NEETMEDICALGK

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The hollow tube inside the neck that starts behind the nose and ends at the top of the trachea (windpipe) and esophagus (the tube that goes to the stomach). The pharynx is about 5 inches long, depending on body size.The pharynx, more commonly known as the throat, is a five cm long tube extending behind the nasal and oral cavities until the voice box (larynx) and the esophagus. Essentially, it forms a continuous muscular passage for air, food, and liquids to travel down from your nose and mouth to your lungs and stomach.Most sensory innervation of the pharynx is derived from the glossopharyngeal nerve, specifically the pharyngeal and tonsillar branches (cranial nerve IX), except for the anterior part of the nasopharynx, which is innervated by a branch of the maxillary nerve (cranial nerve V2) called the pharyngeal nerve.The pharynx is composed of mucous membrane, submucosal connective tissue, glands, lymphoid tissue, muscle and an outermost adventitial coating. The mucous membrane does not possess a muscular layer.The oropharynx and pharynx proper are lined by largely non-keratinizing stratified squamous epithelium. The nasopharynx is mainly lined by ciliated columnar epithelium but stratified squamous epithelium occurs at its lower end where it joins the oropharynx.

The Mendel's laws of inheritance include law of dominance, law of segregation and law of independent assortment. The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring.The Mendel's four postulates and laws of inheritance are: (1) Principles of Paired Factors (2) Principle of Dominance(3) Law of Segregation or Law of Purity of Gametes (Mendel's First Law of Inheritance) and (4) Law of Independent Assortment (Mendel's Second Law of Inheritance).

R.H. Whittaker (1969) proposed a Five Kingdom Classification. The

kingdoms defined by him were named Monera, Protista, Fungi, Plantae

and Animalia. The main criteria for classification used by him include cell

structure, body organisation, mode of nutrition, reproduction and

phylogenetic relationships. Table 2.1 gives a comparative account of different

characteristics of the five kingdoms.

The three-domain system has also been proposed that divides the Kingdom

Monera into two domains, leaving the remaining eukaryotic kingdoms in the

third domain and thereby a six kingdom classification. You will learn about

this system in detail at higher classes.

Let us look at this five kingdom classification.KINGDOM MONERA

Bacteria are the sole members of the Kingdom Monera. They are the most

abundant micro-organisms. Bacteria occur almost everywhere. Hundreds

of bacteria are present in a handful of soil. They also live in extreme habitats

such as hot springs, deserts, snow and deep oceans where very few other

life forms can survive. Many of them live in or on other organisms as

parasites.

Bacteria are grouped under four categories based on their shape: the

spherical Coccus (pl.: cocci), the rod-shaped Bacillus (pl.: bacilli), the

comma-shaped Vibrium (pl.: vibrio) and the spiral Spirillum (pl.: spirilla).KINGDOM PROTISTA

All single-celled eukaryotes are placed under Protista, but the boundaries

of this kingdom are not well defined. What may be ‘a photosynthetic

protistan’ to one biologist may be ‘a plant’ to another. In this book we

include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and

Protozoans under Protista. Members of Protista are primarily aquatic.

This kingdom forms a link with the others dealing with plants, animals

and fungi. Being eukaryotes, the protistan cell body contains a well defined

nucleus and other membrane-bound organelles. Some have flagella or

cilia. Protists reproduce asexually and sexually by a process involving

cell fusion and zygote formation.

Amoeboid protozoans: These organisms live in fresh

water, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine

forms have silica shells on their surface. Some of them such as Entamoeba

are parasites.

Flagellated protozoans: The members of this group are either free-living

or parasitic. They have flagella. The parasitic forms cause diaseases such

as sleeping sickness. Example: Trypanosoma.

Ciliated protozoans: These are aquatic, actively moving organisms because

of the presence of thousands of cilia. They have a cavity (gullet) that opens

to the outside of the cell surface. The coordinated movement of rows of

cilia causes the water laden with food to be steered into the gullet. Example:

Paramoecium (Figure 2.4d).

Sporozoans: This includes diverse organisms that have an infectious

spore-like stage in their life cycle. The most notorious is Plasmodium

(malarial parasite) which causes malaria, a disease which has a staggering

effect on human population.

KINGDOM FUNGI

The fungi constitute a unique kingdom of heterotrophic organisms. They

show a great diversity in morphology and habitat. You must have seen

fungi on a moist bread and rotten fruits. The common mushroom you eat

and toadstools are also fungi. White spots seen on mustard leaves are due

to a parasitic fungus. Some unicellular fungi, e.g., yeast are used to make

bread and beer. Other fungi cause diseases in plants and animals; wheat

rust-causing Puccinia is an important example. Some are the source of

antibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water,

soil and on animals and plants. They prefer to grow in warm and humid

places. Have you ever wondered why we keep food in the refrigerator ? Yes,

it is to prevent food from going bad due to bacterial or fungal infections.

With the exception of yeasts which are unicellular, fungi are

filamentous.Virus.

Blood groups and genetic linkage

Red cell groups act as markers (inherited characteristics) for genes present on chromosomes, which are responsible for their expression. The site of a particular genetic system on a chromosome is called a locus. Each locus may be the site of several alleles (alternative genes). In an ordinary cell of the human body, there are 46 chromosomes arranged in 23 pairs, 22 pairs of which are autosomes (chromosomes other than sex chromosomes), with the remaining pair being the sex chromosomes, designated XX in females and XY in males. The loci of the blood group systems are on the autosomes, except for Xg, which is unique among the blood groups in being located on the X chromosome. Genes carried by the X chromosome are said to be sex-linked. Since the blood groups are inherited in a regular fashion, they can be used as genetic markers in family studies to investigate whether any two particular loci are sited on the same chromosome—i.e., are linked. The genes sited at loci on the same chromosome travel together from parent to child, and, if the loci are close together, the genes will rarely be separated.

Loci that are farther apart can be separated by recombination. This happens when material is exchanged between homologous chromosomes (pair of chromosomes) by crossing over during the process of cell division (mitosis). The reproductive cells contain half the number of chromosomes of the rest of the body; ova carry an X chromosome and spermatozoa an X or a Y. The characteristic number of 46 chromosomes is restored at fertilization. In a classical pedigree linkage study, all the members of a family are examined for a test character and for evidence of the nonindependent segregation of pairs of characters. The results must be assessed statistically to determine linkage. Individual chromosomes are identified by the banding patterns revealed by different staining techniques. Segments of chromosomes or chromosomes that are aberrant in number and morphology may be precisely identified. Other methods for localizing markers on chromosomes include somatic cell hybridization (cell culture with alignment of single strands of RNA and DNA) and use of DNA probes (strands of radiolabeled DNA). These methods are useful in classical linkage studies to locate blood group loci. The loci for many red cell groups have been found on chromosomes and in many cases have been further localized on a particular chromosome.In some of the blood group systems, the amount of antigen produced depends on the genetic constitution. The ABO blood group gene codes for a specific carbohydrate transferase enzyme that catalyzes the addition of specific sugars onto a precursor substance. As a new sugar is added, a new antigen is produced. Antigens in the MNSs blood system are the products of genes that control terminal amino acid sequence. The amount of antigen present may depend on the amount of gene product inherited or on the activity of the gene product (i.e., transferase). The red cells of a person whose genotype is MM show more M antigen than do MN red cells. In the case of ABO, the same mechanism may also play a role in antigen expression, but specific activity of the inherited transferase may be more important.

The amount of antigen produced can also be influenced by the position of the genes. Such effects within a genetic complex can be due to determinants on the same chromosome—they are then said to be cis—or to determinants on the opposite chromosome of a chromosome pair—trans.

In the Rh combination cdE/cde, more E antigen is produced than in the combination cDE/cde. This may be due to the suppressor effect of D on E. An example of suppression in the trans situation is that more C antigen is detectable on the red cells from CDe/cde donors than on those of CDe/cDE people. The inheritance of the Rh system probably depends on the existence of operator genes, which turn the activity of closely linked structural genes on or off.

ANTERIOR TRIANGLE OF THE NECK

The anterior triangle of the neck is bounded anteriorly by the median

line of the neck and posteriorly by the anterior margin of sternocleidomastoid. Its base is the inferior border of the mandible and its projection to the mastoid process, and its apex is at the manubrium sterni. It

can be subdivided into suprahyoid and infrahyoid areas above and

below the hyoid bone, and into digastric, submental, muscular and

carotid triangles by the passage of digastric and omohyoid across the

anterior triangle (see Fig. 29.5).

Digastric triangle

The digastric triangle is bordered above by the lower border of the

mandible and its projection to the mastoid process, posteroinferiorly

by the posterior belly of digastric and by stylohyoid, and anteroinferiorly by the anterior belly of digastric. It is covered by the skin, superficial

fascia, platysma and deep fascia, which contain branches of the facial

and transverse cutaneous cervical nerves. Its floor is formed by mylohyoid and hyoglossus. The anterior region of the digastric triangle contains the submandibular gland, which has the facial vein superficial to

it and the facial artery deep to it. The submental and mylohyoid arteries

and nerves lie on mylohyoid. The submandibular lymph nodes are

variably related to the submandibular gland. The posterior region

of the digastric triangle contains the lower part of the parotid gland.

The external carotid artery, passing deep to stylohyoid, curves above

the muscle, and overlaps its superficial surface as it ascends deep to the

parotid gland before entering it. The internal carotid artery, internal

jugular vein and vagus nerve lie deeper and are separated from the

external carotid artery by styloglossus,stylopharyngeus and the glossopharyngeal nerve.Submental triangle

The single submental triangle is demarcated by the anterior bellies of

both digastric muscles. Its apex is at the chin, its base is the body of the

hyoid bone and its floor is formed by both mylohyoid muscles. It contains lymph nodes and small veins that unite to form the anterior

jugular vein. The structures within the digastric and submental triangles

are described in more detail with the floor of the mouth.

Muscular triangle

The muscular triangle is bounded anteriorly by the median line of

the neck from the hyoid bone to the sternum, inferoposteriorly by the

anterior margin of sternocleidomastoid and posterosuperiorly by the

superior belly of omohyoid. The triangle contains omohyoid, sternohyoid, sternothyroid and thyrohyoid.

Carotid triangle

The carotid triangle is limited posteriorly by sternocleidomastoid, anteroinferiorly by the superior belly of omohyoid and superiorly by

stylohyoid and the posterior belly of digastric. In the living (except the

obese), the triangle is usually a small visible triangular depression,

sometimes best seen with the head and cervical vertebral column

slightly extended and the head contralaterally rotated. The carotid triangle is covered by the skin, superficial fascia, platysma and deep fascia

containing branches of the facial and cutaneous cervical nerves. The

hyoid bone forms its anterior angle and adjacent floor; it can be located

on simple inspection and verified by palpation. Parts of thyrohyoid,

hyoglossus and inferior and middle pharyngeal constrictor muscles

form its floor. The carotid triangle contains the upper part of the

common carotid artery and its division into external and internal

carotid arteries. Overlapped by the anterior margin of sternocleidomastoid, the external carotid artery is first anteromedial, then anterior to

the internal carotid artery. Branches of the external carotid artery are

encountered in the carotid triangle. Thus the superior thyroid artery

runs anteroinferiorly, the lingual artery anteriorly with a characteristic

upward loop, facial anterosuperiorly,occipital,post.auricular,maxillary.

anteriorly in the lower neck, level with the fifth cervical to the first

thoracic vertebrae (see Fig. 29.17). It is ensheathed by the pretracheal

layer of deep cervical fascia and consists of right and left lobes connected by a narrow, median isthmus. It usually weighs 25 g but this

varies. The gland is slightly heavier in females and enlarges during

menstruation and pregnancy. Estimation of the size of the thyroid

gland is clinically important in the evaluation and management of

thyroid disorders and can be achieved non-invasively by means of

diagnostic ultrasound. Mean thyroid volume increases with age (Chanoine et al 1991). No significant difference in thyroid gland volume

has been observed between males and females from 8 months to

15 years.

The lobes of the thyroid gland are approximately conical. Their

ascending apices diverge laterally to the level of the oblique lines on

the laminae of the thyroid cartilage, and their bases are level with the

fourth or fifth tracheal cartilages. Each lobe is usually 5 cm long, its

greatest transverse and anteroposterior extents being 3 cm and 2 cm,

respectively. The posteromedial aspects of the lobes are attached to the

side of the cricoid cartilage by a lateral thyroid ligament (Berry’s ligament). The isthmus connects the lower parts of the two lobes, although

occasionally it may be absent. It measures 1.25 cm transversely and

vertically, and is usually anterior to the second and third tracheal cartilages, although it can be higher or even sometimes lower because its

site and size vary greatly. A conical pyramidal lobe often ascends

towards the hyoid bone from the isthmus or the adjacent part of either

lobe (more often the left). It is occasionally detached or in two or more

parts. A fibrous or fibromuscular band, the levator of the thyroid gland,

musculus levator glandulae thyroideae, sometimes descends from the

body of the hyoid to the isthmus or pyramidal lobe. For further reading,

see Mohebati and Shaha (2012).

Ectopic thyroid tissue is rare but may be found around the course

of the thyroglossal duct or laterally in the neck, as well as in distant

places such as the tongue (lingual thyroid), mediastinum and the subdiaphragmatic organs (Noussios et al 2011). The most frequent location

of ectopic thyroid tissue is at the base of the tongue, in particular at the

region of the foramen caecum; often it is the only thyroid tissue present.

Small, detached masses of thyroid tissue may occur above the lobes or

isthmus as accessory thyroid glands. Vestiges of the thyroglossal duct

may persist between the isthmus and the foramen caecum of the tongue,

sometimes as accessory nodules or cysts of thyroid tissue near the

midline or even in the tongue, where they are called thyroglossal duct cyst.PARATHYROID GLANDS

The parathyroid glands are small, yellowish-brown, ovoid or lentiform

structures, usually lying between the posterior lobar borders of the

thyroid gland and its capsule. They are commonly 6 mm long, 3–4 mm Neck

472SECTION

4

across and 1–2 mm from back to front, each weighing about 50 mg.

Typically, there are two on each side, superior and inferior, but there

may be more or there may be only three or many minute parathyroid

islands scattered in connective tissue near the usual sites. Very occasionally, an occult gland may follow a blood vessel into a groove on the

surface of the thyroid. Normally, the inferior parathyroids migrate only

to the inferior thyroid poles, but they may descend with the thymus

into the thorax or they may be sessile and remain above their normal

level near the carotid bifurcation. The anastomotic connection between

the superior and inferior thyroid arteries that occurs along the posterior

border of the thyroid gland usually passes very close to the parathyroids,

and is a useful aid to their identification.

The superior parathyroid glands are more constant.

External acoustic meatus

The temporal bone contains the bony (osseous) part of the external

acoustic meatus.

Ossification

The four temporal components ossify independently (Fig. 37.2). The

squamous part is ossified in a sheet of condensed mesenchyme from a

single centre near the zygomatic roots, which appears in the seventh or

eighth week in utero. The petromastoid part has several centres that

appear in the cartilaginous otic capsule during the fifth month; as many

as 14 have been described. These centres vary in order of appearance.

Several are small and inconstant, soon fusing with others. The otic

capsule is almost fully ossified by the end of the sixth month. The

tympanic part is also ossified in mesenchyme from a centre identifiable

about the third month; at birth, it is an incomplete tympanic ring,

deficient above, its concavity grooved by a tympanic sulcus for the

tympanic membrane. The malleolar sulcus for the anterior malleolar

process, chorda tympani and anterior tympanic artery inclines obliquely

downwards and forwards across the medial aspect of the anterior part

The superior border of the mastoid part is thick and serrated for

articulation with the mastoid angle of the parietal bone. The posterior

border is also serrated and articulates with the inferior border of the

occipital bone between its lateral angle and jugular process. The

mastoid element is fused with the descending process of the squamous part; below, it appears in the posterior wall of the tympanic

cavity.

Petrous part

The petrous part is a mass of bone that is wedged between the sphenoid

and occipital bones in the cranial base; it contains the labyrinth. It is

inclined superiorly and anteromedially, and has a base, apex, three

surfaces (anterior, posterior and inferior) and three borders (superior,

posterior and anterior).

The base would correspond to the part that lies on the base of the

skull and is separated from the squamous part by a suture. However,

this suture disappears soon after birth. The subsequent development of

the mastoid processes means that the precise boundaries of the base

are no longer identifiable.

The apex, blunt and irregular, is angled between the posterior border

of the greater wing of the sphenoid and the basilar part of the occipital

bone. It contains the anterior opening of the carotid canal and limits

the foramen lacerum posterolaterally.

The anterior surface contributes to the floor of the middle cranial

fossa (Ch. 28) and is continuous with the cerebral surface of the squamous part (although the petrosquamosal suture often persists late in

life). The whole surface is adapted to the inferior temporal gyrus.

Behind the apex is a trigeminal impression for the trigeminal ganglion.

Bone anterolateral to this impression roofs the anterior part of the

carotid canal but is often deficient. A ridge separates the trigeminal

impression from another hollow behind, which partly roofs the internal acoustic meatus and cochlea. This, in turn, is limited behind by the

arcuate eminence, which is raised by the superior (anterior) semicircular canal but is not necessarily directly over it. Laterally, the anterior

surface roofs the vestibule and, partly, the facial canal. Between the

squamous part laterally and the arcuate eminence and the hollows just

described medially, the anterior surface is formed by the tegmen

tympani, a thin plate of bone that forms the roof of the mastoid

antrum, and extends forwards above the tympanic cavity and the canal

for tensor tympani. The lateral margin of the tegmen tympani meets

the squamous part at the petrosquamosal suture, turning down in front

as the lateral wall of the canal for tensor tympani and the osseous part

of the pharyngotympanic tube; its lower edge is in the squamotympanic

fissure. Anteriorly, the tegmen bears a narrow groove related to the

greater petrosal nerve (which passes pos.

SCLERA

The sclera accounts for approximately 93% of the outer coat of the eye.

Anteriorly, it is continuous with the cornea at the corneoscleral junction

(see Fig. 42.1). It is punctured by a number of foramina containing

nerves and blood vessels, most notably the optic foramen, which lies

3 mm medial to the midline and 1 mm below the horizontal, and

houses the optic nerve. Smaller openings contain anterior ciliary arteries

that penetrate anteriorly, vortex veins that cross the sclera equatorially,

and the long and short ciliary nerves and arteries that enter posteriorly.

There is considerable individual variation in scleral dimensions. The

sclera is thickest at the posterior pole (approximately 1 mm) and

decreases anteriorly, reaching a minimum equatorially at about half this

thickness. It also thins approaching the optic nerve. The sclera is thinner

when the eye is elongated in myopia.

The external surface of the sclera is covered by a delicate episcleral

lamina of loose fibrovascular tissue, which contains sparse blood vessels

and is in contact with the inner surface of the fascial sheath of the

eyeball. Anteriorly, the external scleral surface is covered by conjunctiva,

which is reflected on to it from the posterior surfaces of the eyelids. The

scleral internal surface adjacent to the choroid is attached to it by a

delicate fibrous layer, the suprachoroid lamina, which contains numerous fibroblasts and melanocytes. Anteriorly, the inner sclera is attached

to the ciliary body by the lamina supraciliaris. Posteriorly, the sclera is

pierced by the optic nerve. Here, the outer half of the sclera turns back

to become continuous with the dura mater, while the inner half is

modified to form a perforated plate, the lamina cribrosa sclerae. The

optic nerve fascicles pass through these minute orifices, while the

central retinal artery and vein pass through a larger, central aperture.

The lamina cribrosa sclerae is the weakest part of the sclera and bulges

outwards (a cupped disc) when intraocular pressure is raised chronically, as in glaucoma.

Like the cornea, the scleral stroma is composed mainly of densely

packed collagen embedded in a matrix of proteoglycans, which are

mixed with occasional elastic fibres and fibroblasts. However, in contrast to the cornea, scleral collagen fibrils show a large variation in

diameter and spacing, and the lamellae branch and interlace extensively. This arrangement of fibres results in increased light scatter, which

is responsible for the opaque, dull-white appearance of the sclera, and

also imparts a high tensile strength to the sclera to resist the pull of the

extraocular muscles and contain the intraocular pressure. Collagen fibre

bundles are arranged circumferentially around the optic disc and the

orifices of the lamina cribrosa. The fibres of the tendons of the recti

intersect scleral fibres at right angles at their attachments, and then

interlace deeper in the sclera. Collagen fibres of the scleral spur are

orientated in a circular fashion, and there is an increased incidence of

elastic fibres here (Figs 42.2–42.3A). Although the sclera acts as a

conduit for blood vessels, scleral vessels are few and mainly disposed

in the episcleral lamina, especially close to the limbus. Its nerve supply

is surprisingly rich, accounting for the intense pain associated with

scleral inflammation (Watson and Young 2004).

Scleral development is under active regulation to ensure an eye of

the correct axial length to produce a focused image (Wallman and

Winawer 2004).

Filtration angle and aqueous drainage

Aqueous humour is produced by the ciliary epithelium; it passes

through the pupil and circulates within the anterior chamber, supplying

the avascular cornea and lens with nutrients and removing metabolic

waste products. It drains from the eye mainly through the trabecular

meshwork into the canal of Schlemm.

Horner’s syndrome

Any condition or injury that destroys the sympathetic trunk ascending

from the thorax through the neck into the face results in Horner’s syndrome, characterized by a drooping eyelid (ptosis), sunken globe

(enophthalmos), narrow palpebral fissure, contracted pupil (meiosis),

vasodilation and lack of thermal sweating (anhydrosis) on the affected

side. Classically, this is seen in patients with bronchial carcinomas that

have invaded the sympathetic trunk and is also a recognized complication of cervical sympathectomy or a radical neck dissection. Avulsion

of the first thoracic nerve from the spinal cord may be diagnosed by

development of the syndrome after closed traction lesion of the supraclavicular brachial plexus. Congenital Horner’s syndrome has been

reported in association with ipsilateral internal carotid artery agenesis

(Fons et al 2009). Special features of congenital Horner’s syndrome are

iris heterochromia, a difference in colour between the two eyes that

results from interference with melanocyte pigmentation of the iris by a

lack of sympathetic stimulation during development, and unilateral

straight hair.