Which structure prevents spread of infection from middle ear to brain?
What is the distance of the medial rectus from the limbus?
Upper Lid Retractors include
Which nerve is not involved in superior orbital fissure syndrome?
Downward and outward movement of eye is affected in injury of?
Yoke muscle for the right superior rectus is:
Which muscles are responsible for the elevation of the eye?
What is the longest part of the optic nerve?
What is the thinnest portion of the sclera?
Which of the following does not contribute to the formation of the nasal septum?
NEET-PG 2013 - Anatomy NEET-PG Practice Questions and MCQs
Question 91: Which structure prevents spread of infection from middle ear to brain?
- A. Tegmen tympani (Correct Answer)
- B. Cribriform plate
- C. Fundus tympani
- D. Petrous apex
Explanation: ***Tegmen tympani*** - The **tegmen tympani** is a thin plate of bone forming the roof of the middle ear cavity, separating it from the **middle cranial fossa** and the brain. - Its primary function is to act as a **bony barrier**, preventing upward spread of infection from the middle ear space into the intracranial cavity. *Cribriform plate* - The **cribriform plate** is part of the ethmoid bone, located in the anterior cranial fossa, and is perforated by the **olfactory nerves**. - It does not form a boundary to the middle ear cavity and is not involved in preventing infection spread from the middle ear. *Fundus tympani* - This term is not a standard anatomical landmark. The **floor of the tympanic cavity**, or **fundus tympani**, separates the middle ear from the **internal jugular vein**. - It does not prevent the spread of infection to the brain but rather to structures below the middle ear. *Petrous apex* - The **petrous apex** is the very tip of the petrous part of the temporal bone, which houses the cochlea and vestibule. - While part of the temporal bone, it is not the direct barrier between the middle ear cavity and the brain; its involvement in infection spread is typically due to **petrous apexitis**, a distinct complication.
Question 92: What is the distance of the medial rectus from the limbus?
- A. 4.5 mm
- B. 5.5 mm (Correct Answer)
- C. 7.0 mm
- D. 10 mm
Explanation: ***5.5 mm*** - The **medial rectus muscle** inserts into the sclera at an average distance of **5.5 mm** from the limbus [1]. - This distance is an important anatomical landmark in **ophthalmic surgery** and ocular motility studies. - The insertion distances follow the **Spiral of Tillaux** pattern. *4.5 mm* - This distance does **not correspond** to any of the standard rectus muscle insertion points. - The closest insertion is the **medial rectus at 5.5 mm**, followed by the **inferior rectus at 6.5 mm** [1]. *7.0 mm* - This distance corresponds to the insertion point of the **lateral rectus muscle** from the limbus [1]. - It is the **second farthest insertion point** among the recti muscles. *10 mm* - This distance is incorrect for any of the **rectus muscle insertions** from the limbus. - The rectus muscles insert at varying distances following the **Spiral of Tillaux**: medial (5.5 mm), inferior (6.5 mm), lateral (7.0 mm), and superior (7.7 mm).
Question 93: Upper Lid Retractors include
- A. Muller muscle and superior rectus
- B. Levator palpebrae superioris and superior oblique
- C. Superior oblique and superior rectus
- D. Levator palpebrae superioris & Muller muscle (Correct Answer)
Explanation: ***Levator palpebrae superioris & Muller muscle*** - The **levator palpebrae superioris (LPS)** is the primary muscle responsible for lifting the upper eyelid. It is a striated muscle innervated by the oculomotor nerve (CN III). - **Müller's muscle** (also known as the superior tarsal muscle) is a smooth muscle that provides an additional, sustained lift to the upper eyelid. It is sympathetically innervated. *Muller muscle and superior rectus* - While **Müller's muscle** is an upper lid retractor, the **superior rectus** muscle primarily acts to elevate and adduct the eyeball, not the eyelid itself [1]. - The superior rectus muscle has only a minor, indirect role in upper eyelid elevation through its connection with the LPS aponeurosis. *Levator palpabrae superioris and superior oblique* - The **levator palpebrae superioris (LPS)** is a key upper lid retractor. - However, the **superior oblique** muscle is involved in depressing and intorting the eyeball [1], and has no direct role in upper eyelid retraction. *Superior oblique and superior rectus* - Neither the **superior oblique** nor the **superior rectus** muscles are primary upper lid retractors. - The superior oblique depresses and intorts the eye, while the superior rectus elevates and adducts the eye [1]. Both are extrinsic ocular muscles.
Question 94: Which nerve is not involved in superior orbital fissure syndrome?
- A. 1st cranial nerve (Correct Answer)
- B. 3rd cranial nerve
- C. 4th cranial nerve
- D. 6th cranial nerve
Explanation: ***1st cranial nerve*** - The **olfactory nerve (CN I)** is responsible for the sense of smell [2] and passes through the **cribriform plate** of the ethmoid bone, not the superior orbital fissure. - Due to its distinct pathway, it is not affected in **superior orbital fissure syndrome**. *3rd cranial nerve* - The **oculomotor nerve (CN III)** passes through the superior orbital fissure and is frequently involved in the syndrome. - Its involvement leads to ophthalmoplegia, ptosis, and a dilated pupil due to paralysis of most extrinsic ocular muscles [1], [3] and the parasympathetic fibers [1]. *4th cranial nerve* - The **trochlear nerve (CN IV)** also travels through the superior orbital fissure. - Damage to this nerve causes **diplopia** and impaired downward and intorsion movements of the eye due to paralysis of the **superior oblique muscle** [3]. *6th cranial nerve* - The **abducens nerve (CN VI)** enters the orbit via the superior orbital fissure. - Injury to the abducens nerve results in **lateral rectus muscle** palsy, leading to esotropia (medial deviation of the eye) and impaired abduction [3].
Question 95: Downward and outward movement of eye is affected in injury of?
- A. 3rd nerve (Correct Answer)
- B. 4th nerve
- C. 5th nerve
- D. 6th nerve
Explanation: The 3rd cranial nerve (oculomotor nerve) controls most extraocular muscles including the superior rectus, inferior rectus, medial rectus, and inferior oblique, as well as the levator palpebrae superioris. Injury to the 3rd nerve results in paralysis of these muscles, leaving only the lateral rectus (6th nerve) and superior oblique (4th nerve) functioning. This causes the classic "down and out" position of the eye at rest due to the unopposed action of these two muscles [1]. The eye is pulled downward by the superior oblique and outward by the lateral rectus [1]. Additional features include ptosis (drooping eyelid), dilated pupil, and diplopia (double vision) [1][2]. The patient loses the ability to move the eye upward, downward (via inferior rectus), and medially. Incorrect Option: 4th nerve - The 4th cranial nerve (trochlear nerve) innervates the superior oblique muscle. The superior oblique primarily causes depression (downward), intorsion, and abduction of the eye [1]. However, its action is most effective for downward and INWARD movement when the eye is adducted. - 4th nerve palsy results in vertical diplopia (especially when looking down and inward, like reading or descending stairs), hypertropia (upward deviation), and head tilt to the opposite side. This does NOT produce a "down and out" position. Incorrect Option: 5th nerve - The 5th cranial nerve (trigeminal nerve) provides sensory innervation to the face and motor innervation to the muscles of mastication. It has no role in eye movements. Incorrect Option: 6th nerve - The 6th cranial nerve (abducens nerve) innervates the lateral rectus muscle, responsible for abduction (outward movement) of the eye [1]. 6th nerve palsy causes inability to abduct the eye, resulting in esotropia (inward deviation) and horizontal diplopia.
Question 96: Yoke muscle for the right superior rectus is:
- A. Left Inferior Oblique (Correct Answer)
- B. Left Lateral Rectus
- C. Left Superior rectus
- D. Left Inferior rectus
Explanation: ***Left Inferior Oblique*** - Yoke muscles are pairs of synergistic muscles, one in each eye, that act together to produce conjugate eye movements in the same direction of gaze. - The **right superior rectus** and **left inferior oblique** are yoke muscles that work together during **upward and rightward gaze** (dextro-elevation) [1]. - Right SR elevates the **adducted** right eye, while left IO elevates the **abducted** left eye, producing coordinated upward-right movement [1]. - This follows **Hering's Law of Equal Innervation**, where yoke muscles receive equal and simultaneous innervation. *Left Superior rectus* - The left superior rectus is the **contralateral homologous muscle**, not a yoke muscle for the right superior rectus. - Both superior recti work together for **upward gaze in primary position**, but they are versional muscles, not yoke pairs. - Yoke muscles produce conjugate movements in oblique directions, not straight up. *Left Inferior rectus* - The left inferior rectus depresses the left eye and is an antagonist to elevation. - It would pair with the **right superior oblique** for downward-left gaze (levo-depression), not with the right superior rectus. *Left Lateral Rectus* - The left lateral rectus is responsible for **abduction** of the left eye (leftward gaze). - Its yoke muscle is the **right medial rectus** for leftward horizontal gaze (levoversion), not for upward-right gaze.
Question 97: Which muscles are responsible for the elevation of the eye?
- A. SR and IO (Correct Answer)
- B. IO and SO
- C. IR and SR
- D. SO and IR
Explanation: ***SR and IO*** - The **superior rectus (SR)** muscle primarily elevates the eye, especially when the eye is **abducted** [1]. - The **inferior oblique (IO)** muscle also contributes to elevation, particularly when the eye is **adducted** [1]. *IO and SO* - While the **inferior oblique (IO)** elevates the eye, the **superior oblique (SO)** muscle is responsible for **depression** and **intorsion**, not elevation [1]. - Therefore, this combination does not exclusively perform elevation. *IR and SR* - The **superior rectus (SR)** muscle elevates the eye, but the **inferior rectus (IR)** muscle is responsible for **depression** of the eye, not elevation [1]. - This pair has opposing primary actions in vertical movement. *SO and IR* - Both the **superior oblique (SO)** and **inferior rectus (IR)** muscles are primarily involved in **depression** of the eye [1]. - The superior oblique also causes **intorsion**, and the inferior rectus causes **extorsion** [1].
Question 98: What is the longest part of the optic nerve?
- A. Intracanalicular
- B. Intraorbital (Correct Answer)
- C. Intraocular
- D. Intracranial
Explanation: ***Intraorbital*** - The **intraorbital segment** is the **longest portion** of the optic nerve, measuring approximately **25-30 mm**. - It extends from the posterior pole of the eyeball to the **optic canal** and is characterized by a curved, **S-shaped course** within the orbit. - This excess length (approximately 8 mm more than the distance it spans) allows for **free eye movements** without putting tension on the nerve. *Intracranial* - The **intracranial portion** extends from the **optic canal** to the **optic chiasm**, measuring approximately **10-16 mm**. - While often mistakenly thought to be the longest, it is actually the **second longest segment**. - This segment is crucial for the formation of the **optic chiasm** where partial decussation of fibers occurs. *Intracanalicular* - The **intracanalicular portion** passes through the **optic canal** within the sphenoid bone, measuring approximately **4-10 mm**. - This segment is relatively short and constricted, making it vulnerable to compression in conditions like optic nerve gliomas or meningiomas. *Intraocular* - The **intraocular portion** (optic disc) is the **shortest segment**, measuring only about **1 mm**. - It passes through the **lamina cribrosa** of the sclera and is visible on fundoscopy as the optic disc.
Question 99: What is the thinnest portion of the sclera?
- A. Anterior to the rectus muscle insertion
- B. At the posterior pole
- C. At the limbus
- D. Posterior to the rectus muscle insertion (Correct Answer)
Explanation: ***Posterior to the rectus muscle insertion*** - The sclera is thinnest immediately **posterior to the insertion of the rectus muscles**, where it is about 0.3 mm thick. - This area is clinically relevant as it is a common site for globe rupture during trauma. *Anterior to the rectus muscle insertion* - The sclera is relatively thick in this region, measuring around **0.6 mm thick**. - It provides robust support and attachment for the rectus muscles. *At the posterior pole* - At the posterior pole, the sclera is the **thickest**, reaching about 1.0 mm, especially around the optic nerve. - This thickness is necessary to protect the delicate neural structures exiting the eye. *At the limbus* - The sclera-corneal junction, or **limbus**, has an intermediate thickness, around **0.8 mm**. - This area is critical for surgical procedures but is not the thinnest point.
Question 100: Which of the following does not contribute to the formation of the nasal septum?
- A. Nasal bone (Correct Answer)
- B. Vomer
- C. Ethmoid
- D. Septal cartilage
Explanation: ***Nasal bone*** - The **nasal bones** form the bridge of the nose and are part of the external nasal skeleton, not the internal nasal septum. - They articulate with the frontal bone superiorly and the maxilla laterally, forming the **roof of the nasal cavity** anteriorly. *Septal cartilage* - The **septal cartilage**, or quadrangular cartilage, forms the anterior and inferior parts of the cartilaginous nasal septum. - It provides flexibility and support to the anterior nasal cavity. *Vomer* - The **vomer** is a thin, plowshare-shaped bone that forms the posteroinferior part of the bony nasal septum. - It articulates with the sphenoid, ethmoid, palatine, and maxillary bones. *Ethmoid* - The **perpendicular plate of the ethmoid bone** forms the superior part of the bony nasal septum. - It extends downward from the cribriform plate to meet the vomer and septal cartilage.