The pharynx is a wide muscular tube situated behind the nose, the mouth, and the larynx. Clinically it is considered as a part of upper respiratory passage where infections are common. The upper part of the pharynx transmits only air, the lower part(below the inlet of the larynx) only food, but the middle part is a common passage for both air and food(but only one at a time).

Dimensions:-
Length is about 12-14 cm
The upper part is widest (3.5cm) and non collapsible. Middle part ia narrow, the lower part is the narrowest of gastro intestinal tract (except for the appendix).

Boundaries:-
Superiorly : Base of the skull including the posterior part of the nbody of sphenoid and basilar part of the occipital bone in front of the pharyngeal tubercle.
Inferiorly : The pharynx is continuous below with the oesophagus at the level of C6 corresponding to the lower border of the cricoid cartilage.
Posteriorly : The pharynx glides freely on the prevertibral fascia which separates it from the vertebral spine.
Anteriorly : It communicates with the nasal cavity, oral cavity and the larynx. (thus the anterior wall of the pharynx is incomplete). It is attached from above downwards on each side to the medial pterygoid plate, pterygomandibular raphae, mandible, tongue, hyoid bone, and thyroid and cricoid cartilages. Laterally it communicates with the tympanic cavities through the auditory tubes and is in relation with the styloid process and their muscles, the common, internal and external carotid arteries and some of the branches of the latter artery.

The pharynx consists of 3 parts : nasal, oral and laryngeal.
Nasal part of pharynx:-
This is the upper part of pharynx behind the nose and above the lower border of the soft palate.
It resembles the nose structurally and functionally. a)it is respiratory in function. c)its walls are rigid and noncollapsibleb)it is lined by ciliated columnar epithelium. d)its mucous membrane is supplied by trigeminal nerve.
The wall of nasopharynx is formed by pharyngo basilar fascia and the posterior median pharyngeal ligament.
Anteriorly, it communicates with the nasal cavities;
Inferiorly, with the oropharynx.
The lateral wall presents the following a)the pharyngeal opening of the auditory tube b)tubal elevation c)salpingopharyngeal fold d)the levator palate and e)pharyngeal recess.

The roof and the posterior wall form a a continuous slope opposite the posterior part of the body of sphenoid., the basiocciput and the anterior arch of atlas. Under the mucous membrane, opposite the occiput, there is a collection of lymphoid tissue called pharyngeal tonsil (naso pharyngeal tonsil).
The nasopharyngeal tonsil is better developed in children. A pathologically enlarged pharyngeal tonsil is called adenoids. Its presence make nasal breathing impossible.
There is another collection of lymphoid tissue in the nasopharynx behind the tubal opening. It is called tubal tonsil. It is continuous with the lateral part of the pharyngeal tonsil.

Oral part of pharynx (Oropharynx) :-
This is the middle part of pharynx situated behind the oral cavity. Above it communicates with the nasopharynx, in front with the oral cavity. Below it opens into the laryngopharynx at the level of the upper border of epiglottis. Behind it is supported by the body of the axis vertebra and upper part of the body of C3. Its lateral wall presents the palatine tonsil(tonsil) which lies in the tonsillar fossa. This fossa is bounded anteriorly by the palatoglossal arch and posteriorly by the palatopharyngeal arch. The wall of the oropharynx is formed posteriorly by the superior, middle and inferior constrictors of the pharynx.

Waldeyer’s lymphatic ring :-
In relation to the oropharyngeal isthmus, there are several aggregations of lymphoid tissue that constitute Walceyer’s lymphatic ring. The most important aggregations are right and left palatine tonsils. Posteriorly and above there is pharyngeal tonsil. Laterally and above tubal tonsil; inferiorly lingual tonsil. Outer to this ring there are retropharyngeal, jugulodigastric, submandibular and submental lymphnodes.

Palatine tonsil (The tonsil):-
The palatine tonsil occupies the tonsillar sinus(fossa) between the palatoglssal and palatopharyngeal arches. It can be seen through the mouth.
The tonsil is almond shaped. It has 2 surfaces- medial and lateral; 2 borders- anterior and posterior and 2 poles- upper and lower. The medial surface is covered by stratified squamous epithelium which is continuous with that of the mouth.The lateral surface is covered by a sheet of fascia which forms the capsule of the tonsil. The anterior border is related to the palatoglossal arch with its muscle. The posterior border is related to the palatopharyngeal arch with its muscle. The upper pole is related to the soft palate and the lower pole to the tongue.
There are several pitted depressions on the surface of the tonsil, they are called crypts. The intratonsillar cleft is the largest crypt of the tonsil. It is present in its upper part. A peritonsillar abscess(quinsy) often begins in this cleft.
The bed of the tonsil is formed by: a) the pharyngobasilar fascia; b) the superior constrictor and palatopharyngeus muscles; c)the buccopharyngeal fascia; and d) in the lower part the styloglossus and the 9th cranial nerve.

Arterial supply of tonsil:-
1) Main source: tonsillar branch of facial artery.
2) Additional sources: (a)ascending palatine branvh of facial artery; (b)dorsal lingual branches of the lingual artery; (c)ascending pharyngeal branch of the external carotid artery; and (d) the greater palatine branch of the maxillary artery.

Venous drainage:-
One or more veins leave the lower part of the deep surface of the tonsil, pierce the superior constrictor and join the palatine, pharyngeal or facial veins.
Lymphatic drainage:-Lymphatics pass to the jugulodigastric nodes
Nerve supply:-Glossopharyngeal and lesser palatine nerves
Applied anatomy:-
The tonsils are large in children. They retrogress after puberty. The tonsils are frequently sites of infection. Infection may spread to surrounding tissue by forming a peritonsillar abscess.
Laryngeal part of pharynx (Laryngopharynx):- This is the lower part of the larynx situated behind the larynx. It extends from the upper border of epiglottis to the lower border of the cricoid cartilage’
The anterior wall presents a)inlet of larynx b) posterior surfaces of the cricoid and arytenoids cartilages.