NEET MDS Lessons
Anatomy
The Auditory Tube
- This is a funnel-shaped tube connecting the nasopharynx to the tympanic cavity.
- Its wide end is towards the nasopharynx, where it opens posterior to the inferior meatus of the nasal cavity.
- The auditory tube is 3.5 to 4 cm long; its posterior 1/3 is bony and the other 2/3 is cartilaginous.
- It bony part lies in a groove on the inferior aspect of the base of the skull, between the petrous part of the temporal bone and the greater wing of the sphenoid bone.
- The function of the auditory tube is to equalise pressure of the middle ear with atmospheric pressure.
The Temporalis Muscle
- This is an extensive fan-shaped muscle that covers the temporal region.
- It is a powerful masticatory muscle that can easily be seen and felt during closure of the mandible.
- Origin: floor of temporal fossa and deep surface of temporal fascia.
- Insertion: tip and medial surface of coronoid process and anterior border of ramus of mandible.
- Innervation: deep temporal branches of mandibular nerve (CN V3).
- The temporalis elevates the mandible, closing the jaws; and its posterior fibres retrude the mandible after protrusion.
The Meatus of the Nose
Sphenopalatine Recess
- This space is posterosuperior to the superior concha.
- The sphenoidal sinus opens into this recess.
Superior Meatus
- This is a narrow passageway between the superior and middle nasal conchae.
- The posterior ethmoidal sinuses open into it by one or more orifices.
Middle Meatus
- This is longer and wider than the superior one.
- The anterosuperior part of this meatus lead into a funnel-shaped opening, called the infundibulum, through which the frontonasal duct leads to the frontal sinus.
- There is one duct for each frontal sinus and since there may be several, there may be several frontonasal ducts.
- When the middle concha is removed, rounded elevation called the ethmoidal bulla (L. bubble), is visible
- The middle ethmoidal air cells open on the surface of the ethmoidal bulla.
- Inferior to this bulla is a semicircular groove called the hiatus semilunaris.
- The frontal sinus opens into this hiatus anterosuperiorly.
- Near the hiatus are the openings of the anterior ethmoid air cells.
- The maxillary sinus also opens into the middle meatus.
Inferior Meatus
- This is a horizontal passage, inferolateral to the inferior nasal concha.
- The nasolacrimal duct opens into the anterior part of this meatus.
- Usually, the orifice of this duct is wide and circular.
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.
Internal Muscles of the Pharynx
- The internal, chiefly longitudinal muscular layer, consists of 3 muscles: stylopharyngeus, palatopharyngeus, and salpingopharyngeus.
- They all elevate the larynx and pharynx during swallowing and speaking.
The Stylopharyngeus Muscle
- This is a long, thin, conical muscles that descends inferiorly between the external and internal carotid arteries.
- It enters the wall of the pharynx between the superior and middle constrictor muscles.
- Origin: styloid process of temporal bone.
- Insertion: posterior and superior borders of thyroid cartilage with palatopharyngeus muscle.
- Innervation: glossopharyngeal nerve (CN IX).
- It elevates the pharynx and larynx and expands the sides of the pharynx, thereby aiding in pulling the pharyngeal wall over a bolus of food.
The Palatopharyngeus Muscle
- This is a thin muscle and the overlying mucosa form the palatopharyngeal arch.
The Salpingopharyngeus Muscle
- This is a slender muscle that descends in the lateral wall of the pharynx.
- The over lying mucous membrane forms the salpingopharyngeal fold.
- Origin: cartilaginous part of the auditory tube.
- Insertion: blends with palatopharyngeus muscle.
- Innervation: through the pharyngeal plexus.
- It elevates the pharynx and larynx and opens the pharyngeal orifice of the auditory tube during swallowing.
Structure of the Nasal Septum
- This part bony, part cartilaginous septum divides the chamber of the nose into two narrow nasal cavities.
- The bony part of the septum is usually located in the median plane until age 7; thereafter, it often deviates to one side, usually the right.
- The nasal septum has three main components: (1) the perpendicular plate of the ethmoid bone; (2) the vomer, and (3) the septal cartilage.
- The perpendicular plate, which forms the superior part of the septum, is very thin and descends from the cribiform plate of the ethmoid bone.
- The vomer, which forms the posteroinferior part of the septum, is a thin, flat bone. It articulates with the sphenoid, maxilla and palatine bones.
Intramembranous ossification
- Flat bones develop in this way (bones of the skull)
- This type of bone development takes place in mesenchymal tissue
- Mesenchymal cells condense to form a primary ossification centre (blastema)
- Some of the condensed mesenchymal cells change to osteoprogenitor cells
- Osteoprogenitor cells change into osteoblasts which start to deposit bone
- As the osteoblasts deposit bone some of them become trapped in lacunae in the bone and then change into osteocytes
- Osteoblasts lie on the surface of the newly formed bone
- As more and more bone is deposited more and more osteocytes are formed from mesenchymal cells
- The bone that is formed is called a spicule
- This process takes place in many places simultaneously
- The spicules fuse to form trabeculae
- Blood vessels grow into the spaces between the trabeculae
- Mesenchymal cells in the spaces give rise to hemopoetic tissue
- This type of bone development forms the first phase in endochondral development
- It is also responsible for the growth of short bones and the thickening of long bones