LESSON 4 The Skeletal System.

LESSON ASSIGNMENT Paragraphs 4-1 through 4-40.

LESSON OBJECTIVES After completing this lesson, you should be able to:

4-1. Identify and describe functions of major skeletal components.

4-2. Match important skeletal elements with their functions.

SUGGESTION After completing the assignment, complete the exercises at the end of this lesson. These exercises will help you to achieve the lesson objectives.

4-1. INTRODUCTION

The skeleton forms the framework for the human body. It is composed of individual bones. These bones meet (are articulated with) each other at joints.

4-2. GENERAL FUNCTIONS

a. Support.  In general, the skeleton supports the body.

b. Motion and Locomotion.  Because of the joints and the attached skeletal muscles, the parts of the body can move with respect to each other (motion). Also, because of such linkages in the lower members, the entire body can be moved from place to place (locomotion).

c. Protection.  Certain parts of the skeleton are structured to protect vital organs.

d. Hematopoiesis.  The skeleton is also involved in formation of blood (hematopoiesis) cells.

e. Storage.  Moreover, the skeleton stores various minerals.

4-3. CONNECTIVE TISSUES

The skeletal elements are made up of several types of connective tissues. In general, connective tissues tend to connect and/or support. These tissues are characterized by an extracellular material referred to as the matrix.

a. In the formation of the individual organs known as the bones, bone tissues make up the main portion of each bone, on an FCT framework.

b. Certain bone surfaces are covered with cartilage connective tissue.

4-4. PIEZOELECTRIC EFFECT

a. Each bone is built around an FCT framework on which the apatite crystals are deposited in a regular order. Apatite is a mineral, a form of calcium phosphate. (Another mineral found in bones is calcium carbonate.)

b. When compressed, the apatite crystals produce a local electric current. This phenomenon is known as the piezoelectric effect.

c. Presumably, this piezoelectric effect is produced in the bones of the lower limb during walking. We know that tissues respond to local electric current. When walking casts are used, fractured lower members tend to heal much more rapidly than when the patient is bedridden. Bones tend to lose mass when they are not subjected to forces as great as ordinary.

a. The living cells of the bones are osteocytes. When these cells are building up bone tissue, they are called osteoblasts. When they are tearing down bone tissue, they are called osteoclasts.

b. This building up, tearing down, and rebuilding are continuous processes throughout the life of the individual human being. The building and rebuilding respond specifically to the directions of force applied to the body at that particular time. Therefore, throughout the life of the individual, the skeleton can be remodeled and changed continuously in reaction to applied forces.

4-6. DEFINITION

Bones are those individual organs that are elements of the skeletal system.

4-7. TYPES

The individual bones of the skeleton can be categorized into three major groups according to their general shapes:

a. Long.

b. Flat.

c. Irregular.

A "typical" long bone, as the name implies, has more length than width. (See Figure 4-1.)

Figure 4-1. "Typical" long bone section.

a. Shaft (Diaphysis).  In effect, the long bone has a shaft, with proximal and distal ends. The shaft tends to be cylindrical in form.

(1) It has a cortex (outer portion) of dense bony tissue called compact bone tissue. The cortex is usually thickest at the middle of the shaft.

(2) The inside of the shaft is usually hollow, except that it is filled with yellow marrow (in adults, but red marrow in small children and infants).

b. Ends (Epiphyses).  At the ends of the long bone, the cortex is much thinner. Each end is filled with a lattice-or sponge-like network of bony tissue, called cancellous bony tissue. The strands of bone forming this lattice are called trabeculae. The trabeculae are aligned with the lines of applied forces, particularly tension and compression. The spaces within the cancellous bony tissue are filled with red marrow.

c. Some Special Parts.  The skeletal muscles pull and create tensions at their attachments to the bone. These tensions will often cause the bone to react and form spines, tubercles, ridges, and the like.

d. Articular Cartilages.  The surface of each end of the bone is covered by an articular cartilage. This cartilage is located where the bone contacts another bone at a joint. The cartilage is made up of hyaline-type cartilage tissue. The articular cartilage makes the movement between the bones smoother.

e. Periosteum.  The periosteum surrounds the bone, except where the articular cartilages are located. The periosteum is an envelope of the bone and consists mainly of dense FCT. In fact, the periosteum may be considered the outermost portion of the bone.

(1) However, the periosteum has a special layer of cells immediately adjacent to the surface of the bone. Since this layer is able to produce bone material, it is called the osteogenic layer of the periosteum.

(2) When a long bone is fractured or a portion of the bone is lost without losing the periosteum, the fracture is healed by the combined action of the osteogenic layer of the periosteum and the osteoblasts of the bone itself.

f. NAVL.  Associated with the periosteum are the "service tissues." These are the NAVL (nerves, arteries, veins, and lymphatics), which nourish and stimulate the living tissues of the bone and periosteum.

(1) Neurovascular bundle. Branches from the main NAVL of the body go as a unit to the bone. This unit, the neurovascular bundle, consists of NAVL within a common fibrous connective sheath.

(2) Branches of neurovascular bundle.  Portions of these NAVL spread out through the periosteum as periosteal branches over the outer surfaces of the bone. Other branches penetrate through the cortex of the bone to spread out through the medullary (or marrow) cavity. The holes through the cortex are known as the nutrient canals. The branches are known as the nutrient branches.

a. A long bone begins in the fetus as a hyaline cartilage model of the bone.

b. At the appropriate time, the cartilage model is invaded by a mass of material that begins to destroy the cartilage and replace it with bone tissue. This invading mass and the subsequently developed bone structure are called the primary center of ossification, or diaphysis.

c. At about the time of birth or thereafter, a secondary center of ossification, or epiphysis, develops at each end of the developing long bone.

d. A plate of cartilage, called the epiphyseal plate, remains between the diaphysis and each epiphysis. In the early years of life, the cartilage grows faster than the diaphysis can tear it down. This results in gradual lengthening of the long bone.

e. At the proper time, between puberty and adulthood, the bone development overtakes completely destroys the cartilage. After this, the diaphysis and the epiphysis are solidly fused to one another. The dense bony line of fusion between the diaphysis and epiphysis is called the epiphyseal line. The epiphyseal line is easily visible in the radiographs ("x-rays") of young adults.

f. While the bone has been growing in length, it also grows in width. The osteogenic layer of the periosteum gradually adds bony tissue to the outside surface of the bone. At the same time, osteoclastic activity removes bone material from the wall of the marrow cavity.

g. Many factors are involved in the process of bone growth. One of the primary factors is a hormone of the anterior pituitary gland known as somatotropin. Overproduction of somatotropin in a young person (before fusion of the ossification

centers) results in gigantism. Overproduction of somatotropin in adults (after fusion of the ossification centers) results in a condition called acromegaly. Acromegaly involves excessive growth of the jaw, hands, and feet.

h. Throughout the entire life of the individual, the continuous tearing down (osteoclastic activity) and rebuilding (osteoblastic activity) remodel the bony substance. These processes occur in response to the forces or stresses applied to the body.

Another category of bones consists of the flat bones. (See Figure 4-2.)

Figure 4-2. "Typical" flat bone section.

a. The flat bones have two layers of dense bony tissue, called tables. Thus, there is an inner table and an outer table.

b. Generally, between the two tables is a layer of cancellous bony tissue.

(1) The spaces of this cancellous bony tissue are filled with red marrow. In adults, the red marrow of the flat bones is the primary blood-cell forming area of the body.

(2) As with the cancellous tissue of the long bone, the cancellous tissue of the flat bone is organized into trabeculae. The trabeculae are oriented in the same directions as the lines of applied forces, much like the struts of a building.

(3) Adjacent to the nasal cavities, many flat bones are hollowed to form the paranasal sinuses. These hollow spaces take the place of cancellous bony tissue. The development of the mastoid bone is likewise formed by the extension of the air-filled cavity of the middle ear into the mastoid bone.

c. The outer surface of the outer table and the under surface of the inner table are covered with periosteum. The periosteum is similar to that described for the "typical" long bone.

d. At their margins, flat bones are articulated with other flat bones and held together by FCT. These fibrous connections are usually called sutures.

Flat bones generally begin as membranous, FCT models within the fetus. Again, an invasion of material forms an ossification center. This center tears down and

replaces the FCT with bone tissue. The ossification center continues to grow outward. In time, a full plate of bone has been formed. Then, the flat bone grows at its margins until adulthood.

The flat bones of the skull are somewhat special.

a. Curved Shape.  They are generally curved. Together, they form a sphere which surrounds and protects the brain.

b. Healing of Fractures.  When the growth of the cranial flat bones is complete, the osteogenic layer of the periosteum disappears.

(1) Cracks and/or line fractures of cranial flat bones will usually heal by the activity of the osteoblasts within the bone.

(2) However, when bone substance is lost and a spatial defect ("hole") remains, the missing portions of the table(s) will not be replaced. Osteoblastic activity will repair only the margins of the spatial defect ("hole").

c. Variations in Brain Injury.

(1) In a young individual, the flat bones of the skull are not yet fully developed. The cranium as a whole is relatively flexible. An injury to the brain, resulting from a force applied to the cranium, will usually be located immediately below the location of the applied force.

(2) In an older adult, the flat bones of the skull have fully developed and are more or less fused to each other. The cranium is a relatively solid sphere. An injury to the brain, resulting from a force applied to the cranium, will usually be found on the opposite side from the applied force. Often, the applied force will be diverted around the sphere to the base of the cranium. There, the diverted force may cause fractures of the cranium at the apertures (openings) in its base.

The sesamoid bones are another kind of bone. Sesamoid bones develop in place within tendons of skeletal muscles where the tendons sustain excessive pressures. Since the sesamoid bone absorbs these pressures, it protects the tendon from wear and tear.

The primary example of sesamoid bones is the patella (kneecap). In the form of a simple pulley mechanism, the tendon of the quadriceps femoris muscle passes over the distal end of the femur. Located at this point within the tendon is the patella.

a. Where two bones meet each other, this junction is referred to as a joint or articulation.

b. The joints of the human skeleton may be characterized, in general, in three different ways.

First, they are characterized by the type of material that holds the bones together at the joint.

a. If the bones are fused together with bony tissue, the articulation is called a synosteosis.

b. Thus, in a synchondrosis, the bones are held together by cartilage tissue.

c. In a syndesmosis, the bones are held together by FCT.

NOTE: A synovial articulation is somewhat different and will be described in detail in the next section.

A second way of categorizing joints of the human skeleton is according to relative mobility.

a. The junctions of some bones are nonmobile, such as a synosteosis.

b. Others are semimobile, as seen with some syndesmoses.

c. Being structured to facilitate motion, synovial articulations (see the next section) are mobile to various degrees.

The term degrees of freedom refers to the number of planes in which movement is permitted. This also equals the number of axes around which motion can take place at a particular joint.

a. One Degree of Freedom.  One degree of freedom means that the joint is uniaxial. Motion can take place in a single plane around one axis only. An example is a "hinge" joint.

b. Two Degrees of Freedom.  Two degrees of freedom mean that the joint is biaxial. Motion can take place around two different axes.

c. Three Degrees of Freedom.  With three degrees of freedom, we say that the joint is multiaxial. Motion can take place around the three axes in all three planes. An example is "ball and socket" type joints.

A synovial joint is structured to facilitate freedom of motion in one or more of the three planes around the three axes of any given joint. The "typical" synovial joint (Figure 4-3) is a schematic representation rather than an actual synovial joint, but it contains the structural features common to all synovial joints.

The synovial articulation is formed between two bones. These bones are parts of the skeleton. They are levers of motion. To them are attached skeletal muscles, which provide the forces for motion.

Figure 4-3. A "typical" synovial joint--diagrammatic.

Covering a portion of each bone is an articular cartilage. The portions covered are the ends that would otherwise be in contact during the motions of the joint. Each articular cartilage has a relatively smooth surface and some ability to act as a shock absorber.

The joint area is surrounded by a dense FCT capsule that encloses the joint area.

The inner surface of this fibrous capsule is lined with a synovial membrane. The synovial membrane secretes a synovial fluid into the synovial cavity, or joint space. The synovial fluid is a very good lubricant. Thus, it minimizes the frictional forces between the moving bones.

The bones of the synovial joint are held together by ligaments. Ligaments are very dense FCT structures that keep the bones from being pulled apart. These ligaments may occur as either discrete, individual structures or as thickenings of the fibrous capsule.

The skeletal muscles cross the synovial joint from one bone to the other. They are attached to the bones. The tonic (continuous) contraction of these skeletal muscles holds the opposing surfaces of the bones tightly together. When properly stimulated, these muscles contract and cause motion of the bones around the joint.

Synovial joints are often referred to by their geometric or mechanical structure.

a. Ball-and-Socket Joint.  The ball-and-socket synovial joint has one bone with a rounded head, a "ball." The other bone has a corresponding cavity, the "socket." The ball-and-socket joint is usually multiaxial.

b. Hinge Joint.  In the hinge joint, the geometry of the bony surfaces and the disposition of the ligaments are such as to allow the parts to fold on each other, around a single axis only.

c. Others.  There are other special arrangements of the synovial joints to produce specific motions. An example: Rotation of the head at the pivot-type joint of atlas and axis (the upper two vertebrae).

As a whole, the human skeleton (Figure 4-4) is the supporting framework of the body. The skeleton is composed of the individual bones and the articulations

between them. The human skeleton is generally considered in two major subdivisions: the axial skeleton and the appendicular skeleton.

Figure 4-4. Anterior view of the human skeleton.

The axial skeleton (Figure 4-5) is the central supporting framework of the body. Its major components are the vertebral column (spine), the thoracic cage, and the skull.

Figure 4-5. Midsagittal section of skull and vertebral column with

CNS and meninges in place.

The skull is the skeleton of the head region. It is located on the top of the vertical vertebral column. It has two major functional subdivisions: the cranium and the facial (visceral) skeleton.

a. Cranium.  The cranium is a spherical container that protects the brain. At the base of the cranium is a series of openings. Blood vessels and nerves enter and leave the cranial cavity through these openings.

b. Facial Skeleton.  The facial skeleton is also referred to as the visceral skull. It is attached to the anterior and inferior surfaces of the cranium. It is the skeleton of the entrances of the respiratory and digestive systems and the orbits containing the eyes.

The vertebral column is a series of individual segments, the vertebrae, and one on top of the other.

The upper part of the vertebral column, the neck region, and associated muscles provide the head with its various motions. The upper two vertebrae are specifically constructed for head motions.

a. The articulation between the occipital base of the skull and the atlas (the first cervical vertebra) is specially constructed for anterior-posterior motions of the head ("nodding").

b. Between the atlas (the first cervical vertebra) and the axis (the second cervical vertebra) is a special pivotal-type joint. This joint facilitates rotary (turning) motions of the head.

a. The vertebral bodies and the associated intervertebral discs are the primary mechanism for supporting the body weight.

b. In the lumbar and lumbosacral regions, the articular processes of the vertebrae is also weight bearing. (A bony projection extends upward and another extends downward from each right and left side of the neural arch of each of these vertebrae.) These projections are the articular processes. Through them, as well as through the vertebral bodies and discs, adjacent vertebrae are articulated with each other.

c. The specially constructed sacrum, at the lower end of the vertebral column, receives the body weight from above and transfers it to the pelvic bones of the lower members.

Whereas the cranium protects the brain, the neural arches protect the spinal cord and its membranes (meninges). The neural arches of the individual vertebrae arch over the spinal cord and its membranes. The continuous series of neural arches forms a continuous spinal canal.

Together, the vertebrae, the intervertebral discs, and the associated ligaments form a semiflexible rod. This allows a certain amount of motion to the vertebral column in addition to its supporting role.

a. Role of Processes. The spinous and transverse processes of the neural arches serve as attachments for skeletal muscles. By acting as levers, these processes enable the skeletal muscles to move the vertebrae.

b. Role of Intervertebral Discs. The intervertebral discs between adjacent vertebrae serve several functions.

(1) First, they allow motion to occur between adjacent vertebrae. The relative thickness of the individual intervertebral disc determines the amount of motion possible between the adjacent vertebrae. The total movement of the vertebral column (spine) is the sum of the motions of the individual intervertebral discs.

(2) Secondly, the intervertebral disc acts as a shock absorber. As such, it minimizes the shocks that are transmitted to the vertebral column by the contact of the heels with the floor during walking, jumping, etc.

(3) During the course of a day standing and sitting, the individual becomes about an inch shorter than he was at the beginning of the day. This is less true of older individuals. After a good night's rest in a horizontal position, these discs regain their original thickness. As an astronaut works at zero gravity, he retains his full height.

(4) With age, individuals tend to lose height. This is because the intervertebral discs shrink somewhat over the years. Since these discs also become less flexible, there is less compression from morning until night. Thus, the height in the evening is closer to the morning height than with a younger person.

c. Role of Curvatures of Vertebral Column.  As a whole, the vertebral column has four curvatures. Two of these are concave to the front; two are concave to the rear. As do the intervertebral discs, these curvatures function as shock absorbers for the body.

The thoracic cage consists of the ribs, the sternum, and thoracic vertebrae. The 12 pairs of ribs are attached posteriorly to the thoracic vertebrae. Anteriorly, the upper 10 pairs of ribs attach directly or indirectly (via costal cartilages) to the sternum.

a. Motion.  Because of the segmentation of the thoracic cage into vertebrae and ribs, motion can occur in the thoracic region of the body.

b. Costal Breathing.  The special construction of the ribs and their costal cartilages allows costal breathing to take place.

c. Protection.  In addition, the rib cage encloses such vital structures as the lungs, the heart, and the liver and gives them protection.

The appendicular skeleton consists of the bones of the upper and lower members.

Each member is attached ("appended") to the axial skeleton by a skeletal element called a girdle.

a. Pelvic Girdles.  The girdle of each lower member is called the pelvic girdle. Each pelvic girdle is attached firmly to the corresponding side of the sacrum. With their ligaments, the two pelvic girdles and sacrum together form a solid bony circle known as the bony pelvis.

b. Pectoral Girdles.  The girdle of each upper member is called the pectoral girdle. Unlike the pelvic girdles, each pectoral girdle is very loosely attached to the axial skeleton. The sole attachment is by the sternoclavicular joint, which in turn is constructed to increase the degrees of motion.

Both the upper and lower members have limbs arranged in three segments. The proximal segment has one bone. The middle segment has two bones. The distal segment has many bones arranged in a five-rayed (pentadactyl) pattern.

a. Body Support.  The skeleton of the lower member is strongly constructed in a columnar fashion for body support. The foot at the lower end of the lower limb extends at a 90° angle. Therefore, the foot forms a base for the body during the erect, standing posture.

b. Locomotion.  At the same time, the lower limb has a series of linkages that enable the body to move from place to place.

The grasping hand is the distal segment of the upper member. The flexible construction of the pectoral girdle and the bones of the upper limb serve to place the grasping hand into as many positions as possible. This is particularly helpful in grasping food and placing it into the mouth. The grasping hand also serves as a tool-holding device. (When we study the nervous system, we shall see that a significant portion of the brain and special pathways are present in order to control the movements of this grasping hand.)