SKELETAL ANATOMY
The skeletal system has many important functions, including support of the body and its structures, protection of body organs, production (along with muscles) of movement, storage of calcium, phosphate, and fat, and blood cell production in the bone marrow. Bone is a living tissue consisting of bone cells, osteocytes, found in lacunae, spaces in the hard calcium phosphate matrix or framework. There are two types of bone, compact and spongy.
Compact bone covers the outside of bones and is very hard. It is arranged in units called osteons or Haversian units. In cross-section, these units look something like tiny dart boards, with the bull's-eye being a canal (osteonic or Haversian canal) through which blood vessels and a nerve run. Concentric rings of hard calcareous matrix surrounding the canals are called lamellae; osteocytes reside in lacunae between successive lamellar rings. The osteocytes are living cells which must obtain nutrients and oxygen from the blood and eliminate wastes. They do so by sending cytoplasmic projections through tiny canals (canaliculi) which pass through the hard matrix, interconnecting the bone cells and eventually contacting the blood vessels in the Haversian canals.
Spongy bone is less hard and brittle and has a different structural arrangement. It is arranged in a fine network of thin plates called trabeculae. Spongy bone is found sandwiched between compact bone in the skull bones, clavicles (collar bones), and scapulae (shoulder blades), and in numerous other sites. In the arms and legs, it is found inside the ends of the bones. Spongy bone contains red bone marrow, the site of blood cell production.
Arm and leg bones are called long bones. They consist of a shaft (diaphysis) and two expanded ends (epiphyses). Inside the diaphysis is a cavity containing yellow bone marrow for fat storage, while the expanded epiphyses contain red marrow in spongy bone. The long bones contain many small holes through which blood vessels pass.
Observe long bones of the arms and/or legs. Identify the diaphysis, epiphyses, regions of compact and spongy bone, and regions of yellow and red bone marrow. (Refer to Figure 11.1 in the Mader text.)
Observe a microscope slide of (ground) compact bone. Identify the Haversian canals, lamellae, lacunae, and canaliculi. Sketch below.
Observe a microscope slide of (decalcified) spongy bone. Identify the trabeculae and bone marrow cells. Sketch below.
The skeleton
The skeleton consists normally of about 206 bones. (Some people have tiny extra skull bones or extra bones embedded in ligaments or tendons.) Bones have a number of bumps, knobs, projections, pits, canals, and holes, and of course, these have technical names. For example, condyles are rounded knobs which form joints with other bones. Tubercles and trochanters are projections for the attachment of ligaments, which attach bone to bone at joints, or tendons, which connect muscle to bone. A hole is called a foramen and is for the passage of blood vessels, nerves, and/or ligaments.
The skeleton is divided into two major subdivisions: (I) the axial skeleton consists of the 80 bones of the head and trunk which protect the central nervous system, and (II) the appendicular skeleton consists of the 126 bones of the pectoral and pelvic girdles, arms and legs. The following is a short description of the major bones and markings of the skeleton. Identify these on the skeleton models in the laboratory.
I. Axial skeleton
A. Skull (Label the photograph that follows.)
1. Cranium: surrounds the brain
a. frontal bone: forms forehead
b. parietal bones: form top of cranium
c. occipital bone: forms rear of cranium. Note the foramen magnum,
the large hole through which the spinal cord passes to the brain.
d. temporal bones: form the sides of the cranium. Note:
i. external auditory canals: canals for sound to pass to the eardrums
ii. mastoid processes: large bumps behind ears, to which neck
muscles attach; larger in males than females; hollow inside;
ear infection may spread here, causing mastoiditis
iii. styloid processes: sharp processes to which throat and tongue
muscles attach
iv. zygomatic processes: to which chewing muscles attach
e. sphenoid bone: butterfly-shaped bone forming the floor of the
cranium. Note the saddle-shaped depression (sella turcica or
Turk's saddle) in which the pituitary gland sits.
f. ethmoid bone: a small bone in the front of the cranium with the
sharply-pointed crista galli (cock's comb) to which the connective
tissues of the brain attach. A perpendicular plate projecting
down from this bone forms the upper part of the nasal septum
2. Facial bones
a. maxilla: forms the upper jaw; a plate projecting to the posterior
forms most of the hard palate.
b. nasal bones: form the bridge of the nose (Most of the nose is com-
posed of hyaline cartilage.)
c. zygomatic bones: form cheeks.
d. mandible: the lower jaw; the only movable bone of the skull.
e. vomer: forms the lower part of the nasal septum.
f. other facial bones include: lacrimals, nasal conchae, palatines
3. Other features of the skull:
a. orbits or eye sockets
b. paranasal sinuses: hollow areas in the maxilla, frontal, ethmoid,
and sphenoid bones to reduce the weight of the skull and to help
project the voice.
c. middle ear bones: for sound amplification
B. Vertebral column: The vertebral column consists of individual vertebrae, each having a weight-bearing body, a large hole for the spinal cord to pass, and various processes (projections) to form joints and for ligament and tendon attachment. Vertebrae are cushioned by pads of tough fibrous cartilage between the bodies of vertebrae. These may become ruptured with the center pushing outward to one side, applying pressure to a spinal nerve. This herniated disk is very painful. There are five regions of the vertebral column: (See Figure 11.7 in the text.)
1. cervical: 7 vertebrae of the neck. The first two are modified.
The first, the atlas, articulates with the skull. The next, the
axis, has an upward projecting process, the dens, which forms a
pivot joint with the atlas and allows rotation of the head. In
severe whiplash, the dens is driven into the medulla of the brain,
resulting in death.
2. thoracic: 12 vertebrae which articulate with the ribs.
3. lumbar: 5 vertebrae of the lower back.
4. sacral: fused vertebrae form the sacrum, the posterior portion of
the pelvis.
5. coccygeal: fused vertebrae form the coccyx, the remnant of a tail.
C. Ribs: (See Figure 11.8 in the Mader text.) 12 pairs articulate with the thoracic vertebrae posteriorly and join the sternum or breast bone in the front via costal cartilages. The first 7 pairs are called true ribs because they each have their own cartilage to attach to the sternum. The last 5 pairs are called false ribs. The first 3 of these have costal cartilages that join the cartilage of the last true rib before joining the sternum. The last 2 false ribs are floating ribs; they do not attach to the sternum.
Muscles of breathing attach between ribs. Rib fractures generally
occur at the point of greatest curvature; the fractured bone can pene-
trate the underlying lung.
D. Sternum: breastbone. Notice the lower, pointed xiphoid process.
When CPR (cardio-pulmonary resuscitation) is performed, this process
is identified, then the hands are placed several inches superiorly
to avoid breaking off the process and forcing it into the liver. Bone
marrow can be "conveniently" removed by inserting a needle into the sternum, a process known as sternal puncture.
Label the indicated bones of the skull.
II. Appendicular skeleton
(Refer to Figures 11.4, 11.9, and 11.10 in Mader.)
A. Pectoral girdle: consists of two pairs of bones forming the supporting
framework for the arms.
1. scapula: the "shoulder blade". Notice the prominent ridge on the
posterior surface, serving for muscle attachment. The shallow cup,
the glenoid cavity, forms a joint with the head of the humerus.
This is an example of a ball-and-socket joint.
2. clavicle: the "collarbone" articulates with the scapula and sternum.
The clavicle fractures easily when one falls on an outstretched arm.
B. Humerus (upper arm): possesses a rounded head and short neck. You may notice a roughened "V"-shaped region along the diaphysis; this is called the deltoid tuberosity, the region where your shoulder muscles attach. At the lower or distal end, two rounded processes, condyles, form a joint with the lower arm bones (radius and ulna). Notice the large cavity for receiving a process of the ulna when the arm is straightened.
C. Lower arm (forearm)
1. Ulna: a "wrench-shaped" bone on the "little finger" side of the arm.
It has a deep groove and large process (olecranon) at its upper
(proximal) end. This hooks into the distal humerus and forms the
elbow. At the distal end of the ulna, there is a styloid process
that forms a bump on the "little finger" side of your wrist.
2. Radius: located on the "thumb side" of your lower arm. The radius
has a flat proximal head which articulates with the humerus and
ulna. The articulation with the ulna forms a pivot joint allowing
forearm rotation. The radius has a styloid process at the distal
end which can be felt as a bump on the "thumb side" of the wrist.
D. Wrist: formed by two rows of four carpal bones.
E. Hand: formed by five metacarpal bones.
F. Fingers: formed by 14 phalanges (3 per finger, 2 in thumb).
G. Pelvic girdle: formed by 2 coxal bones. Each coxal bone consists of three regions, each region existing as a separate bone in childhood.
These regions are the ilium, which forms the hip, the ischium, on which you sit, and the pubis in the groin region. The coxal bones articulate with the sacrum and fuse anteriorly at a region known as the pubis symphysis. This entire structure is the pelvis. Males and females differ in pelvic structure. In males the pelvis is taller, deeper, and narrower, and the angle between the two pubis bone regions is less than 90 degrees. In women, the pelvis is shorter, shallower, and broader, and the pubic angle is > 90o.
Notice the obturator foramen, the large hole in each coxal bone.
These are the largest holes in the skeleton. Ligaments, blood vessels, and nerves pass through these holes. Also notice the cup-like depression, the acetabulum, in each coxal bone. It receives the head of the femur, the thigh bone. Compare this ball-and-socket joint with the shoulder joint (scapula-humerus). Which would you predict might be dislocated more readily? Why might the shoulder joint be structured the way it is?
H. Upper leg (thigh)
1. Femur: the largest bone of the skeleton, it has a large round head
and a long neck. The neck is the region where hip fractures tend
to occur. Two large projections called trochanters extend from the
proximal end of the femur; these are for ligament attachment. At
the distal end, two condyles form a joint at the knee with the tibia
of the lower leg. Notice the smooth surface along which the patella
or kneecap lies. The patella is embedded in a ligament and serves
to protect the knee joint.
I. Lower leg (shank)
1. Tibia: the large bone of the lower leg, forming the shin. Inflam-
mation of the outer covering or periosteum of the tibia is called
shin splints. The medial malleolus at the distal end of the tibia
is the bump on the inside of the ankle.
2. Fibula: this long thin bone has a head which articulates with the
tibia, not the femur. This bone bears no weight, but aids in the
rotation of the lower leg. At the distal end is the lateral malleolus,
the bump on the outside of the ankle.
J. Ankle: formed by 8 tarsal bones, 2 of which are the following:
1. Talus: forms joints with the tibia and fibula.
2. Calcaneus: bears the weight of the body and forms the heel.
K. Foot: formed by 5 metatarsals.
L. Toes: formed by 14 phalanges.
Some additional anatomy
Identify the following on the human torso model :
1. larynx 11. large intestine
2. thyroid 12. spleen
3. heart 13. kidneys
4. lungs 14. ureters
6. liver 16. urinary bladder
7. gallbladder 17. aorta
8. stomach 18. vena cavae
9. pancreas 19. jugular veins
10. duodenum 20. carotid arteries
Label the indicated structures on the following photos:
Internet Resources:
For further information on the musculoskeletal system and non-invasive methods for visualizing internal structures, look at these "Virtual Hospital" Web sites at the University of Iowa:
http://www.vh.org/Patients/IHB/OrgSys/Musculoskeletal.html and
http://www.vh.org/adult/provider/radiology/MuscleInjuries/MuscleInjuries.html.
Bone up on the skeleton at http://orthocenter.com/anatomy.htm.
The National Institutes of Health has lots of information about arthritis, osteoporosis, and other muskuloskeletal disorders at http://www.niams.nih.gov/hi/index.htm.
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