Essentials of Anatomy and Physiology

3. Hormones—produced by endocrine glands; con-
   cerned with cell division, protein synthesis, calcium
   metabolism, and energy production (see Table 6–1).


4. Exercise or stress—weight-bearing bones must
   bear weight or they will lose calcium and become
   brittle.

The Skeleton—206 bones (see Fig. 6–4);
bones are connected by ligaments

1. Axial—skull, vertebrae, rib cage.
   
   
   • Skull—see Figs. 6–5 through 6–8 and Table 6–2.
     
     
     • Eight cranial bones form the braincase, which
       also protects the eyes and ears; 14 facial bones
       make up the face; the immovable joints
       between these bones are called sutures.
     
     
     • Paranasal sinuses are air cavities in the maxil-
       lae, frontal, sphenoid, and ethmoid bones; they
       lighten the skull and provide resonance for
       voice (see Fig. 6–9).
     
     
     • Three auditory bones in each middle ear cav-
       ity transmit vibrations for the hearing process.
     
     
     
     
   
   
   • Vertebral column—see Fig. 6–10.
     
     
     • Individual bones are called vertebrae: 7 cervi-
       cal, 12 thoracic, 5 lumbar, 5 sacral (fused into
       one sacrum), 4 to 5 coccygeal (fused into one
       coccyx). Supports trunk and head, encloses
       and protects the spinal cord in the vertebral
       canal. Discs of fibrous cartilage absorb shock
       between the bodies of adjacent vertebrae, also
       permit slight movement. Four natural curves
       center head over body for walking upright (see
       Table 6–5 for joints).
     
     
     
     
   
   
   • Rib cage—see Fig. 6–11.
     
     
     • Sternum and 12 pairs of ribs; protects thoracic
       and upper abdominal organs from mechanical
       injury and is expanded to contribute to inhala-
       tion. Sternum consists of manubrium, body,
       and xiphoid process. All ribs articulate with
       thoracic vertebrae; true ribs (first seven pairs)
       articulate directly with sternum by means of
       costal cartilages; false ribs (next three pairs)
       articulate with 7th costal cartilage; floating
       ribs (last two pairs) do not articulate with the
       sternum.
       2. Appendicular—bones of the arms and legs and the
       shoulder and pelvic girdles.
     
     
     
     
   
   
   
   

• Shoulder and arm—see Fig. 6–12 and Table 6–3.


• Scapula—shoulder muscles are attached; gle-
  noid fossa articulates with humerus.


• Clavicle—braces the scapula.


• Humerus—upper arm; articulates with the
  scapula and the ulna (elbow).


• Radius and ulna—forearm—articulate with
  one another and with carpals.


• Carpals—eight—wrist; metacarpals—five—
  hand; phalanges—14—fingers (for joints, see
  Table 6–5).


• Hip and leg—see Figs. 6–13 and 6–14 and Table
  6–4.


• Pelvic bone—two hip bones; ilium, ischium,
  pubis; acetabulum articulates with femur.


• Femur—thigh; articulates with pelvic bone
  and tibia (knee).


• Patella—kneecap; in tendon of quadriceps
  femoris muscle.


• Tibia and fibula—lower leg; tibia bears weight;
  fibula does not bear weight, but does anchor
  muscles and stabilizes ankle.


• Tarsals—seven—ankle; calcaneus is heel bone.


• Metatarsals—five—foot; phalanges—14—toes
  (see Table 6–5 for joints).

Joints—Articulations

1. Classification based on amount of movement:
   
   
   • Synarthrosis—immovable.
   
   
   • Amphiarthrosis—slightly movable.
   
   
   • Diarthrosis—freely movable (see Table 6–5 for
     examples; see also Fig. 6–15).
   
   
   
   


2. Synovial joints—all diarthroses have similar struc-
   ture (see Fig. 6–16):
   
   
   • Articular cartilage—smooth on joint surfaces.
   
   
   • Joint capsule—strong fibrous connective tissue
     sheath that encloses the joint.
   
   
   • Synovial membrane—lines the joint capsule;
     secretes synovial fluid that prevents friction.
   
   
   • Bursae—sacs of synovial fluid that permit ten-
     dons to slide easily across joints.
   
   
   
   

132 The Skeletal System