NEET MDS Lessons
Anatomy
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
The Medial Wall of the Orbit
- This wall is paper-thin and is formed by the orbital lamina or lamina papyracea of the ethmoid bone, along with contributions from the frontal, lacrimal, and sphenoid bones (L. papyraceus, "made of papyrus" or parchment paper).
- There is a vertical lacrimal groove in the medial wall, which is formed anteriorly by the maxilla and posteriorly by the lacrimal bone.
- It forms a fossa for the lacrimal sac and the adjacent part of the nasolacrimal duct.
- Along the suture between the ethmoid and frontal bones are two small foramina; the anterior and posterior ethmoidal foramina.
- These transmit nerves and vessels of the same name.
Initially, four clefts exist; however, only one gives rise to a definite structure in adults.
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1st pharyngeal cleft |
Penetrates underlying mesenchyme and forms EAM. The bottom of EAM forms lateral aspect of tympanic cavity. |
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2nd pharyngeal cleft |
Undergoes active proliferation and overlaps remaining clefts. It merges with ectoderm of lower neck such that the remaining clefts lose contact with outside. Temporarily, the clefts form an ectodermally lined cavity, the cervical sinus, but this disappears during development. |
The Oral Cavity
- The oral cavity (mouth) consists of two parts: the vestibule and the mouth proper.
- The vestibule is the slit-like spaced between the cheeks and the lips and the teeth and gingivae.
- It is the entrance of the digestive tract and is also used for breathing.
- The vestibule communicates with the exterior through the orifice of the mouth.
- The oral cavity is bounded:
- Externally: by the cheeks and lips.
- Roof of oral cavity: formed by the palate.
- Posteriorly: the oral cavity communicates with the oropharynx.
Muscles of the larynx
Extrinsic muscles
suprahyoid: raise larynx, depress mandible for swallowing
infrahyoid: lower larynx for swallowing
both stabilize hyoid for tongue movements
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.
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Pharyngeal Arch |
Arch Artery |
Cranial Nerve |
Skeletal elements |
Muscles |
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1 |
Terminal Branch of maxillary artery |
Maxillary and mandibular division of trigemenial (V) |
Derived from arch cartilages (originating from neural crest): From maxillary cartilages: Alispenoid, incus From mandibular: Mackel’s cartilage, malleus
Upper portion of external ear (auricle) is derived from dorsal aspect of 1st pharyngeal arch.
Derived by direct ossification from arch dermal mesenchyme: Maxilla, zygomatic, squamous portion of temporal bone, mandible
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Muscles of mastication (temporalis, masseter, and pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini (originate from cranial somitomere 4) |
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2 |
Stapedius artery (embryologic) and cortiotympanic artery (adult) |
Facial nerve (VII) |
Stapes, styloid process, stylohyoid ligament, lesser horns and upper rim of hyoid (derived from the second arch cartilage; originate from neural crest).
Lower portion of external ear (auricle) is derived from 2nd pharyngeal arch. |
Muscles of facial expression (orbicularis oculi, orbicularis oris, auricularis, platysma, fronto-ooccipitalis, buccinator), posterior belly of digastric, stylohyoid, stapedius (originate from cranial somitomere 6) |
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3 |
Common carotid artery, most of internal carotid |
Glossopharyngeal (IX) |
Lower rim and greater horn of hyoid (derived from the third arch cartilage; originate from neural crest cells) |
Sytlopharyngeus (originate from cranial somitomere 7) |
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4 |
Left: Arch of aorta; Right: Right subclavian artery; Original sprouts of pulmonary arteries |
Superior laryngeal branch of vagus (X) |
Laryngeal cartilages (Derived from the 4th arch cartilage, originate from lateral plate mesoderm) |
Constrictors of pharynx, cricothyroid, levator veli palatine (originate from occipital somites 2-4) |
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6 |
Ductus arteriosus; roots of definitive pulmonary arteries |
Recurrent laryngeal branch of vagus (X) |
Laryngeal cartilages (derived from the 6th-arch cartilage; originate from lateral plate mesoderm) |
Intrinsic muscles of larynx (originate from occipital somites 1 and 2) |