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Class of 11-12 Animal Nutrition Tonsils NEET/AIIMS

Tonsils

Tonsils

  • These are lymphoid organs, and of three types –
  • Pharyngeal tonsils (=adenoids) located in the posterior wall of the nasopharynx.
  • Tubal tonsils are located around the openings of Eustachian tube in the nasopharynx.
  • Palatine tonsils are embedded in the posterior lateral wall of the throat.
  • Lingual tonsils are located on the posterior part of the tongue.
  • Tonsils produce defensive lymphocytes, which form immunoglobulin.

Inflammation of tonsils is called tonsillitis :

DENTITION

  • The study of teeth is known as odontology.
  • Cyclostomes have false teeth (= ectodermal teeth).
  • Mammalian teeth originate from ectodermal and mesoderm.
  • Teeth are present in both jaws as semicircular arches of vertebrates.      

Frogs have teeth only on the upper jaw. Vomerine teeth are on the roof of the buccal cavity, Vomarine teeth kill the prey :         

Structure of a Tooth

  • The crown exposed part of a tooth.
  • Enamel (ectodermal in origin) which covers the crown, is secreted by ameloblasts.
  • Enamel is the hardest substance in the vertebrate body and is made up of calcium phosphate, calcium carbonate and fluorine.

Tar is the white and cream coloured deposit on enamel :

  • Dentine (mesodermal in origin) which lies beneath the enamel, is secreted by odontoblasts.
  • Root embedded in the socket is fixed to the jaw bone by cement and the vascular periodontal membrane.
  • The neck is the zone between the crown and root. It is surrounded by muscular gum.
  • The pulp cavity is a narrow cavity of the tooth and filled with a gelatinous substance (pulp). blood vessels and nerves.           

Pyorrhoea is inflammation of the periodontal ligament and adjacent gums. Dental caries (tooth decay) involve gradual demineralization of enamel and dentine. It causes the formation of cavities. Two bacteria – Lactobacillus acidophilus and Streptococcus mutans, are responsible for tooth decay.

Replacement of teeth    

       Polyphyodont – If teeth are replaced indefinitely. Example – Reptiles, Frog.

       Diphyodont – If teeth grow twice. That is, two sets of teeth develop –

                        – Milk (= deciduous) teeth

                        – Permanent teeth

                        Example  – Mammals.

        Monophyodont ­–  If teeth are not replaced, the animal has one set of teeth.

       Example  – Platypus, toothed whale.

Shape of Teeth

       Homodont (= isodont) – If teeth are of similar shape.

             Example – Reptiles, Toothed whale.

       Heterodont – If teeth are more than one type.

             Example – Mammals.

Types of Teeth

       Heterodonts have four types of teeth –

  • Incisor – For biting and cutting.
  • Canine – For cutting and tearing.
  • Premolars and molars  – for grinding.

The last molars in men are called Wisdom teeth, which are vestigial :

  • Incisor and canine have one root.
  • Premolars of the upper jaw have two roots and of the lower jaw have only one root.
  • Molars of the upper jaw have three roots and the lower jaw has only two roots.

Attachment of Teeth

Acrodont – When teeth are not directly attached to the jaw (as in frog).

Pleurodont – When teeth are attached to the lateral surface of the jaw bone (as in lizard).

Thecodont – When teeth have roots implanted in sockets (theca) of the jaw bone (as in mammals, crocodiles, fossil birds and some fishes).    

Dental Formula

  • A maximum number of teeth were found in Opossum (50).
  • Horses and pigs have 44 teeth.
  • The dental formula indicates variety and number of teeth which vary in different mammals.

A space left between the two types of teeth is called diastema.

  • Tusks of elephants are enormously enlarged upper incisors and grow continuously.
  • Tusks of walrus are modified canines.
  • Teeth of sloths and armadillos have no enamel.
  • Some fishes (sea horse, pipefish and Sturgeon), toads (Bufo and Pipa), Chelonia (reptiles)and modern birds lack teeth.
  • Among mammals, spiny anteaters, scaly anteaters and some whales are toothless.
  • Fangs are specialized teeth attached to maxillary bones in poisonous snakes. When a functional fang is lost and damaged, it is replaced by one of the reserve fangs.

Types of Cheek Teeth

Premolars and molars are called cheek teeth.

Bunodont – Crown with blunt and round cusps (as in man, monkey, pig etc.)

Secondment – Crown with pointed cutting edges (as in carnivores)

Selenodont – Crown with vertical virescent cusps (as in cow, sheep and goat).      

Lophodont – Crown with transverse ridges (lophos) (as in elephants).

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