Anatomy of Tonsil

A ring of lympnoid tissue surround the naso pharynx and oro pharynx. These lymphoid tissue are collectively known as the waldayer's ring. Waldayer's ring has two components, namely the inner and outer rings. The cervical lymph nodes constitute the outer ring, while the inner ring is constituted by 1. adenoid at the roof of nasopharynx, 2. tubal tonsils or tonsil of Gerlac which surround the pharyngeal ends of eustachean tube. These lymphoid tissue surround the naso pharynx.

The lymphoid tissue surrounding the oropharynx also constituting the components of the inner Waldayer's ring are 1. Lingual tonsil in the posterior 1/3 of the tongue, 2. Palatine tonsils on either side of oro pharynx, and 3. sub epithelial lymphoid tissue found in the posterior pharyngeal wall. All these structures of the inner Waldayer's ring are inter linked.

Functions of Waldayer rings: These Waldayer's rings consitute an antigen sampling center where the extraneous antigens are caught and sampled stimulating the immune mechanism. Antigens from inspired air are trapped by the adenoid and the tubral tonsils. These antigens in turn stimulate release of immunoglobins by the B lymphocytes. To facilitate eposure and trapping of antigens the mucosa covering the adenoid is thrown in to grooves called as furrows. These furrows serve to increase the surface area of the adenoid tissue, similarly antigens from ingested food are captured and sampled by the lingual and palatine tonsils. The mucosa covering the palatine tonsils are thrown in to numerous crypts about 18 - 20 in each tonsil. These crypts serve to increase the surface area of mucosa covering the tonsil.


Image of tonsil showing plica semilunaris

Image showing the components of Waldayer's ring

Anatomy of palatine tonsils: Palatine tonsils are the largest member of the inner waldayer's ring. It is almond shaped and lie on either side of the oropharynx. Develomentally tonsils arise from the ventral portion of the second pharyngeal pouch, i.e. ideally named as sinus tonsillaris. The trace of this sinus is present in tonsil as supra tonsillar cleft.

Figure showing embryology of Tonsil

Tonsil is lodged in the tonsillar fossa on either side of orophayrnx. The tonsillar fossa lies between two pillars, anterior and the posterior pillars. The anterior pillar is formed by palato glossus muscle, the posterior pillar is formed by palato pharyngeus muscle. The outer aspect of tonsil is lined by condensed capsule formed by the pharyngobasilar fascia a specialised portion (it is also known to course the surface of the tonsil and extend into it to form septa that conduct nerves and vessels), deep to which lie the superior constrictor muscle, lateral to which is the bucco pharyngeal fascia. The glossopharyngeal nerve and the stylohoid ligament pass downwards and forwards beneath the lower edge of the superior constrictor in the lower part of the tonsillar fossa. These structures collectively constitute the tonsillar bed. The tonsil is virtually inseparable from its capsule, but the capsule is united by loose connective tissue to pharyngeal muscles, hence the tonsillar dissection is carried out in this plane.
The medial surface of the tonsil is free and faces the oropharynx. It is covered by non-keratinizing stratified squamous epithelium which is continuous with that of the lining of the oropharynx. A triangluar fold of mucous membrane extends back from the paltoglossal fold to cover the anteroinferior part of the tonsil. This fold of mucous membrane is known as plica triangularis. In childhood, this fold is usually invaded by lymphoid tissue and becomes incorporated into the tonsil.A semilunar fold of mucous membrane passes from the upper aspect of the palatopharyngeal arch towards the upper pole of tonsil, thus separating it from the base of the uvula.

The main artery of the tonsil is the tonsillar branch of the facial artery which enters the tonsil near its lower pole by piercing the superior constrictor just above the styloglossus muscle. Other arteries supplying the tonsil are lingual artery through its dorsal lingual branches, ascending palatine branch of facial artery, and ascending pharyngeal vessels.

Venous drainage occurs through the para tonsillar vein, and the vessels also pass through to the pharyngeal plexus or facial vein after piercing the superior constrictor.

Lymphatic vessels from the tonsil pierce through the buccopharyngeal fascia and pass to the upper deep cervial group of nodes, particularly to the jugulodigastric group.

Nerve supply to the tonsil is from the glossopharyngeal nerve.