Anatomy and histology
It is a long partially coiled tubular structure extending from mouth till anus where food we intake pass all along to digest and absorb and remaining forms feces to be egested.
It is 8 to 10 M long having various girths in various regions.
The whole canal can be studied under following headings :
Mouth and buccal cavity .
Oesophagus / esophagus
Mouth and buccal cavity
Anterior most opening of alimentary canal is surrounded by lips dorsally and ventrally and cheeks laterally.
Orbicularis oris is muscle surrounding mouth.
Median region depression is called philitrum.
Lips are with mucosal lining and separates it from skin by vermillion border.
Going a little inside ,vestibule are narrow vertical recesses or space in between outer lips and inner gums /gingiva .
Buccal cavity :
Cavity with teeth in upper and lower zones called jaws . The roof of this cavity is called a palate that separates buccal with nasal cavity .
Palate possess two regions:
Hard palate : anterior zone supported by palatine and maxillary bone.
Soft palate : not supported by bone and made up of cles and connective tissues. This terminates into an lymphoid structure called uvula / velum palati .
Two arches are in the cavity called palatoglossal and palatopharyngeal.
Palatoglossal : lateral to tongue .
Palatopharyngeal : behind palate till pharynx; possess palatine tonsils.
Tongue : located on the floor of buccal cavity which is a muscular, glandular and sensory organ .
Three parts of the tongue :
Root : posterior most region that is attached to unsocial hyoid bone through many muscles.
Body : middle larger part .
Tip : anterior ,freely movable region .
Tongue is attached to the cavity by lingual frenulum.
Ankyloglossia : when lingual frenulum is short and tied making speech impossible in just born babies.
They are removed by surgical cuts.
Tongue possess papillae which indeed possess nerve endings or taste buds.
Papillaes in tongue :
Vallate /circumvallate : formed as an inverted V shape, 10 - 12 in number in posterior zones of the tongue which is mostly 1/3rd back.
Fungiform : mushroom-like scattered in 2/3rd of body.
Filiform : smallest papillae scattered in 2/3rd zone with fungiform.
Foliate : absent or vestigial in the human body . but is seen in rabbits.
Vallate, foliate and fungiform are for tastes as they possess buds while filiform are just with nerve endings so more than for taste they are for sensation.
Tastes in tongue :
Sour : by the sides of tongue .
Sweet, salty and umami (MSG :Monosodiumglucamate’s taste ): by tip of tongue
Bitter : by posterior tongue.
Glands in tongue :
Anterior lingual / glands of Blandis and Nuhn : It is located near the apex of the tongue . Ducts open on the ventral surface of the tongue near frenulum . approx. 5 small ducts are there .
Posterior lingual glands :
Webner glands : pure mucous , located lateral and posterior to vallate papillae .
Their ducts open into the dorsal surface of the tongue .
Ebner gland : pure serous gland ,located in between muscles of tongue and foliate papillae or below circumvallate and open into the trough of the vallate papilla.
Functions of tongue :
It helps in locating food position during mastication.
For Speech ability.
Glandular secretion , taste perception and sensation role.
Teeth : singular tooth is a calcified conical structure found in jaws of mainly gnathostomes.
Origin : ecto-mesodermal (ecto: enamel , meso : rest of dentine pulp…)
Note : Hardest tissue in human: dentine
Hardest substance in human : Enamel
Characters of mammalian teeth :
Heterodont : more than one type of teeth morphologically.
Diphyodont : two sets of teeth in entire life. Which are :
Milk teeth : erupts at 6 months of birth, finishes to erupt in 2 years and gets replaced within 12 years old.
Dental formula : arrangement of teeth in a half of upper or lower zone .
For milk teeth it is : 2102 ( no premolars ).
Permanent teeth : replacement begins at 6 and gets completed at 12 except wisdom or 3rd molar which may or may not appear and if it does it will at 17- 25 age.
Dental formula : 2123
Hence , it is inferred all are diphyodont but premolars and 3rd molars.
Thecodont : embedded in sockets or alveoli of the jaw .
Bunodont : having cusps i.e projected parts in crown except incisors .
Structure of a tooth
Externally it can be divided into 3 parts :
Crown : outer exposed part of teeth out of gingiva , whitish and shining in colour.
Neck : constricted part covered by gingiva.
Root : portion of teeth found generally embedded in the socket of jaw.
Mostly a single root is present in teeth.
Composition of teeth :
Enamel : it is the outermost exxposed shiny, whitish part covering the crown which is the hardest of all in the body.
It is a non living component secreted by ameloblast cells from the pulp cavity via pulp canal. The hardness of enamel is due to the presence of fluorides.
Dentine : it is the hardest tissue of the body even more than bones .They are secreted by odontoblast cells. Mainly it’s 70% inorganic and 30% organic substituents. Hydroxyapatite is the main constituent of this.
Pulp cavity : innermost composition with jelly like matrix supplied by blood vessels.
Root Canal Treatment (RCT) : This in dentistry is a way to clean out this composition; PULP CAVITY.
Cementum : this is a specialized calcified substance that covers roots.
In between cementum and jaw bone, there is connective tissue called periodontal ligament . This is important for movement during mastication.
Root canal’s blood vessels enter through apical foramen .
Functions of teeth :
Incisors : cisled, acuspid shaped so for cutting and biting
Canine : pointed/dagger shaped so, for tearing flesh .
Premolar and molar : plane crown so, in grinding and chewing.
Aids in speech also.
Tusk of an elephant : modified incisors
Tusk of walrus : modified canines
Fangs of teeth : modified maxillary teeth
musculo - membranous tube lies behind the nasal cavity , oral cavity and larynx.
From upper skull up to cricoid cartilage level which is up to 6th cervical vertebrae.
It is 12.5 cm in length.
It is lined by stratified squamous epithelium.
Overall , it is conical in shape.
Parts of conical pharynx :
Nasopharynx : from upper skull to uvula . The eustachian tube ends here.Cells here are pseudo stratified...Around here ,we have Waldeyer's ring which is ;
Ring of tonsils which is of adeno tonsils in pharynx wall, tubal tonsils, palatine tonsils ; (this is only in oropharynx, others all belong here) and lingual tonsils.
Oropharynx : from uvula to upper portion of epiglottis . cells here are non keratinised squamous epithelium.
Laryngo pharynx : from upper epiglottis to level of 6th cervical vertebrae. Here it is also non keratinised squamous epithelium.
Posterior communicates with 2 opening glottis and gullet .glottis is guarded by epiglottis and gullet is by sphincters.
This is a common passage for food and air.
Lies back of the trachea in the upper zone and back of the heart in the lower region.
Collapsible part lining of muscular region.
Lies in level of c6 to t12 level.
Narrowest tube of alimentary canal which is 25 cm long.
Possess 3 sphincters two at ends and one at middle.
Upper one third is skeletal muscle while the rest is smooth muscles which are involuntary.
Most dilated sac of J shaped is located covering the epigastric, left hypochondriac and umbilical region ; which is just below the diaphragm on the left side of abdomen.
This is also 25 cm long.
Parts of stomach :
Cardia : small part where bollus enter through cardiac sphincter.
Fundus :Above orifice and is of dome shape and mainly contains gases this portion is basically absent in frogs.
Body: it is the large portion of the stomach where food is temporarily stored and has a lot of rugae.
Pylorus :it is a funnel shaped portion connected with duodenum via pyloric sphincter. .
Curves and omentum in stomach :
Lesser curvature: it is the medial small concave surface of the stomach .
Greater curvature : this is a larger outer convex surface of the stomach.
lesser omentum :it is mesentery from lesser curvature connecting stomach with liver.
Bigger omentum :it is mesentery from Greater curvature joining stomach to coelom wall.
This is also called policeman of the abdomen as it localises infection in abdomen .
The main purpose of these structures is insulation by storing fat and helding the structure firmly and preventing sticking of visceral and parietal peritoneum.
Cells in mucosa of stomach
Following are the cells in the stomach:
Parietal/ oxyntic cells : Creates hydrochloric acid and Castle intrinsic factor this factor protects vitamin B12 .so in efficiency of this factor eventually lead to Pernicious anaemia.
Peptic/ chief/zymogen cells : they secrete in-active pepsin term as pepsinogen.
G cells / argentaffin/enteroendocrine cell : they are named so because they take colour of Silver and produce gastrin and ghrelin secretion.
Mucus/ Goblet cells : as the name suggests, they secrete mucus for Lubrication.
Functions Of stomach
Temporary stories of food
mechanical and chemical digestion
HCL plays a vital role in creating an acidic medium to activate pepsinogen Pro rennin in babies and kill microorganisms .
Long coiled tube extending from pylorus to ileum of about 3 M in an alive person and double of that in cadaver.
Can be divided into 3 parts :
Duodenum : first portion that is widest among the other two .
25 cm long C Shaped extending from pylorus till duodeno jejunal flexure.
Jejunum : empty region after death ; 2.5 m long in dead; in between duodenum and ileum
Ileum : 3.5 m long; from end of jejunum to ileocecal junction.
Mucosal cells of intestine in crypts of lieberkuhn :
Mucous cells : secretes mucous
Epithelial cells : secretes enzymes like trypsinogen
Paneth cells: secretes lysozyme
Entero endocrine / argentaffin cells:also in stomach mucosa; secretes hormones like secretin, duocrinin
Stem cells : undifferentiated cells that supplements dying cells.
Large intestine :
Wide tube extending from ileocaecal junction till before anus .
1.5 m long; shorter than small intestine but named large because of its large cross section.
Divided into following parts :
Caecum: also called blind pouch with a semi vestigial vermiform appendix as it’s immunological aid is being discovered . But this is functional in case of rabbits ; for cellulose digestion via bacterias. Here in the human gut 3 to 10 times more bacteria are present than total cells in our body. For instance : Bacteroides fragilis.
Colon : question mark ? shaped . they possess taenia coli i.e. 3 muscle bands of muscle in wall of colon. The walls also are with sacculations called haustra.
Epiploic appendages are small pouches of the peritoneum filled with fat .
4 sub parts :
Ascending colon : from caecum towards liver.
Descending colon : moves away fro spleen.
Transverse colon : straight joining both above colons.
Sigmoid colon : U shaped structure connecting to rectum.
Rectum: distal portion from recto sigmoid to anal canal. 12 cm long for temporary storage of feces, muscles here contract under reflex.
Anal canal :
Terminal part of GI TRACT .
3.8 cm long with internal involuntary and external voluntary sphincters
Bursting of veins due to inflammation by bacteria so that stool is seen in stools called Piles.
You can refer to following video too: