The Paranasal Sinuses

• These sinuses are air-filled extensions of the respiratory part of the nasal cavity.
• They are in the following bones, frontal, ethmoid, sphenoid and the maxilla.

The Frontal Sinuses

• These are located between the outer and inner tables of the frontal bone, posterior to the superciliary arches.

The Ethmoidal Sinuses

• These comprise of several small cavities, called ethmoidal air cells, within the ethmoidal labyrinth (G. labyrinthos, a maze) of the lateral mass of the ethmoid bone.

The Sphenoidal Sinuses

• These occupy a variable amount in the body of the sphenoid bone and may extend into the wings.

The Maxillary Sinuses

• These are the largest pair of paranasal sinuses.
• They are pyramidal-shaped cavities that may occupy the entire bodies of the maxillae.

The Auditory Ossicles

The Malleus

• Its superior part, the head, lies in the epitympanic recess.
• The head articulates with the incus.

• The neck, lies against the flaccid part of the tympanic membrane.
• The chorda tympani nerve crosses the medial surface of the neck of the malleus.

• The handle of the malleus (L. hammer) is embedded in the tympanic membrane and moves with it.
• The tendon of the tensor tympani muscle inserts into the handle.

The Incus

• Its large body lies in the epitympanic recess where it articulates with the head of the malleus.
• The long process of the incus (L. an anvil) articulates with the stapes.
• The short process is connected by a ligament to the posterior wall of the tympanic cavity.

The Stapes

• The base (footplate) of the stapes (L. a stirrup), the smallest ossicle, fits into the fenestra vestibuli or oval window on the medial wall of the tympanic cavity.

Functions of the Auditory Ossicles

• The auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane.

•  Provides a rigid support system
• Protects delicate structures (e. g., the protection provided by the bones of the vertebral column to the spinal cord)
• Bones supply calcium to the blood; are involved In the formation of blood cells (hemopoiesis)
• Bones serve as the basis of attachment of muscles; form levers in the joint areas, aIlowing movement

The Walls of the Orbit

• Each orbit has four walls: superior (roof), medial, inferior (floor) and lateral.
• The medial walls of the orbit are almost parallel with each other and with the superior part of the nasal cavities separating them.
• The lateral walls are approximately at right angles to each other

Muscles Around the Nose

The Nasalis Muscle

• This muscle consists of a transverse (compressor naris) and alar (dilator naris) parts.
• It is supplied by the buccal branch of the facial nerve.

Endochondral ossification

• A cartilage model exists
• Through intramembraneous ossification in the perichondrium a collar of bone forms around the middle part of the cartilage model
• The perichondrium change to a periostium
• The bone collar cuts off the nutrient and oxygen supply to the chondrocytes in the cartilage model
• The chondrocytes then increase in size and resorb the surrounding cartilage matrix until only thin vertical septae of matrix are left over
• These thin plates then calcify after which the chondrocytes die
• The osteoclasts make holes in the bone collar through which blood vessels can now enter the cavities left behind by the chondrocytes
• With the blood vessels osteoprogenitor cells enter the tissue
• They position themselves on the calcified cartilage septae, change into osteoblasts and start to deposit bone to form trabeculae
• In the mean time the periosteum is depositing bone on the outside of the bone collar making it thicker and thicker
• The trabeculae,consisting of a core of calcified cartilage with bone deposited on top of it, are eventually resorbed by osteoclasts to form the marrow cavity
• The area where this happens is the primary ossification centre and lies in what is called the diaphysis (shaft)
• This process spreads in two directions towards the two ends of the bone the epiphysis
• In the two ends (heads) of the bone a similar process takes place
• A secondary ossification centre develops from where ossification spreads radially
• Here no bone collar forms
• The outer layer of the original cartilage remains behind to form the articulating cartilage
• Between the primary and the secondary ossification centers two epiphyseal cartilage plates remain
• This is where the bone grows in length

• From the epiphyseal cartilage plate towards the diaphysis a number of zones can be identified:

 Resting zone of cartilage

 Hyaline cartilage

 Proliferation zone

 Chondrocytes divide to form columns of cells that mature.

Hypertrophic cartilage zone

 Chondrocytes become larger, accumulate glycogen, resorb the surrounding matrix so that only thin septae of cartilage remain 

Calcification and degeneration zone

The thin septae of cartilage become calcified.

The calsified septae cut off the nutrient supply to the chondrocytes so subsequently they die.

Ossification zone.

Osteoclasts make openings in the bone collar through which blood vessels then invade the spaces left vacant by the chondrocytes that died.

Osteoprogenitor cells come in with the blood and position themselves on the calcified cartilage

septae, change into osteoblasts and start to deposit bone.

 When osteoblasts become trapped in bone they change to osteocytes.

Growth and remodeling of bone

Long bones become longer because of growth at the epiphyseal plates

They become wider because of bone formed by the periosteum

The marrow cavity becomes bigger because of resorbtion by the osteoclasts

Fracture repair

When bone is fractured a blood clot forms

 Macrophages then remove the clot, remaining osteocytes and damaged bone matrix

The periosteum and endosteum produce osteoprogenitor cells that form a cellular tissue in the fracture area

 Intramembranous and endochondral ossification then take place in this area forming trabeculae.

Trabeculae connect the two ends of the broken bone to form a callus

Remodelling then takes place to restore the bone as it was

Joints

The capsule of a joint seals off the articular cavity,  

The capsule has two layers

 fibrous (outer)

synovial (inner)

The synovial layer is lined by squamous or cuboidal epithelial cells,  Under this layer is a layer of loose or dense CT, The lining cells consists of two types:

- A cells

- B cells

They secrete the synovial fluid

They are different stages of the same cell, They are also phagocytic., The articular cartilage has fibres that run perpendicular to the bone and then turn to run parallel to the surface