NEET MDS Lessons
Anatomy
->The two parietal bones (L. paries, wall) form large parts of the walls of the calvaria.
->On the outside of these smooth convex bones, there are slight elevations near the centre called parietal eminences.
->The middle of the lateral surfaces of the parietal bones is crossed by two curved lines, the superior and inferior temporal lines.
->The superior temporal line indicates an attachment of the temporal fascia; the inferior temporal line marks the superior limit of the temporalis muscle.
->The parietal bones articulate with each other in the median plane at the sagittal suture. The medial plane of the body passes through the sagittal suture.
->The inverted V-shaped suture between the parietal bones and the occipital bones is called the lambdoid suture because of its resemblance to the letter lambda in the Greek alphabet.
->The point where the parietal and occipital bones join is a useful reference point called the lambda. It can be felt as a depression in some people.
->In addition to articulation with each other and the frontal and occipital bones, the parietal bones articulate with the temporal bones and the greater wings of the sphenoid bone.
->In foetal and infant skulls, the bones of the calvaria are separated by dense connective tissue membranes at sutures.
->The large fibrous area where several sutures meet are called fonticuli or fontanelles.
->The softness of these bones and looseness of their connections at these sutures enable the calvaria to undergo changes of shape during birth called molding. Within a day or so after birth, the shape of the infant’s calvaria returns to normal.
->The loose construction of the new-born calvaria also allows the skull to enlarge and undergo remodelling during infancy and childhood.
->Relationships between the various bones are constantly changing during the active growth period.
->The increase in the size of the cranium is greatest during the first 2 years, the period of most rapid postnatal growth of the brain.
->The cranium normally increases in capacity until about 15 or 16 years of age; thereafter the cranium usually increases only slightly in size as its bones thicken for 3 to 4 years.
Digastric Muscle
- Origin:
- Anterior Belly: Digastric fossa of the mandible.
- Posterior Belly: Mastoid notch of the temporal bone.
- Insertion: Intermediate tendon attached to the body of the hyoid bone.
- Nerve Supply:
- Anterior Belly: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
- Posterior Belly: Facial nerve (CN VII).
- Arterial Supply:
- Anterior Belly: Branch of the submental artery.
- Posterior Belly: Muscular branch of the posterior auricular artery and occipital artery.
- Action: Raises the hyoid bone and base of the tongue, steadies the hyoid bone, and opens the mouth by lowering the mandible.
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.
Stylohyoid Muscle
- Origin: Posterior border of the styloid process of the temporal bone.
- Insertion: Body of the hyoid bone at the junction with the greater horn.
- Nerve Supply: Facial nerve (CN VII).
- Arterial Supply: Muscular branches of the facial artery and muscular branches of the occipital artery.
- Action: Elevates the hyoid bone and base of the tongue.
|
Pharyngeal Arch |
Arch Artery |
Cranial Nerve |
Skeletal elements |
Muscles |
|
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
|
Muscles of mastication (temporalis, masseter, and pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini (originate from cranial somitomere 4) |
|
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) |
|
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) |
|
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) |
|
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) |
|
Mesodermal Origin |
Muscles |
Innervation |
|
Somitomeres 1, 2 |
Superior, medial and ventral recti |
Oculomotor (III) |
|
Somitomere 3 |
Superior oblique |
Trochlear (IV) |
|
Somitomere 4 |
Jaw-closing muscles |
Trigeminal (V) |
|
Somitomere 5 |
Lateral rectus |
Abducens (VI) |
|
Somitomere 6 |
Jaw-opening and other 2nd arch muscles |
Facial (VII) |
|
Somitomere 7 |
Stylopharyngeus |
Glossopharyngeal (IX) |
|
Somites 1, 2 |
Intrinsic laryngeals |
Vagus (X) |
|
Somites 2-5 |
Tongue muscles |
Hypoglossal (XII) |
The Submandibular Glands
- Each of these U-shaped salivary glands is about the size of a thumb and lies along the body of the mandible.
- It is partly superior and partly inferior to the posterior 1/2 of the base of the mandible.
- It is partly superficial and partly deep to the mylohyoid muscle.
- The submandibular duct arises from the portion of the gland that lies between the mylohyoid and hyoglossus muscle.
- The duct passes deep and then superficial to the lingual nerve.
- It opens by one to three orifices on a small sublingual papilla beside the lingual frenulum.
- The submandibular gland is supplied by parasympathetic, secretomotor fibres from the submandibular ganglion (preganglionic fibres from the chorda tympani via the lingual nerve).