscopic structure of bone and analyze bone development, growth and the aging process. Once we cover this groundwork, we will then investigate specific bones and their organization in the human skeleton. We will conclude our studies with an overview of joints, or articulations, and the integration of the skeletal system with other organ systems of the human body.

Figure 4-1 Human skeleton.

 ■Functions of the Skeletal System

The skeletal system provides five distinct functions1:

•Support

•Protection

•Movement

•Storage (mineral homeostasis)

•Hematopoiesis

The bones of the skeletal system provide a supporting framework for the whole body; all of the soft tissues in the body use the bones as a scaffold from which to suspend. The bones also serve as a protective barrier for the body’s most vital structures; for example – the skull protects the brain, while the sternum and ribs protect the heart and lungs.

Bones aid in the mechanics of movement. While muscles provide the force via contraction, the bones provide the vehicle for the movement. Muscles are attached to bones, and during contraction, they shorten, pulling on the bones and moving them. Bones also perform a significant role in the storage and maintenance of certain metabolic functions, such as calcium and phosphate regulation.

Figure 4-2 Red blood cells.

Lastly, bones are responsible for one of the most vital physiological processes in the body. That process is called hematopoiesis, or blood cell formation. Blood cell production is carried out in the red bone marrow, a soft connective tissue found deep inside the medulla, or center, of specific bones.

 ■Classification of Bones

Six different types of bones are defined and named according to their shape2:

•long bones

•short bones

•flat bones

•sesamoid bones

•sutural bones

•irregular bones

Examples of long bones are the humerus (bone of the upper arm), and the femur (bone of the upper leg).

Short bone examples include the carpals (wrist bones) and the tarsals (ankle bones).

Examples of flat bones can be seen in the skull, such as the frontal bone or in the pelvis, such as the ilium.

Sesamoid bones are small, rounded bones implanted with a tendon (a type of connective tissue that attaches skeletal muscles to bones); the most easily recognizable one being the patella, or knee cap.

Sutural bones are small and flat bones that sit between the flat bones of the skull; they vary in size and number.

Finally, irregular bones are bones that cannot be classified as any other type of bone and have varying shapes that can be found in the vertebrae, or spinal bones and in the skull, such as the sphenoid and ethmoid bones.

We will take a thorough look at the structural components of a long bone, which will allow us to recognize all of the fundamental characteristics of the entire collection of bones.

Bone structure (gross)

Gross anatomy defines a branch of anatomy focusing on macroscopic (large enough to be observed by the naked eye) structures of organs and tissues.

Seven main components to the gross structure of a long bone are discussed below:

•Diaphysis – shaft; the cylindrical mid-section of a long bone,3 constructed out of solid compact bone (discussed later). It is dense, hard, and extremely strong.

•Medullary cavity – the hollow area inside and deep to the diaphysis of a bone. It contains yellow bone marrow, the fatty, inactive form of marrow found in the adult skeleton.

•Epiphysis – the outer ends of bone, constructed of spongy bone, which houses red bone marrow, the active form of marrow that is responsible for hematopoiesis. The epiphysis is also the location of secondary ossification during development (discussed later).

•Periosteum – A thick, strong, outer fibrous membrane that covers most of a long bone, except for the ends of joint surfaces.

•Endosteum – A thin, interior membrane of connective tissue that lines the medullary cavity.

•Articular cartilage – a thin layer of cartilage that covers the ends of bones, at the outermost regions of the epiphysis; they function like shock absorbers that cushion the ends of bones, where a joint is formed.

•Physis – the epiphyseal plate, also called the growth plate. This is the site of active primary endochondral ossification in growing bone (discussed later).

Figure 4-3 Bone structure.

 ■Bone Histology

Histology defines structure and organization, so in regard to the skeletal system, bone histology means a branch of anatomy that focuses on the minute structures of human (or animal) tissues that is discernable through a microscope.

Several types of bone cells4 are found in the human body:

•Osteoblasts – bone-forming cells

•Osteocytes – responsible for the maintenance of mineral and organic elements in bone

•Osteoclasts – involved in breakdown of bone tissue and involved with resorption processes

Figure 4-4 Bone cells.

The skeletal system contains two main types of connective tissue: bone and cartilage. Further, each bone is comprised of two types of osseous tissue: compact bone, and spongy bone.

Compact bone is a dense and solid type of bone. Spongy bone, also called cancellous bone,5 is a relatively weaker and porous type of bone (hence, resembling a sponge). It is composed of “needle-like threads” of bone, called trabeculae encompassed by gaps of space filled with red marrow.

Compact bone, because it does not contain a network of gaps or open spaces, is a dense and rigid structure, organized into a matrix of several functional units of bone called osteons, or Haversian systems.6

All osteons are packed tightly together and oriented the same way, creating immense strength and providing a great basis for support. The Haversian systems are circular structures composed of a hardened medium arranged in multiple layers, resembling the rings of an onion.

Each concentric ring or layer in the osteon is called a lamella. The lamellae (plural) surround and encircle the central canal, also known as the Haversian canal, named after Dr. Clopton Havers (1657–1702),7 which contains a blood vessel.

In contrast to compact bone, spongy bone often lacks complete osteons or Haversian systems due to extremely thin trabeculae. However, spongy bone is more metabolically active than its denser counterpart, due to its much larger surface area.

Bones, because of their hard, rigid matrix, are often thought to be lifeless structures. However, wedged in between the hard layers of the lamellae in tiny pockets called lacunae are living bone cells; the aforementioned osteocytes.

Lacunae are connected to one another and to the central canal via microscopic channels or canals, called canaliculi. Because of this interconnection, nutrients from the blood vessel in the central canal are able to pass through perforating Volkmann’s canals (named after Alfred Volkmann: 1800–1877) to other osteocytes and maintain their viability. It should be noted here that several blood vessels enter the bone within the periosteum and ultimately make their way throughout the Haversian systems.

Figure 4-5 Interior of bone.

Cartilage is both similar to and different from bone. Both structures consist more of an intracellular substance than that of actual cells. They both contain a countless number of collagenous fibers that strengthen their intracellular matrices. However, the type of collagenous fibers that form the matrices embedded in each structure is different.

In cartilage, Type II collagen is used to form the firm gel-like substance found in its matrix, while Type I collagen is used to form the harder, cement-like and calcified matrix found in that of bone. This is the reason for the flexibility found in cartilage, whereas bone is much more rigid. Cartilage cells, called chondrocytes, are located in lacunae – just as with the osteocytes. Conversely, no blood vessels are found in cartilage. Lacunae are suspended in the gel-like firm matrix. Therefore, nutrients must disperse throughout the matrix of cartilage to reach the chondrocytes.

 ■Bone Development, Growth, and Aging

The human skeleton begins to form early during embryogenesis (beginning of an embryo), approximately six weeks after fertilization. However, at this stage the skeleton is basically a model of cartilage. During the process of bone development, called osteogenesis8 (osteo=bone; genesis=origin), the bones undergo a significant increase in size and shape. This development is continuously monitored and carefully regulated.

The growth process of bone development, such as the cartilage in an embryo with osseous tissue (bone tissue) is called ossification. It should also be noted here that the term ossification is reserved specifically to the formation of bone. While the term calcification (the accumulation of calcium salts that leads to the hardening of structures) also occurs during ossification, it can also take place in other tissues. Two types of ossification9 take place in the human body:

•Intramembranous ossification

•Endochondral ossification

Intramembranous ossification, also called “dermal ossification”, is the term applied to the process of forming bone directly from mesenchymal or fibrous connective tissue. Intramembranous ossification normally only occurs in the deep layers of the dermis, resulting in development and growth of dermal bones. Examples of dermal bones include the flat bones of the skull, the mandible (jaw bone), and the clavicle (collar bone).

In response to abnormal stresses, such as a fracture, the human body relies on intramembranous ossification to form bone in other dermal areas, tendons, joints, and even skeletal muscle.

The second type of ossification, endochondral ossification, is the process of forming bone that has been modeled from cartilage; most bones in the human body are formed this way. As an example, the development of a long bone in one of the limbs.

During embryogenesis, which occurs approximately six weeks after conception, the proximal portion of a limb is present – albeit completely composed of cartilage. Chondrocytes, or cartilage cells, perform an essential role during this stage of the process, providing a platform for longitudinal growth by means of an arrangement of proliferation, extracellular matrix (ECM) secretion, and hypertrophy.

Chondrocytes grow, but eventually disintegrate and die, leaving behind a cavity invaded by blood vessels, providing a medium for the differentiation of fibroblasts into osteoblasts, or bone-forming cells. It is at this time, during the primary center of ossification, where actual bone development proceeds and ultimately results in a diaphysis (shaft) of a long bone.

A second site of bone development, called the secondary ossification center, takes place in the epiphysis or end of a long bone. Located between the diaphysis and epiphysis is a cartilaginous structure called the epiphyseal plate.10 As long as the epiphyseal plate remains between the diaphysis and epiphysis of a long bone, growth will continue to occur.

Eventually, when the entire epiphyseal cartilage is converted into bone, growth stops and all that remains of the epiphyseal plate is an epiphyseal line demarcating the location where the two ossification centers have merged together. It should be noted that the timing of the epiphyseal closure differs from bone to bone, and individual to individual.

The human skeleton is a dynamic, living tissue – continuously and internally monitored and regulated for nutrition, growth, and repair. Osteolysis, or bone resorption, is an erosion process constantly occurring via osteoclast (bone-resorbing cells) secretion of acids and proteolytic enzymes.

Essentially, osteoclasts eliminate the bone matrix, while osteoblasts enhance it. The management and balance of both of these types of cells is extremely important in bone regulation and in the control of calcium and phosphate concentrations in the human body. As the human body ages, so does the skeleton; the bones become thinner and weaker over time. This is normal aging. The actual physiological phenomenon that occurs is deficient ossification process, known as osteopenia.

Usually, this process begins to occur between the ages of 30 and 40, when osteoclastic activity disproportionately increases in comparison to osteoblastic processes. Once this reduction in ossification, women tend to lose approximately eight percent of their skeletal mass every ten years, while men only lose approximately three percent of their skeletal mass in the same amount of time – mainly through hormonal decreases. Not all components of the skeleton are affected equally – the epiphysis, vertebrae, and mandible lose a higher percentage of their portions – resulting in easily broken limbs, a decrease in height, and the loss of teeth, respectively.

 ■Divisions of the Skeleton

The human skeleton contains a total of 206 individual bones and several accompanying cartilages and ligaments. The skeletal system is divided into two main divisions:

•the axial skeleton

•the appendicular skeleton

The axial skeleton, which encompasses the main mechanical core of the human body, contains exactly 80 bones, while the appendicular skeleton, which includes the bones of the limbs and their respective attachments to the pectoral and pelvic girdles, contains 126 bones. We begin our discussion with a closer look at the axial skeleton.

 ■Axial Skeleton

The axial skeleton offers a support system that functions to ensure the protection of the vital organs located in the ventral and dorsal body cavities.

Figure 4-6 Axial skeleton structure.

It also serves as a platform for the attachment of skeletal muscles responsible for:

•movement of the head, neck, and trunk

•aiding respiratory processes

•stabilization and security of inserting components of the appendicular skeleton

While movement is limited in the axial skeleton, the bones and joints are solidly fortified with ligaments, and therefore are exceptionally strong and durable. The principal components and their included subsidiaries of the axial skeleton include:

•The skull (8 cranial bones and 14 facial bones), as well as bones associated with the skull (6 auditory ossicles and the hyoid bone).

•The thoracic cage (the sternum and 24 ribs).

•The vertebral column (24 vertebrae, the sacrum, and the coccyx).

The skull is the most composite bony structure in the human body. It is formed by a total combination of 22 cranial and facial bones. An additional seven bones associated with the skull that are technically not part of the skull include the six auditory ossicles11 (the stapes, the incus, and the malleus – located bilaterally in the inner ear), and the hyoid bone – situated in a floating position, posterior to the mandible and bilaterally connected to the inferior border of the temporal bones by a pair of ligaments. (For trivia enthusiasts out there, the hyoid bone is the only bone in the human body that does not articulate, or join, with another bone).

 ■Cranial Bones

The eight cranial bones fuse together to form the cranium, or braincase, protecting the fragile brain and providing stability for the attachment of head and neck muscles.

Figure 4-7 Cranial bones.

The individual bones making up the cranium12 are the:

•Frontal

•Occipital

•Sphenoid

•Ethmoid

•Pairs of parietal and temporal bones

Collectively, the cranial bones form the cranial cavity, a fluid-filled compartment that encases and shields the brain. Articulations, or joints, are formed at the junction of wherever two bones intersect. Except for where the mandible connects to the skull, all joints in the adult human skull are immoveable structures, or seams, called sutures.

Bones are bound tightly together by dense fibrous connective tissue, allowing for the expansion and protection of delicate brain tissue during growth and development. At a young age, ossification of the sutures results in the fusion of the skull bones into a single component, providing an immense protective barrier.

The individual bones of the cranium are important to distinguish because they are anatomically associated with specific blood vessels and corresponding areas of the brain responsible for specific central nervous system functions.

The frontal bone is a shell-shaped structure that forms the anterior aspect of the skull, better known as the forehead. It articulates (joins) with the laterally paired parietal bones via the pronounced coronal suture. The parietal bones are rounded, semi-circular shaped structures that form the lateral and superior aspects of the skull. They are fused together and assemble superiorly at the midline of the cranium.

The occipital bone is a convex, trapezoid-shaped structure that forms the majority of the posterior aspect and base of the skull. It joins anteriorly with the parietal and temporal bones, and articulates inferiorly with the sphenoid bone at the base of the skull.

The two paired temporal bones are relatively smaller cranial bones located on the lateral aspects of the skull. They lie inferior to the parietal bones and constitute the inferiolateral aspect of the skull and a portion of the cranial floor.

Temporal bones were named due to their Latin origin, temporum, meaning “time”; a sign of aging, or time passing, is the appearance of gray hairs – which usually first become visible in the region of the temporal bones.

The sphenoid bone is a centrally positioned, butterfly-shaped structure that shapes the middle cranial fossa. The sphenoid bone is unique in that it articulates with all the other cranial bones in the skull. Because of its rather irregular shape, the sphenoid bone is a difficult structure to study. Its configuration is essential to many of the central nervous system’s fundamental components, and a thorough understanding of the human body’s neuroanatomy would necessitate a complete investigation of this structure.

Finally, the ethmoid bone, similar to the sphenoid bone, has a complex and irregular shape. It is positioned anterior to the sphenoid bone and posterior to the nasal bones of the face, making it the most deeply placed bone in the skull. The ethmoid bone performs an important role in the formation and integrity of the nasal cavity and nasal septum,13 as well as the human body’s sense of smell. At the superior surface of the ethmoid bone are found a pair of bony structures called the cribiform plates. The cribiform plates are perforated with tiny holes called olfactory foramina, which allow for the olfactory nerves to pass through from the brain to the olfactory receptors located in the nasal cavities, making the sense of smell possible.

Craniofacial bones

Fourteen facial bones serve to guard and sustain the openings of two other major organ systems, the digestive and respiratory systems. The facial bones also form the contour of the face, provide supporting cavities for the special sensory organs of sight, taste, and smell, teeth, and allow for the attachment of facial muscles. The fourteen bones of the facial skeleton include the:

•unpaired mandible and vomer

•paired maxillae

•zygomatic bones

•nasal bones

•lacrimal bones

•palatine bones

•inferior conchae bones

The mandible, or lower jaw bone, is the largest and most durable bone of the face. The body of the mandible forms the chin, and the two lateral and upright stems, called rami (or singularly, ramus), intersect the body at its posterior aspect. The rami articulate with the temporal bones at their superior aspects, forming what is referred to as the temporomandibular joint (commonly abbreviated as TMJ).

A major function of the mandible is to support the oral cavity, as well as serve as an anchor for lower teeth. Because of its location, the mandible is one of the most commonly dislocated bones of the face. Although usually not an emergency, dislocation of the mandible commonly presents with other, more serious traumatic injuries depending on the nature of the injury. Treatment of a simple mandibular dislocation (closed reduction) is not recommended in the field, and should be assessed in a proper clinical setting.

Figure 4-8 Craniofacial and skull bone structure.

The paired maxillary bones demarcate the upper jaw and form the central division of the facial skeleton; all facial bones, except the mandible, articulate with the maxillary bones. The maxillae anchor the upper teeth. The specific structure of the maxillae also serves as a passageway for requisite blood vessels and nerves of the face.

The zygomatic bones are a pair of irregularly shaped facial bones commonly referred to as the “cheek bones”. They individually articulate with the zygomatic processes (bony projections originating from a particular bone) of the adjacent temporal bones posteriorly, the zygomatic processes of the frontal bone superiorly, and the zygomatic processes of the maxillary bones anteriorly.

The remaining facial bones together collectively comprise the nose and nasal cavity. The two nasal bones are thin, rectangular-shaped bones, fused together at the midline and materialize the bridge of the nose. At their inferior aspect, the nasal bones attach to cartilaginous structures that make up the frame of the external nose.

The lacrimal bones are small, delicate bones that form the medial aspect of the orbital socket. Each lacrimal bone includes a deep channel called the lacrimal fossa, which houses a structure known as the lacrimal sac. This structure provides a passageway for tears to drain from the eyes into the nasal cavity.

The palatine bones are L-shaped structures that form the posterior aspect of the hard palate. Together with the inferior portions of the maxillary bones, they form the roof of the oral cavity. The vomer (which literally means “plow”) is a thin, aptly named plow-shaped structure that provides the framework for the nasal septum. Finally, the paired inferior nasal conchae are narrow, slightly bowed structures that form the lateral walls of the nasal cavity. The superior and middle nasal conchae are subdivisions of the ethmoid bone.

 ■The Thoracic Cage

To be technically and anatomically correct, the thorax refers exclusively to the chest and the bony substructures that serve to protect the chest, hence the name thoracic cage. Particular divisions of the thoracic cage include the thoracic vertebrae posteriorly, the sternum and costal cartilage anteriorly, and the ribs laterally.

The function of the ribs is two-fold: first and most obvious is protection of the thoracic cavity. Second, they serve as an attachment site for the intercostal muscles – the muscles of respiration. Acting as a hinge, the ribs are both extremely durable and flexible – and their movement can increase or decrease the volume of the thoracic cage. While the ribs can bend and cushion blows, sudden and/or severe force can cause rib fractures.

Amazingly, because they are securely fastened to connective tissue, a fractured rib can heal without the placement of a cast or splint. However, compound rib fractures can propel fragments of bone into the thoracic cavity, possibly piercing vital internal organs.

Cooperatively, the thoracic cage provides two main functions:

•Guards the heart, lungs, major blood vessels, and thymus located in the thoracic cavity

•Provides a platform or scaffold for the fixture of the skeletal muscles involved in breathing, movements of the pectoral girdle and limbs, and secure positioning of the vertebral column.

The sternum, commonly referred to as the breast bone, is positioned in the anterior midline of the chest. Technically, it’s a composite structure consisting of three fused bones14:

•the manubrium

•body

•xiphoid process

The manubrium is the broadest and most superior segment of the sternum; triangular in shape, it resembles a knight’s shield. The manubrium articulates with the first pair of ribs laterally and the clavicles superiorly. The central depression of the manubrium (which is easily palpated), is located between the articulations of the clavicles and is referred to as the jugular notch.

This is an important anatomical landmark, as it designates the point at which the left common carotid artery branches from the aorta. Inferior to the manubrium is the second segment of the sternum, called the sternal body. The horizontal groove between the manubrium and the body of the sternum is referred to as the sternal angle; another important anatomical landmark, it designates the level of the second ribs. This landmark serves as an essential tool in physical examination and is utilized as a reference point for listening to sounds made by the aortic and pulmonic heart valves.

Figure 4-9 The thoracic cage.

The xiphoid process shapes the inferior border of the sternum and serves as an attachment station for some of the abdominal muscles. It is worth mentioning that ossification of the sternum is not complete until at least the age of 25, with the xiphoid process generally being the last section to fuse. This makes proper hand placement during cardiopulmonary resuscitation (CPR) rather crucial, and therefore is strongly emphasized during CPR training.

The ribs, or costae, are long, rounded bones that form the bell-shaped sides of the thoracic cage.15 Each of the twelve pairs of ribs attach posteriorly to the twelve thoracic vertebrae. From their posterior point of attachment, the ribs curve slightly inferiorly and spread anteriorly to reach the anterior body wall.

The superior seven pairs of ribs attach directly to the sternum by individual cartilaginous annexes called costal cartilages. Because they attach directly to the sternum, the first seven ribs are called true ribs. The inferior, or lower, five pairs of ribs are referred to as false ribs, since they do not attach directly to the sternum; rib pairs 8, 9, and 10 (the vertebrochondral ribs) attach directly to the cartilaginous extension of rib pair number 7 before they make their way to the sternum. The last two pairs of ribs, ribs 11 and 12, are referred to as vertebral ribs, or simply, floating ribs, considering that they have no connection to the sternum or any other rib.

An important anatomical characteristic to note is that the pronounced furrow that runs along the inferior border of each individual rib (called the costal groove) designates the pathway of nerves and blood vessels.