A NOTE ON ANATOMY OF PAROTID GLAND

Parotid gland

The parotid gland is the largest of the salivary glands.

The parotid, a serous compound tubulo-alveolar gland, is yellowish, lobulated, and irregular in shape.

It occupies the interval between the sternomastoid muscle and the mandible.

Average Wt - 25gm (varies in weight from 14 to 28 gm)

Surface anatomy

The parotid gland lies inferior to the zygomatic arch, anteroinferior to the external acoustic meatus, anterior to the mastoid process, and posterior to the ramus of the mandible.

Relations

The parotid gland is enclosed in a sheath (parotid fascia) and is shaped roughly like an inverted pyramid, with three (or four) sides (fig A).

It has a base (from which the superficial temporal vessels and auriculotemporal nerve emerge),apex (which descends inferior and posterior to the angle of the mandible),and lateral, anterior, and posterior (or posterior and medial) surfaces.

The lateral surface is superficial and contains lymph nodes.

The anterior surface is grooved by the ramus of the mandible and masseter (fig.B), producing a medial lip (from which the maxillary artery emerges) and a lateral lip, under cover of which the parotid duct, branches of the facial nerve, and the transverse facial artery emerge (see fig. C).

The posterior surface is grooved by  the mastoid process and the sternomastoid and digastric muscles and  more medially by the styloid process and its attached muscles.

Medially, the superior part of the gland is pierced by the facial nerve and the inferior part by the external carotid artery.

The following structures lie partly within the parotid gland, from superficial to deep:

1. The facial nerve forms the parotid plexus within the gland and separates the glandular tissue partially into superficial and deep layers ("lobes"). In surgical excision of the parotid gland (e.g., for a tumor), damage to the facial nerve is a possibility.

2. The superficial temporal and maxillary veins unite in the gland to form the retromandibular vein, which contributes in a variable manner to the formation of the external jugular vein (see fig. D).

3. The external carotid artery divides within the parotid gland into the superficial temporal and maxillary arteries.

Parotid duct

The parotid duct is about 7 cm long

The parotid duct, emerging under cover of the lateral surface, runs anteriorward on the masseter and turns medially to pierce the buccinator.

The branching of the duct can be examined radiographically after injection of a radio-opaque medium. 

The parotid duct, which is palpable, opens into the oral cavity on the parotid papilla opposite the upper second molar tooth.

Innervation of parotid gland ( fig. E)

Preganglionic parasympathetic secretomotor fibers (from the glossopharyngeal, tympanic, and lesser petrosal nerves) synapse in the otic ganglion.

Postganglionic fibers travel with the auriculotemporal nerve and so reach the gland.

Cranial nerves VII and IX communicate, so that secretory fibers to each of the three major salivary glands may travel in both the facial and glossopharyngeal nerves.

The sympathetic supply to the salivary glands includes vasomotor fibers.

Blood supply

The arteries supplying the parotid gland are derived from the external carotid, and from the branches given off by that vessel in or near its substance. The veins empty themselves into the external jugular, through some of its tributaries.

Lymphatics

The lymphatics end in the superficial and deep cervical lymph glands, passing in their course through two or three glands, placed on the surface and in the substance of the parotid.

A NOTE ON PTERYGOPALATINE FOSSA

PTERYGOPALATINE FOSSA

The pterygopalatine fossa—

           A small, pyramid-shaped space.

           Situated between the maxilla, sphenoid, and palatine bones.

           It communicates via canals, fissures, and foramina with various regions of the skull.

          The contents of the pterygopalatine fossa include

                     The terminal portion of the maxillary artery;

                     The pterygopalatine ganglion;

                     The maxillary division of the trigeminal nerve; and branches of these structures.

Maxillary Artery

The third, or pterygopalatine portion, of the maxillary artery enters the pterygopalatine fossa from the infratemporal fossa via the pterygomaxillary fissure

Maxillary artery and its distribution in the deep face

Branches of the pterygopalatine portion of the maxillary artery are the posterosuperior alveolar, infraorbital, greater palatine, pharyngeal, and sphenopalatine arteries as well as the artery of the pterygoid canal.

The posterior superior alveolar artery branches from the maxillary artery as that vessel enters the pterygomaxillary fissure. It travels on the maxillary tuberosity and enters the posterior superior alveolar foramen accompanied by the like-named nerve. The vessel ramifies within the maxilla to vascularize the maxillary sinus, molars, and premolars as well as the neighboring gingiva.

The infraorbital artery, a continuation of the maxillary artery, enters the orbit through the inferior orbital fissure, lies in the infraorbital groove, leaves the orbit via the infraorbital canal, and enters the face by way of the infraorbital foramen. Branches of the infraorbital artery are the orbital branches, serving the lacrimal gland and the inferior oblique and inferior rectus muscles; the anterior superior alveolar branches, which vascularize the anterior teeth and the maxillary sinus; and the facial branches.

The greater palatine artery and its branch, the lesser palatine artery, pass through the pterygopalatine canal and gain entrance to the palate via the greater palatine and lesser palatine foramina, respectively, to vascularize the hard and soft palates as well as associated structures. The pharyngeal branch passes dorsally, through the pharyngeal canal, to vascularize the auditory tube, sphenoidal sinus, and portions of the pharynx. The sphenopalatine artery leaves the pterygopalatine fossa via the sphenopalatine foramen on its medial wall to enter the nasal fossa. The distribution of this vessel and its branches is discussed later in this chapter. The small artery of the pterygoid canal passes through the posterior wall of the pterygopalatine fossa via the pterygoid canal. It supplies part of the auditory tube, pharynx, middle ear, and sphenoidal sinus.

Maxillary Nerve

The maxillary division of the trigeminal nerve enters the pterygopalatine fossa at its posterior boundary via the foramen rotundum. While in the fossa, it gives off the zygomatic nerve, which, passing into the orbit through the inferior orbital fissure, will bifurcate to form the zygomaticotemporal and zygomaticofacial nerves.

The maxillary division of the trigeminal nerve

The posterior superior alveolar nerves also branch from the maxillary nerve, exit the fossa via the pterygomaxillary fissure, and enter the maxillary tuberosity to serve the maxillary sinus, molars, and adjacent gingiva and cheek. The maxillary nerve then enters the orbit by way of the inferior orbital fissure and is referred to as the infraorbital nerve.

While in the pterygopalatine fossa, the maxillary nerve communicates with the pterygopalatine ganglion via two small trunks, the pterygopalatine nerves; however, these nerves do not bear a functional relationship with the ganglion. Postganglionic parasympathetic fibers derived from the ganglion ride along and distribute with branches of the maxillary division of the trigeminal nerve.

Pterygopalatine ganglion and associated nerves and arteries

Orbital branches are slender nerves that supply the periosteum of the orbit and the mucoperiosteum of the ethmoidal and sphenoidal sinuses. The greater palatine nerve and its branches, the lesser palatine and posterior inferior nasal branches, descend through the pterygopalatine canal to supply regions of the palate, gingiva, tonsil, and lateral wall of the nasal fossa.

Posterior superior nasal branches leave the pterygopalatine fossa via the sphenopalatine foramen to serve the posterior aspect of the nasal fossa and part of the ethmoidal sinus. Its nasopalatine branch grooves the vomer bone in its path to the incisive foramen of the anterior hard palate, which it supplies. The pharyngeal nerve traverses the pharyngeal canal to innervate part of the nasopharynx.

Pterygopalatine Ganglion

The pterygopalatine ganglion seems to be functionally associated with the maxillary division of the trigeminal nerve because it is suspended by the pterygopalatine nerves within the fossa. It is, however, a parasympathetic ganglion of the facial nerve (cranial nerve VII).

This ganglion receives its parasympathetic preganglionic root by way of the pterygoid canal, which opens onto the posterior wall of the fossa. The preganglionic parasympathetic fibers synapse with postganglionic parasympathetic cell bodies within the ganglion. Postsynaptic parasympathetic fibers leave the ganglion and distribute with branches of the maxillary division of cranial nerve V. These fibers are secretomotor in function. They provide parasympathetic flow to the lacrimal gland and mucosal glands of the nasal fossa, palate, and pharynx.

         

   

An Illustrative Note & PowerPoint presentation on Cementum

PERIODONTIUM

PERIODONTIUM

TEETH IN-SITU

Periodontium (forms a specialized fibrous joint called Gomphosis)

•         Cementum

•         Periodontal Ligament

•         Alveolar bone

•         Gingiva facing the tooth

Histology of periodontium

Cementum

It is a hard avascular connective tissue that covers the roots of teeth

Role of Cementum

1.    It covers and protects the root dentin (covers the opening of dentinal tubules)

2.    It provides attachment of the periodontal fibers

3.    It reverses tooth resorption

Varies in thickness:  thickest in the apex and In the inter-radicular areas of multirooted teeth, and thinnest in the cervical area 10 to 15 mm in the cervical areas to 50 to 200 mm (can exceed > 600 mm) apically

        
Cementum simulates bone

•          Organic fibrous framework, ground substance, crystal type, development

•          Lacunae

•          Canaliculi

•          Cellular component

•          Incremental lines (also known as “resting”  lines; they are  produced by continuous but            phasic, deposition of cementum)

Differences between cementum and bone

•         Not vascularized – a reason for it being resistant to resorption

•          Minor ability to remodel

•          More resistant to resorption compared to bone

•          Lacks neural component – so no pain

•          70% of bone is made by inorganic salts (cementum only 45-50%)

•          2 unique cementum molecules: Cementum attachment protein (CAP) and IGF

Clinical Correlation

Cementum is more resistant to resorption: Important in permitting orthodontic tooth movement

Development of Cementum

• Cementum formation occurs along the entire tooth
• Hertwig’s epithelial root sheath (HERS) –Extension of the inner and outer dentalepithelium
• HERS sends inductive signal to ectomesenchymal pulp cells to secrete predentin by differentiating into odontoblasts
• HERS becomes interrupted
• Ectomesenchymal cells from the inner portion of the dental follicle come in with predentin by differentiating into cementoblasts
• Cementoblasts lay down cementum 

PowerPoint presentation on Cementum

A Note On Basic Nature Of Polymers

Basic Nature Of Polymers

CHEMICAL COMPOSITION

The  term  polymer  denotes  a  molecule  that  is made  up of  many(poly) parts(mers). The  mer ending represents the simplest repeating chemical  structural  unit  from  which  the  polymer  is composed. Thus poly(methy1 methacrylate) is  a  polymer having chemical structural units derived from  methyl  methacrylate, as  indicated  by  the simplified reaction and structural formula I.

The  molecules  from  which  the  polymer  is constructed  are  called  monomers  (one  part). Polymer molecules may be prepared from a mixture  of  different  types  of  monomers.  They  are called  copolymers if  they  contain  two  or  more different chemical units  and  telpolymers if  they contain three  different units, as indicated by the structural formulas 11  and 111.

As  a convenience  in expressing the structural formulas of  polymers, the mer units are enclosed in  brackets,  and subscripts such as  n,  m,  and p represent the average number of the various mer units  that make  up the polymer molecules. Notice  that  in  normal  polymers  the  mer  units  are spaced in  a random  orientation  along the polymer  chain.  It is possible, however,  to  produce copolymers  with  mer  units  arranged  so  that  a large number of  one mer type are connected to a large  number  of  another mer  type. This  special type of  polymer is called a blockpolymer. It also is possible to produce polymers having mer units with a special spatial arrangement with respect to the adjacent units; these  are called stereospeczfic polymers.

MOLECULAR WEIGHT

The molecular weight of  the polymer molecule, which equals the molecular weight of  the various mers multiplied by the number of  the mers, may range  from  thousands  to  millions  of  molecular weight units, depending on the preparation conditions. The higher the molecular weight  of  the polymer made from a single monomer, the higher the degree of  polymerization. The term polymerization is often used in a qualitative sense, but the degree  of  polymerization  is  defined  as  the  total number  of  mers in a polymer  molecule. In general,  the  molecular  weight  of  a  polymer  is  reported as the average molecular weight because the number of  repeating units  may  vary  greatly from  one molecule to another. As  would be  expected, the fraction  of  low-, medium-, and high- molecular-weight molecules  in  a  material  or, in other words, the  molecular  weight  distribution, has  as  pronounced  an  effect  on  the  physical properties as the average molecular weight does.

Therefore  two  poly(methy1  methacrylate)  samples can have the same chemical composition but greatly different physical properties because one of  the  samples  has  a  high  percentage  of  low- molecular-weight molecules,  whereas  the other has a high  percentage of  high-molecular  weight molecules. Variation in the molecular weight distribution  may  be  obtained  by  altering  the  polymerization  procedure. These materials therefore do  not  possess  any  precise  physical  constants, such  as  melting  point,  as  ordinary  small  molecules do. For example, the higher the molecular weight,  the  higher  the  softening  and  melting points  and the stiffer the plastic.

SPATIAL  STRUCTURE

In  addition to chemical  composition and molecular  weight, the  physical  or  spatial  structure  of the  polymer  molecules  is  also  important  in  determining  the  properties  of  the  polymer. There are  three  basic  types  of  structures:  linear, branched,  and cross-linked. They are illustrated in Figure as segments  of  linear, branched,  and cross-linked polymers. The linear homopolymer has mer units of  the same type, and the random copolymer  of  the  linear  type  has  the  two  mer units randomly  distributed  along the chain. The linear block  copolymer has segments, or blocks, along the chain where the mer units are the same. The  branched  homopolymer  again  consists  of the  same mer units, whereas the graft-branched copolymer consists of one type of mer unit on the main  chain  and  another  mer  for  the  branches. The  cross-linked  polymer  shown  is  made  up of  a  homopolymer  cross-linked  with  a  single crosslinking  agent.

The linear  and branched  molecules are separate and discrete, whereas the cross-linked  molecules are a network structure that may result in the polymer's becoming one giant molecule. The spatial  structure  of  polymers  affects  their  flow properties,  but  generalizations  are  difficult  to make  because  either  the  interaction  between linear  polymer  molecules  or  the  length  of  the branches  on  the  branched  molecules  may  be more important  in a particular  example. In  general, however, the cross-linked  polymers flow at higher  temperatures  than  linear  or  branched polymers.  Another  distinguishing  feature  of some  cross-linked  polymers  is  that  they  do not absorb  liquids  as  readily  as  either  the  linear  or branched  materials.

An  additional method  of  classifying polymers other than by  their  spatial structure is  according to  whether  they  are  thermoplastic  or  thermosetting.  The  term  thermoplastic  refers  to  polymers that may be softened by heating and solidify  on  cooling,  the  process  being  repeatable.

Typical  examples  of  polymers  of  this  type are  poly(methy1  methacrylate),  polyethylene- polyvinylacetate, and polystyrene. The term thermosetting  refers  to  plastics  that  solidify  during fabrication but cannot be softened by reheating. These  polymers  generally  become  nonfusible because  of  a  crosslinking  reaction  and the  formation  of  a spacial  structure. Typical dental examples  are  cross-linked  poly(methy1 methacrylate),  silicones, cis-polyisoprene, and bisphenol A-diacrylates. Polymers  as  a  class  have  unique  properties, and  by  varying  the  chemical  composition,  molecular weight, molecular-weight  distribution, or spatial arrangement of the mer units, the physical and mechanical properties  of  polymers  may  be altered.

A Note on Muscles of the Face and Scalp.....With A Video

A Video on Muscles of the Face and Scalp

Muscles of the Facial Expression

Muscles of Facial Expression are unique in that they migrate to their destinations about the scalp, neck, and mostly about the face from second pharyngeal arch mesenchyme and thus receive their motor innervation via the facial nerve (CN VII), the nerve of the second arch. Although most of these muscles originate on bone, most do not insert on bone; rather, they insert into the dermis of the skin and freely intermingle with muscles in their vicinity. Upon contraction, this arrangement and groupings of muscles about the orifices of the face convey movements about these orifices that we interpret as emotions.

The muscles of the face (and scalp) are derived from the second pharyngeal arch (hyoid arch) mesenchyme that migrates to its final destination.

Muscles of the Face and Scalp

Considering the origin of these muscles, it is not surprising that they receive motor innervation from branches of the facial nerve (CN VII).

Rather than inserting into bone, these muscles insert into the dermis of the skin, thus their orchestrated contractions convey various shapes to the face that we interpret as emotions. It is important to understand that fascicles of these muscles intermingle with each other, and they tend to act in groups to control the orifices around which they are grouped, such as the orbit, nose, and mouth. It is according to this grouping that they are described.

Muscles of the Face and Scalp
Muscle	Location	Origin
Scalp
Frontalis	Forehead	Procerus, corrugator, orbicularis oculi
Occipitalis	Back of the head	Mastoid process and superior nuchal line
Temporoparietalis	Temple	Temporal fascia
Ear
Auricularis anterior	Anterior to ear	Temporal fascia
Auricularis superior	Above ear	Temporal fascia
Auricularis posterior	Behind ear	Mastoid process
Nose
Procerus
Nasalis
Depressor septi
Eye
Orbicularis oculi	Around the orbit	Nasal process of frontal bone, frontal process of maxilla, medial palpebral ligament, and lacrimal bone
Corrugator	Deep to the orbicularis oculi	Medial aspect of superciliary arch
Mouth
Levator labii superioris	Upper lip	Zygoma and maxilla just above infraorbital foramen
Levator labii superioris alaque nasi	Upper lip and side of nose	Maxilla, frontal process
Levator anguli oris	Corner of mouth	Canine fossa of maxilla
Zygomaticus major	Cheek and corner of mouth	Temporal process of zygoma
Zygomaticus minor	Cheek and corner of mouth	Maxillary process of zygoma
Risorius	Cheek	Masseteric fascia
Depressor labii inferioris	Lower lip	Oblique line of mandible
Depressor anguli oris	Corner of mouth	Oblique line of mandible
Mentalis	Chin	Incisive fossa of mandible
Orbicularis oris	Circumscribes the mouth	Muscles in the vicinity, maxilla, nasal septum, mandible
Buccinator	Cheek	Pterygomandibular raphe, alveola arches of mandible and maxilla
Neck
Platysma	Neck and chin	Pectoral and deltoid fascia

Muscles of the Ear and Nose

The three external muscles of the ear are the auricularis anterior, superior, and posterior. Similarly, the three muscles of the nose are the procerus, nasalis, and depressor septi. These two groups of muscles are fairly inconsequential.

Muscles Surrounding the Orbit

Orbicularis Oculi

The orbicularis oculi muscle is composed of two parts, the palpebral portion and the orbital portion. The former originates from the medial palpebral ligament (attached to the medial aspect of the orbit) and inserts into the lateral palpebral raphe (attached to the lateral aspect of the orbit). The orbital portion of the muscle describes an oval around the orbit.

The orbicularis oculi is innervated by the temporal and zygomatic branches of the facial nerve and acts to close the eyelid completely. Forceful closure is mediated by the orbital portion, whereas the palpebral portion is responsible for light closure, as in blinking.

Corrugator

The corrugator (supercilii) muscle is located deep to the superomedial aspect of the orbicularis oculi, at the medial aspect of the eyebrow. It originates at the medial extent of the superciliary arch and inserts into the skin of the eyebrow.

It is innervated by the temporal and zygomatic branches of the facial nerve; the combined actions of the paired muscles approximate the eyebrows, producing frowns.

Muscles Surrounding the Mouth

Orbicularis Oris

The orbicularis oris completely encircles the mouth. Its fibers are positioned at various depths and angles in the two lips. Fascicles of this muscle, some of which are derived from those of neighboring muscles—especially the buccinator—freely intermingle with fascicles of other muscles acting on the lips, permitting extensive movability. Many of the fibers of the buccinator cross over each other at the angle of the mouth so the upper fibers proceed to the lower lip and the lower fibers to the upper lip. Hence, the origin of the orbicularis oris is complex and is usually considered to be from the fibers of the surrounding muscles as well as from the alveolar portion of the maxilla, the septum of the nose, and the area lateral to the incisive fossa of the mandible. Insertion is into the skin and into itself, forming an ellipse around the mouth.

Buccal branches of the facial nerve innervate this complex muscle, which closes the lips and, during stronger contraction, purses them, as in osculation and whistling.

Risorius

The risorius is a small, horizontally placed muscle that originates in the masseteric fascia and inserts in the skin of the corner of the mouth. This is the smiling muscle; it is responsible for drawing the corners of the mouth laterally. The risorius is innervated by buccal and mandibular branches of the facial nerve.

Depressors of the Lip

The depressor labii inferioris is quadrangular in shape. It originates on the medial extent of the oblique line of the mandible and inserts into the skin of the lower lip. It acts to depress the lower lip.

The depressor anguli oris (triangularis) originates on the oblique line of the mandible and inserts into the skin of the corner of the mouth and depresses it, expressing sadness.

The mentalis is a small muscle of the chin. Its origin is in the incisive fossa of the mandible, and it inserts into the skin of the chin to wrinkle it and also to protrude the lower lip, as in drinking.

The platysma was previously detailed in Chapter 7. All of the muscles of this group, except the platysma, are innervated by the buccal and mandibular branches of the facial nerve.

Elevators of the Lip

Five muscles elevate the lip and corner of the mouth. The levator labii superioris alaque nasi is the most medial of these muscles, originating from the frontal process of the maxilla passing inferiorly along the  side of the nose. It then splits into a medial and a lateral portion to insert into the wing of the nose and into the upper lip. This muscle functions in dilating the nostril and raising the upper lip.

The levator labii superioris originates from the maxilla and zygoma just inferior to the orbit. Its fibers pass across the infraorbital foramen to insert into the upper lip, lateral to and intermingling with the fibers of the levator labii superioris alaque nasi. The levator labii superioris elevates and protrudes the upper lip.

The levator anguli oris lies deep to the levator labii superioris. It originates below the infraorbital foramen, from the canine fossa of the maxilla, to insert into the corner of the mouth. This muscle elevates the angle of the mouth and assists in the formation of the nasolabial furrow.

The zygomaticus minor, a slender muscle arising from the maxillary process of the zygomatic bone, inserts just lateral to the insertion of the levator labii superioris muscle. This muscle elevates the upper lip. It also assists in the formation of the nasolabial furrow.

The zygomaticus major is the lateral-most muscle of this group. It originates on the temporal process of the zygomatic bone and inserts into the corner of the mouth. This muscle elevates the corner of the mouth and pulls it laterally.

All of the five muscles acting to elevate the lips are innervated by the buccal branches of the facial nerve.

Muscle of the Cheek

The buccinator, a quadrangule-shaped muscle occupying the space between the mandible and the maxilla, is the primary muscular component of the cheek. It lies deep to the muscles of facial expression and is separated from them by the buccopharyngeal facia and the buccal fat pad. The parotid duct pierces the substance of this muscle to enter the oral vestibule.

The buccinator originates on the maxilla and mandible, specifically on the buccal surfaces of the alveolar processes in the vicinity of the three molars, and from the pterygomandibular raphe, a collagenous tendinous inscription attached to the pterygoid hamulus and the mylohyoid line of the mandible. This raphe is interposed between the buccinator and superior pharyngeal constrictor muscles.

The buccinator inserts into the fleshy corner of the lip in such a fashion that the upper fascicles and the lower fascicles decussate at the corner of the mouth and insert into the lower and upper lips, respectively, becoming fibers of the orbicularis oris. The highest and lowest fascicles, however, continue without decussation into the upper and lower lips, respectively.

The buccinator muscle acts to press the mucosa of the cheek against the teeth, thus aiding in mastication and deglutition. In addition, it assists in distending the oral vestibule and forcefully expelling air, as in blowing dust particles off a surface. The buccal branch of the facial nerve innervates this muscle.