Know these facts

*All mammals have 7 cervical vertebrae except 6 in manatee, 8 in ant bear (Tamandua) and 9 in sloth (Bradypus).

*Elephants can hear infrasonic sounds of 14 hertz. from hundreds of km and whales can hear ultrasonic sounds of 123,000 hertz from thousands of km.

*Each second brain receives 100 billion impulses and fires 5000 commands but we are conscious of one millionth activity of our brain.

*Human brain loses about 100,000 neurons each day but the loss is insignificant because of the presence of billions of neurons in brain.

*Snakes auditory lobes are highly enlarged to hear faint sounds received through ground as they possess no external ear.

* Other than mammals, in all vertebrates spinal cord has more control over body functions than the brain.

*In humans brain is marvelously packed as 1800 sq cm of brain surface area is packed in only 15% of skull space.

*Nerve impulse travels in our body @ 400 km per hour.

*Crab eating frog (Rana cancerivora) is the only amphibian that can survive in the intertidal marine water.

*Monotremes and marsupials do not possess aprostate gland.

*In sharks and caecilian kidney is calledopisthonephros because it extends up to the posterior end of the body.

*Birds have only the left ovary and oviduct.

*Systolic blood pressure decreases in arteries from 130 mmHg in systemic arch, 30 mmHg in arterioles, 20 mmHg in capillaries, 12 mmHg in venules, 5 mmHg in smaller veins and zero mmHg in larger veins.

*Aortic blood pressure in giraffe is 260/160 to supply brain sitting on top of the long neck, but in brain it is only 120/70.

*Largest RBCs (75 microns) are found in the urodele, Amphiuma, and the smallest (2.5 microns) in the musk deer.

*Whales store air in huge nasal chambers and not in lungs.

*Elephant’s trunk has 40,000 muscles.

*No animal breathes free air. Oxygen from air must diffuse through fluid to the blood and hence a water film is necessary over the respiratory surface in all animals.

*Sharks jaws are loosely attached to the skull, so that when shark bites its jaws come out of the oral cavity to seize prey.

*Plethodont salamanders possess a tongue that is three-fourth the length of the body.

*Tongue of an elephant weighs 12 kg while that of a whale weighs 1500 kg and 50 persons can stand on it.

*Incisors of rodents and elephants continue to grow throughout life. A single tusk of an African elephant weighs about 100 kg.

*In hind gut fermenters such as horses, elephants and rabbits, large amount of food eaten goes undigested and hence they must eat large quantities to obtain enough nutrients.

*Gastric juices in snake’s stomach are so strong that they dissolve even the bones of the swallowed prey.

*In humans intestine is 28 feet long whereas in cow it is 165 feet long.

*Batrachotoxin and bufotoxin are such potent poisons secreted by frogs and toads that only 200 microgram can kill a man.

*Penguins breed in Antarctic and hatch their eggs at minus 50 degrees.

*Density of sweat glands is highest in the skin of Indians (738/sq cm) followed by Negros (709/sq cm), while in Europeans it is only 550/sq cm.

*The American pronghorn(Antilocapra americana) is the only antelope that sheds its horn annually.

*Antlers are found only in male deer and are shed after breeding season is over. Reindeers are the only deers that possess permanent antlers in both males and females.

In echidna and duckbilled platypus both male and female possess active mammary glands and feed young babies.

Sharks retain 100 times more urea in blood as compared to mammals. Their organs cannot function without such a high amount of urea in body.

Humans can sweat away as much as 3 litres of water per hour in summer. A person can collapse if loss of body fluid is 10% of body weight.

Digestive system in vertebrates

Digestive system includes alimentary canal and the associated glands. Alimentary canal or gut shows four distinct regions, namely, ingressive zone that includes mouth, lips and jaws for capturing and handling food; progressive zone includes oral cavity, pharynx and oesophagus through which food passes with little digestion; degressive zone contains stomach and small intestine in which digestive process is accomplished and the egressive zone includes colon and rectum where undigested food stays before expulsion and excess water from it is absorbed. Each part of the alimentary canal is described below one by one in a sequence.

 MOUTH

Mouth is the anterior opening of the alimentary canal, which may be terminal, ventral or slightly dorsally directed. It is guarded by suckers in cyclostomes and by jaws and teeth in gnathostomes. Lips are horny in fishes but fleshy and suctorial in mammals. Birds and turtles possess horny beak in place of lips.

ORAL CAVITY

This is the anterior most chamber of alimentary canal meant for handling food. Depending upon the kind and size of food, its size is highly variable and it contains three important organs for handling food material, namely, oral glands, tongue and teeth.

ORAL GLANDS

Cyclostomes possess mucous glands in oral cavity but Petromyzon also has salivary glands that secrete an anticoagulant enzyme. Fishes and perennibranch amphibians have no particular oral glands except simple mucous glands. In tailed amphibians and apoda oral glands are almost nonexistent but in anurans there are. Oral glands are poorly developed in turtles and crocodiles but well developed in lizards and snakes. Poison glands of snakes are modified labial glands and those of Heloderma,the only poisonous lizard that has fangs in the lower jaw, has sublingual glands modified as poison glands.

Birds possess sublingual glands that open into the floor of the oral cavity. Mammalian oral cavity is very wet as it contains two types of glands – salivary glands and mucous glands. The compound acinar submaxillary or submandibular glands lie in the posterior part of the lower jaw. Sublingual glands are smaller than the other two salivary glands. Molar glands are mucous glands that are well developed in herbivores and open near the upper molars. Another kind of mucous glands are Orbital glands which occur in cat and dog family.

TONGUE

Tongue is a fleshy and highly mobile organ in the oral cavity that is used in various ways in vertebrate groups.

Cyclostomes possess a thick and fleshy primary tongue on the floor of the oral cavity. In fishes the tongue is primary and merely a fleshy fold on the floor of anterior end of pharynx supported by the extension of hyoid arch.

In perennibranch urodeles such as Necturus,tongue is similar to fishes and is not put to much use. Frogs and toads having a predilection for insectivorous diet are gifted with a highly flexible tongue that consists of a basal primary tongue and the anterior glandular and muscular secondary tongue. Turtles and crocodiles being amphibious in nature have a small non-protrusible tongue but snakes and lizards possess a highly movable tongue that is bifurcated at the apex and supplied with olfactory cells. 

Bird tongue is short and hard and practically lacks muscles and lateral lingual swellings. Such incapable tongue is of no handicap to these beaked creatures as the food does not stay in the mouth for longer duration. However, some birds are gifted with long and flexible tongue such as woodpeckers.

Mammalian tongue is the best developed of all vertebrates, except in the aquatic cetaceans. It is derived from 5 portions—paired fleshy ridges of hyoid arch, a median secondary tongue called tuberculum impar and paired lateral lingual swellings, which provide it extraordinary mobility and flexibility in the oral cavity. Tongue of anteaters is suitably long and sticky to feed on termites which they dig out by their strong fossorial front legs.

 TEETH

Teeth are hard bony structures in the oral cavity that are variously modified to capture, tear, cut or grind food material before it is swallowed. Epidermal teeth are hard cornified epidermal structures of rare occurrence, as in the buccal funnel of cyclostomes and on the edges of tadpole jaws. 

 TYPES OF TEETH

Polyphyodont dentition involves replacement of teeth from time to time several times in lifetime so that jaws are never left without teeth.

Diphyodont dentition is a characteristic of mammals in which milk teeth appear in the young ones but as they grow and jaw becomes larger, milk teeth are replaced by larger permanent ones.

Monophyodont teeth appear only once in lifetime and if they fall they are never again replaced by new ones. 

Based on the type of attachment of teeth on the jaw bone the following three types are found in vertebrates:

Acrodont teeth are attached on the top surface of the jaw bone as in fish and amphibians.

Pleurodont teeth are attached on the inner side of the jawbone that brings larger surface area of tooth in contact with jawbone and hence attachment is stronger, as in lizards and urodeles.

Thecodont dentition is found in mammals in which root of the tooth is firmly fixed in a socket of the jawbone, making the attachment strongest in vertebrates. 

Based on the kinds of teeth found there are two types of dentition:

Homodont dentition is found in the majority of vertebrates such as fish, amphibia and reptiles in which all teeth are functionally of the same type, although their size may be variable depending on the location.

Heterodont dentition occurs in mammals in which there are 4 functionally different types of teeth, namely, flat incisors for cutting, long and pointed canines for tearing flesh and large and broad premolars and molars with flat grinding surface. 

There are also some other type of teeth as follows:

Secodont teeth have sharp cutting edges that function like scissors to cut flesh.

Bunodont teeth are small with smaller cusps or tubercles on the surface for handling soft diet as in man, monkeys, rodents etc.

Brachydont teeth are smaller and low crowned suitable for feeding on soft diet.

Hypsodont teeth possess larger crown that can resist wear and tear of feeding on tough and fibrous diet as in ungulates.

Selenodont teeth, as found in horses and other ungulates, deposit silica around cusps and in the depressions of the grinding surface.

Lophodont teeth are found in elephants which feed on the roughest diet that any mammal can feed on.

DEVELOPMENT OF TOOTH

Teeth develop over the jaw bone where certain malpighian cells start actively multiplying forming a mass of cells called dental lamina or enamel organ. A dental papilla made of group of dermal cells appears below the dental lamina that supplies nourishment to the growing mass of cells. Cells of the outer layer of dermal papilla arrange themselves in a row and get differentiated into odontoblast cells.

Epidermal cells of the dental lamina that cover the growing dentine are called ameloblasts. The tooth gradually grows outwards and eventually gets exposed by penetrating through the skin covering the jaw bone. The dental papilla inside the pulp cavity remains active along with its blood supply and nerve intact. This development of tooth is identical to the development of dermal scales in fishes. Hence shark teeth are also called modified placoid scales.

COMPARATIVE ACCOUNT OF DENTITION

Cyclostomes have only epidermal teeth. Some fishes are toothless such as sturgeons, sea horse and pipe fish and others like lung fishes and Chimaera have teeth modified into crushing plates. Majority of fishes possess Polyphyodont, Acrodont and homodont dentition suitable for seizing prey.

In amphibians teeth are located on jaw bones, palatine and vomer bones and are Polyphyodont. Tadpoles lack true teeth and their jaws have horny epidermal ridges which are used to scrape algae on which they feed.

Among reptiles, turtles lack teeth and have horny beak. In others, teeth are generally confined to jaw bones but in snakes and lizards may occur on palatine and pterygoid bones. Fangs of snakes are modified upper maxillary teeth.

Modern birds lack teeth but Archaeopteryx had thecodont dentition and so were the toothed birds Ichthyornis and Hesperornis.

DENTITION IN MAMMALS 

Mammals as a rule possess heterodont, diphyodont and thecodont dentition. However, some mammals lack teeth as given below in detail.

Among monotremes, the spiny anteater or echidna (Tachyglossus and Zaglossus) lacks teeth.

Edentates, as the name suggests are toothless such as the giant anteater of South America. Among the aquatic Cetacea baleen whales have no teeth, such as the blue whale, Balaenoptera musculus  and, the whalebone whale, 

Among humans, and astonishingly, males in “Bhudas” tribe of Hyderabad Sindh in Pakistan are genetically so predisposed that they never grow teeth all their lives.

The Dental Formula

Mammals have heterodont dentition having four types of teeth meant for different function in handling food in the oral cavity. Incisors in front are flat teeth designed for cutting food into pieces and the canines next to them are generally long and pointed spike-like used for tearing flesh by carnivore animals. Premolars and molars are located on the posterior side of the jaw, have flat surface with tubercles called cusps and are used for grinding food of plant origin. They are therefore well developed in herbivore animals. Number and arrangement of teeth in mammals is specific in different groups of animals so much so that mammalian orders can be identified by their teeth and dental formula, which is written for one half of the upper and lower jaw as follows:

3 – 1 – 4 – 3 x 2 = 44. This dental formula belongs to horse and pig.

 3 – 1 – 4 – 3

PRIMATES.  Primates are basically arboreal animals whose ancestors were insectivores and some of them still continue with their original diet.  

UNGULATES. Ungulates belong to two orders, the even-toed Artiodactyla and the odd-toed Perissodactyla that includes horses and rhinoceroses. They are all herbivores and fleet footed grazers and browsers, with teeth adapted for grinding tough vegetation. Musk deer also has large upper canines hanging on either side of the jaws. Hippopotamus has large canines too with sharp edges meant for defense against the lurking crocodiles in their amphibious habitat.

Pigs and horses have full set of 44 teeth as depicted in the dental formula given above. Premolars and molars are similar in shape and size, have flat grinding surface, with silica deposits between cusps. Such grinders are called selenodont and are designed to grind tough grasses. 

INSECTIVORES. In insectivores such as shrews, hedgehogs and moles, all teeth are pointed and grinding teeth possess peg-like cusps for crushing hard exoskeleton of insects on which they feed. In the mole genera Scalopus andCondylura milk teeth are retained throughout life.

CHIROPTERA. In bats milk teeth are shed before birth and they are born with permanent teeth. Insectivorous bats have conical cusps on the grinding teeth for crushing insects.

 RODENTIA. Rodents include rats, mice, squirrels and guinea pigs which possess chisel-shaped front incisors for gnawing nuts and hard objects. These teeth grow throughout life due to the wide opening of the pulp cavity but they are worn out equally fast.

 LAGOMORPHA. In rabbits also the upper incisors are chisel shaped adapted for gnawing, canines are absent and cheek teeth are modified for grinding. As in other herbivores, a big diastema is present between incisors and premolars.

PROBOSCIDIA.  The order includes elephants which have upper incisors modified as long tusks which are used for digging roots of plants, for removing barks from trees or for offence and defense. Premolars and molars are alike in appearance and they have broad and lophodont surface, in which cusps unite to form circular lophs of ridges with silica deposits in the depressions. 

CARNIVORES.  Carnivores include cats, tigers, lions, dogs, wolves, jackals and bears and the aquatic seals, sea lions and walruses. Their canines are long and pointed, dagger like for tearing flesh of the prey. Canines of walruses are modified into long tusks. Carnassial teeth are enlarged teeth with pointed cusps. These teeth also have sharp cutting edges and are called secodont teeth used for shearing flesh.

CETACEA.  There are no teeth in Mysticeti or baleen whales which possess baleen plates hanging from the palate that are suitable for straining planktons from sea water. In Odontoceti or toothed whales, teeth are homodont and monophyodont which are used to seize fish or other prey.

 MARSUPIALS. Generally marsupials retain milk teeth except the last premolars. Herbivores have a diastema and premolars and molars modified for grinding.

PHARYNX

Pharynx is part of the alimentary canal between oral cavity and oesophagus and is primarily concerned with respiration. In fishes pharynx exhibits paired gill pouches containing gill lamellae and gill slits opening to the exterior, whereas in terrestrial vertebrates trachea opens into the pharynx. Nasal passage opens into the oral cavity in all other vertebrates except mammals and crocodiles in which nasal passage opens far backwards into the pharynx, allowing the oral cavity to handle food while breathing can go on uninterrupted. At this time larynx is pulled forward to lie against a flap of tissue called epiglottis, which closes the tracheal opening called glottis. The nasal opening is closed by the soft palate and uvula which is a fleshy elongation hanging downward from the soft palate. In all other vertebrates except mammals as long as the food remains in the oral cavity breathing has to stop. 

 OESOPHAGUS

Oesophagus is a narrow tube that connects pharynx with stomach and is generally as long as the length of neck. It has no serous coat and inner mucosa bears longitudinal folds that give it enormous power of distension to allow large food to pass through it by peristalsis. Fish oesophagus is very short bearing longitudinal folds but in birds and mammals it may be very long as in giraffe where it has to match the whole length of neck.

STOMACH

Stomach is a muscular chamber or a series of chambers that serves for storage of food swallowed, macerating and churning it into pulp by peristalsis and secrete and mix certain digestive juices with it for digestion of nutrients. Primary function of stomach continues to be storage of large quantity of food that has been swallowed.

Cyclostomes.  In larval cyclostomes stomach is ciliated which is quite useful in pushing detrivorous diet backwards on which they feed but in adult lampreys stomach is indistinguishable.

 Fishes. In most of the fishes stomach continues to be narrow and long to adjust inside the elongated body cavity but in elasmobranchs it is J-shaped and measures about half the length of the entire digestive tract.

 Amphibia. Urodeles have straight stomach with hardly any digestive function assigned to it. But in frogs and toads cardiac end is wide and pyloric small.

Reptiles. Snakes and lizards have elongated stomachs that fit inside their elongated abdominal cavity but in turtles the stomach is narrow U-shaped tube. Crocodiles have highly specialized stomach that is highly curved. Except for the tortoises, digestive glands are strongly developed in the stomachs of reptiles.

Birds. Bird stomach is modified greatly into a glandular proventriculus and horny gizzard that is necessary to grind seeds and other types of food that they need to swallow whole in the absence of teeth.

Mammals. In monotremes stomach is sack-like but lacks glands and in ungulates and cetaceans the glands occur in pyloric portion only. Stomach is a large sac meant to accommodate large quantity of food that must be swallowed quickly when available before the arrival of predators and competitors. 

Ruminant (cud-chewing) mammals have a complex stomach having four parts, namely, rumen or pounch, reticulum with honey-comb like rough lining, omasum or psalterium and abomasum or rennet. Gastric juice is secreted by the lining of abomasums and pylorus for further digestion. Carnivore stomach is clearly divided into cardiac, fundus and pyloric portions, of which fundus always remains empty and accommodates gases. The so called hourglass stomach is found in primates and rodents in which cardiac and pyloric parts are divided by a constriction.

INTESTINE

It is part of the alimentary canal between stomach and cloaca or anus and the primary site of digestion and absorption. Digestion is alkaline as bile and pancreatic juices are released into duodenum which is attached to the pyloric part of stomach.

Cyclostomes. Intestine is straight, slightly enlarged on the posterior side to form rectum and terminates into anus that opens in a cloacal depression. 

 Fishes. Intestine of fishes is short, wide and almost straight, although some teleosts possess a spirally twisted intestine. Lungfishes also have a cloacal caecum to increase the absorptive surface area. Bony fishes do not have a spiral valve but have many hollow finger-like pyloric caeca instead, attached between pyloric stomach and duodenum, which perform the same function of increasing the digestive area of intestine.

 Amphibia. In limbless amphibians, intestine is almost straight and is not differentiated into small and large intestine. In urodeles and anurans small intestine is long and coiled but large intestine is short and straight and separated from small intestine by an ilio-colic valve. 

Reptiles. Small intestine is elongated, coiled and uniform in diameter and large intestine is almost straight. Ilio-colic valve is present between the two portions.

Birds. Length of intestine increases in birds but large intestine is short, straight and terminates into cloaca. Most of the birds possess one or two colic caeca that increase the absorptive surface of intestine. 

Mammals. Intestine is more elaborate; the small intestine is divided into duodenum, jejunum and ileum and the large intestine differentiated into colon and rectum. Jenjunum (means empty) is about 8 feet long in man and has leaf-like large villi. Ileum is about 12 feet long and has finger-like one millimeter long villi for aiding absorption. Carnivore intestine measures only 5-6 times the length of body while in herbivores it is 20-28 times the body length. In carnivores including man caecum is reduced and appendix rudimentary. Rectum is small and opens to the exterior by anal opening, except in monotremes where there is a shallow cloaca.

 LIVER

The main function of liver is assimilation and treatment of food after it has been digested and absorbed in intestine. New types of proteins and fats are synthesized in liver. But liver also produces bile juice that contains bile pigments. Often a gall bladder that opens by cystic duct into the bile duct for quick release whenever required.

Cyclostomes. Liver is unusually small and single lobed in lampreys but bilobed in hagfishes. Gall bladder is absent in lampreys but present in hagfishes but in larval stages both gall bladder and bile ducts are present in all.

 Fishes. Liver is large and lobed in fishes. Gall bladder is always present except in some sharks.

 Amphibia. Liver is large for the body size, lobed and with a gall bladder.

 Reptiles. There is no important deviation from amphibians in reptiles. In sakes there is a single elongated lobe. Gall bladder is always present in reptiles.

Birds. Liver is always lobed. Gall bladder is sometimes absent as in pigeon, in which bile ducts open directly.

Mammals. There are many more variations in mammals as compared to other groups. There are two main lobes which are subdivided into as many as 6-7 sub-lobes. Gall bladder is generally absent in those whose diet does not include much fat. 

 PANCREAS

This is the second largest digestive gland that arises from the endoderm of embryonic gut and can be divided into head, body and tail portions that lie in the loop of duodenum. It performs both exocrine and endocrine functions; the former contributes to about 99% of the secretions that are responsible for the digestion of proteins, fats and carbohydrates in small intestine.

Cyclostomes. There is no well-defined pancreas in adult lampreys but pancreatic tissue is embedded in liver and intestine. Hagfishes possess small pancreas that lies near the bile duct.

 Fishes. Cartilaginous fishes have 2-lobed, well-defined pancreas that has a single duct. In lungfishes and some teleosts pancreas is diffused and its endocrine portion is separated.

Amphibia, reptiles & Aves. There is no noteworthy feature in these groups. There is a well-developed pancreas with one or several ducts which may open in duodenum either directly or may join the bile duct.

Mammals. Generally two pancreatic ducts are present in mammals. Both ducts are functional in horse and dog. When bile duct joins the pancreatic duct there is a sphincter of Boyden at the junction. A small sac-like hepatopancreatic ampulla or ampulla or Vater is found in man in which bile duct and pancreatic duct open.

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DIGESTIVE SYSTEM IN HUMAN BEING

Digestion in vertebrates occurs in the digestive tract or alimentary canal. The various parts involved in digestion can be broadly grouped in two groups –

(1) Digestive tract or alimentary canal

(2) Digestive glands

(3) Digestive tract or alimentary canal

On the basis of the embryonic origin, the alimentary canal of vertebrates can be divided into three parts –

(1) Fore gut / Stomodaeum : Ectodermal. It includes buccal cavity / oral cavity, pharynx, oesophagus, stomach and small part of duodenum.

(2) Mid gut / Mesodaeum : Endodermal. It includes small intestine, and large intestine.

(3) Hind gut / Proctodaeum : Ectodermal. It includes anal canal and anus.

Parts of alimentary canal and its histology

Mouth

The mouth is a transverse slit bounded by two movable lips or labia, upper lip and lower lip. Upper lip has small ridges on the sides, a tubercle in the middle and a vertical groove (philtrum) above.

Vestibule

It is a narrow space between lips and gums in front and gums and cheeks on the sides. Its lining contains mucous glands. In the vestibule, a small median fold of mucous membrane, the superior labial frenulum, connects the middle of the upper lip to the gum and usually a similar but sma0ller inferior labial frenulum connects the middle of the lower lip to the gum.

Buccopharyngeal cavity

It includes anterior buccal cavity lined by stratified squamous epithelial cell and posterior pharyngeal cavity lined by columnar epithelial cell. It is distinguished into three region. Pharynx is a vertical canal beyond the soft palate. The food and air passages cross here. Pharynx may be divided into three parts; Nasopharynx, Oropharynx and Laryngopharynx.

Main structures of Buccopharyngeal cavity are –

(1) Fauces : A triangular area present between buccal cavity and pharynx in human.

(2) Palate : The roof of buccal cavity is called Palate. In crocodiles and mammals horizontal shelf like processes of premaxilla and maxilla and the palatine bones of upper jaw fused and form a secondary palate. Which separates the buccal cavity from nasal cavity. Palate is distinguished into three regions –

(i) Hard palate : Anterior, bony portion formed of maxilla and palatine bones in human and premaxilla, maxilla and palatine bones in rabbit. Hard palate have transverse ridges called palatine rugae. Such rugae or ridges are more develop in carnivorous mammals because their function is to firmly grip the food and prevent it from slipping out the cavity.

(ii) Soft palate : Posterior soft part, made up of connective tissue and muscles.

(iii) Vellum palati/uvula : Posterior most part of soft palate, which hangs in the region of pharynx. It closes the internal nostrils during degglutition.

(3) Palatine glands : Numerous mucous glands. Chiefly present in soft palate, secretes mucous for lubrication.

(4) Naso-palatine duct : One pair, present in rabbit, extends from nasal passage to the buccal passage, contains Jacobson’s organ concerned with olfaction.

(5) Vibrissae : A tuft of hairs on upper lip of rabbit.

(6) Hare-cleft : A cleft on the upper lip of rabbit, which makes it bilobed.

(7) Tongue (linguae) : Ectodermal, single, pinkish, oval, elongated highly muscular (mesodermal) and protrusible present on the floor of buccopharyngeal cavity the cells present are stratified squamous epithelial cells. A furrow termed the sulcus terminalis divides the oral part and pharyngeal part of the tongue. The limbs of the sulcus terminalis run laterally and forward from a median pit, named the foramen caecum.

Posterior part of tongue (endodermal) is attached with hyoid, middle one with the floor of buccopharyngeal cavity with the help of frenulum lingum and anterior part is free. The tongue is provided with two specialized structure viz. lingual papillae and lingual glands or weber’s gland. Lingual glands are the mucous glands, which secretes mucous. Lingual papillae are numerous, minute projections chiefly present on the dorsum of the tongue. All these lingual papillae can be grouped as simple lingual papillae and taste papillae. Taste papillae are of following types –

(i) Circumvallate : Circular largest  in number, present in the posterior part of the tongue extending from one side to another. They possess taste buds. These are the largest of all the papillae.

(ii) Fungiform : Mushroom shaped (Fungi - shaped), numerous, present at the anterior margins and tip of the tongue. They have 200 taste buds.

(iii) Foliate : Leaf like flat, less  in number, present at the posterior margin of the tongue. They are absent in human and found in rabbit.

(iv) Filiform : Conical shaped, smallest and most numerous distributed throughout tongue. They are without taste buds.

Hence, in human taste is recognized with the help of circumvallate and fungiform taste papillae. In man the anterior end of tongue feels sweet taste, posterior part feel bitter taste, sides feel sour taste and a small part behind the anterior end feel salty taste.

Functions of tongue : Important function of tongue are as follows –

(i) Acts as universal toothbrush, as it helps in tooth cleaning.

(ii) Helps in speaking.

(iii) Helps in degglutition.

(iv) Helps in mixing saliva with food.

(v) Acts as a curry comb in many animals, hence help in body cleaning.

(vi) Helps in taste detection.

(vii) In dog helps in regulation of body temperature. The phenomenon is called as “Panting”.

(viii) In frog and other animals, it helps in prey capturing

(8) Teeth : Teeth is a living structure. On the basis of embryonic origin, teeth in vertebrates are of following two types-

(i) Horny/ectodermal/epidermal/false teeth :The teeth which develops only from ectoderm. Examples -

Cyclostomes, tadpole larva of frog, prototherian mammals etc.

(ii) True teeth : The teeth which develops from both ectoderm and mesoderm. Examples – Fishes, amphibians, reptiles, eutherian mammals etc.

Differentiation of teeth : Morphologically, teeth can be distinguished as homodont or heterodont.

(i) Homodont : When all the teeth are structurally and functionally similar. Examples – Vertebrates except metatherian and eutherian mammals.

(ii) Heterodont : When the teeth are different in structure and functions. They are distinguished into four types incisors, canines, premolars and molars. Examples – metatherian and eutherian mammals.

(a) Incisors : These are the front teeth borne by the premaxillae in upper jaw and tips of dentaries in lower jaw. They are single-rooted monocuspid and long, curved and sharp-edged. They are adapted for cutting or cropping and biting.

(b) Canines : There is one pointed canine in each maxillary of upper jaw and each dentary of lower jaw next to the incisors. They are meant for piercing, tearing and offence and defence. They are single rooted and monocuspid.

(c) Premolars : They have one root (only in upper first PM two roots) and two cusps (bicuspid). They are meant for crushing, grinding and chewing.

(d) Molars : They have more than two roots (upper molars have three roots and lower molars have two roots) and 4 cuspid.

Attachment of teeth : On the basis of attachment of teeth at their bases with the jaw bones, teeth can be differentiated into –

(i) Acrodont : Teeth are attached to the free surface or summit of the jaw bone, as in a shark or frog. Such teeth are apt to break off easily but are replaced.

(ii) Pleurodont : In this condition, common in urodeles and lizards, teeth are attached to the inner side of jaw bone by their base as well as one side.

(iii) Thecodont : Such teeth are characteristic of mammals. Teeth have well developed roots implanted in deep individual pits or socketes called alveoli or theca, in the jaw bone. These type of teeth also present in crocodilians, fossil toothed bird (Archeaeopteryx).

Succession of teeth : According to their replacement (succession), teeth can be divided into 3 categories: polyphyodont, diphyodont and monophyodont.

(i) Polyphyodont : In lower vertebrates, teeth can be replaced an indefinite number of times during life. e.g., – Fishes, amphibia, reptilia.

(ii) Diphyodont : In most mammals teeth develop during life in two successive sets, a condition known as diphyodont. Teeth of the first set are known as deciduous teeth or milk teeth or lacteal teeth whereas the second set is called permanent teeth.

(iii) Monophyodont : In some mammals such as platypus, marsupials, moles, sirenians, toothed whale etc. only one set of teeth develops known as monophyodont condition.

Types of cheek teeth

(i) Bunodont : Crown with small, blunt and round cusps as in man, monkey, pig etc. found in mixed diet mammals.

(ii) Secodont : With sharp cutting edges for tearing flesh as in carnivores.

(iii) Lophodont : Only one cusp is present with transverse ridges called lophos, e.g., Elephant.

(iv) Selenodont : With vertical crescentic cusps as in grazing mammals like cow, sheep and goat. Selenodont teeth are two types –

(a) Brachyodont : Normal low crowned selenodont teeth with large roots are termed brachyodont. e.g. Ground squirrel, cattle.

(b) Hypsodont : In large grazing mammals teeth are elongated, prism shaped with high crown and low roots. e.g. Horse.

Structure of teeth : Teeth divided into three parts –

(i) Root : Inner most, attached to the bone with help of cement (hyaluronic acid).

(ii) Neck : Middle, small, covered with gum. Gum provides strength to the teeth.

(iii) Apex or crown : External exposed part of teeth. Longest part, white in colour.

A small cavity present inside teeth called as pulp cavity or dentine pulp cavity. It contains blood vessels, lymphatic vessels, nerve fibres, connective tissue etc. and provides nutrition to odontoblast cells or osteoblast cells. The odontoblast cells are mesodermal in embryonic origin forming immediate covering of the pulp cavity. The cells secrete dentine/ivory. Bulk of tooth in a mammal is formed of dentine. Dentine is a layer of inorganic substances 

(62−69

 which surrounds the odontoblast cells. It is mesodermal in origin. Enamel, secreted by Ameloblast/Enameloblast cells, forms the outermost covering. It is ectodermal and made up of 92% of inorganic substances, hence considered as hardest part of the body. The inorganic substances present are 

[Ca3(PO4)2,Ca(OH)2.H2O]

  Calcium phosphate (85%), Calcium hydroxide and Calcium Carbonate. Cement/Cementum attaches the tooth root to the bone.

Dental formula : Each mammalian species is characterized by its own specific dentition with a definite number and arrangement of teeth. Hence, dentition is of taxonomic importance. It is expressed by a dental formula as below –

Rabbit : 

i21,c00,pm32,m33=86×2=28

 or briefly,

20331023=2+0+3+31+0+2+3×22=1612=28

(i = incisors; c = canines; pm = premolars; m = molars)

Dental formulae of some common mammals

Horse and pig	3.1.4.33.1.4.3×2=44	Cat	3.1.3.13.1.2.1×2=30
Dog	3.1.4.23.1.4.3×2=42	Squirrel	1.0.2.31.0.1.3×2=22
Lemur	2.1.3.32.1.3.3×2=36	Rat	1.0.0.31.0.0.3×2=16  .
Man (adult set)	2.1.2.32.1.2.3×2=32	Elephant	1.0.0.30.0.0.3×2=14
Cow	0.0.3.33.1.3.3×2=32	Human set (milk set)	2.1.0.22.1.0.2×2=20

Oesophagus (food tube)

(1) Morphology : Single, ectodermal, dorsal to trachea, approximately 25 cm long. passes through thoracic cavity and opens into stomach present in abdominal cavity. Oesophagus anteriorly opens into pharynx through gullet and posteriorly into stomach through cardiac orifice.

(2) Histology : Serosa is absent but outermost layer of connective tissue is called as tunica adventitia. Muscular layer are striated/voluntary in anterior region and unstriated/involuntary in posterior part. The epithelial lining is made up of non-keratinized stratified squamous epithelial cells. Goblet cells are present.

Function : Conduction of food.

Stomach

(1) Structure : Single oval, elongated, unilobed present within abdominal cavity below diaphragm. It consists of three parts as cardiac/fundic (anterior), corpus/body (middle, chief part) and pyloric (posterior part) in human, whereas in rabbit stomach is bilobed and consists of three parts as cardiac (Anterior), fundic (middle, chief part) and pylorus (posterior). Two types of valves are present in the stomach viz. Cardiac sphincter valve between oesophagus and stomach and pyloric sphincter valve between stomach and duodenum. It new born baby cardiac sphynctor is much less developed that is why regurgitation of gastric contents is very common. Inner surface of stomach raised into numerous longitudinal folds called gastric rugae. In case of ruminant mammals (cud chewing mammals) oesophagus consists of only skeletal or voluntary muscles.

Oesophagus lack digestive glands but multicellular glands are found, which extends upto submucosa. Due to the presence of these submucosal mucous glands, submucosa of oesophagus is thickest than other parts of alimentary canal.

(2) Histology : Outermost layer is serosa. Muscular layer is three layered with outer longitudinal, middle circular and inner oblique. Muscles are involuntary and unstriated. Epithelial lining is made up of simple columnar epithelial cells and specialized cells present in the gastric glands. The nomenclature of gastric glands is according to the parts of the stomach. The various type of gastric glands and the cells present in them are as follows –            

(i) Anterior part : Cardiac gastric glands in rabbit and human cells present are mucous neck cells secreting mucous.

(ii) Middle part : Fundic gastric/Main gastric glands glands in rabbit and corpus in human has at least four distinct types of cells –

(a) Peptic or zymogenic or chief or central cells : Secretes two digestive proenzymes pepsinogen and prorennin.

(b) Oxyntic or parietal cells : Secretes 

HCl

  and castle's intrinsic factor required for the absorption of vitamin B12. Hyperacidity is a abnormally high a degree of acidity due to the secretion of large quantity of 

HCl

  i.e. gastric juice.

(c) Mucous neck cells : Secretes alkaline mucous.

(d) Argentaffin cells or Kultchitsky or enterochromaffin cells : Responsible for the secretion of vasoconstrictor seratonin.

(iii) Posterior part : Pyloric gastric glands in rabbit and human-cells are mucous neck cells secreting mucous and some cells, called “gastrin” or “G” cells, secrete a hormone, named gastrin, which increases the motility of gastric wall and stimulates gastric glands for active secretion.

Functions

(1) Storage of food.       

(2) Trituration or churning of food to mix with gastric juice.

(3) Functions of gastric juice (discussed along with gastric juice).

Stomach of ruminants (cud-chewing mammals) : The stomach of cattles have four parts, as rumen (paunch), reticulum(honeycomb), omasum (psalterium) and abomasum (rennet). Some authors believe that first three chambers are parts of the oesophagus, the fourth chamber is the real stomach secretes 

HCl

  and enzymes. The embryological studies have proved that all the chambers are parts of the real stomach. Camel and deer lack omasum. Reticulum is the smallest part and its cells are provided with water pockets for the storage of metabolic water.

In the rumen, food undergoes mechanical and chemical breakdown. Mechanical breakdown results from through churning brought about by muscular contractions and aided by cornified surface of villi. Chemical breakdown is caused by symbiotic microorganisms (bacteria and ciliates) that release enzyme cellulase, which act on cellulose and simplify it into short-chain fatty acids, such as acetic acid, butyric acid, propionic acid. This is called microbial digestion.

Small intestine

(1) Structure : Endodermal, longest part of alimentary canal present in the abdominal cavity, supported by a peritoneal membrane called mesentery. Wall of jejunum and ileum has circular or spiral internal fold called fold of kerckring or valvulae conniventes. Also numerous finger like projection called villi project from the wall of lumen, increasing internal surface area about ten time. The distal end of ileum is leads into the large intestine by ileo-caecal valve in man but in rabbit sacculus rotundus and ileo-coecal valve both are present.

(2) Parts : It is approximately 3 metres in human. It is divisible into three parts. In man small intestine divided into three parts :

Parts of small intestine

Duodenum (Proximal part)	Jejunum (Middle part)	Ileum (Posterior part)
25 cm. Long Forming U-shaped loop before leading to jejunum, pancreas lies in the loop.	About 1 m long and about 4 cm. wide. Wall is thicker and more vascular. Villi thicker and tongue-like. Plicae best developed. Peyer's patches are lacking.	About 2 m long and about 3.5 cm. wide. Wall is thinner and less vascular. Villi thinner and finger-like. Plicae less developed. Peyer?s patches are present.

(3) Histology : Serosa is the outer most covering. Muscular layer is generalized with involuntary, unstriated muscles. The cells present in the epithelial lining are simple columnar epithelial cells, which are brush- boardered i.e. provided with villi and microvilli to increase the surface area. The folds present are longitudinal and are called folds of kerckring or valvulae canniventes. Goblet cells secrete mucous. Payer’s patches are the oval, rounded masses of lymphatic tissue present in between lamina propria and epithelial lining. They produce lymphocytes. Brunner’s glands or Duodenal glands are the multicellular mucous glands present in the submucosa of duodenum only. They secrete mucous. In addition there are also found granular arogyrophil cell.

(4) Glands of small intestine : Various glands found in small intestine. Each gland has three types of cells : (1) Undifferentiated epithelial cell (2) Zymogenic cell (paneth cell) and (3) Argentaffin (Enterochromaffin cell).

Glands of small intestine

Brunner's glands	Payer's patches	Crypts of Leiberkuhn
Found in duodenum only. Mucus secreating gland as so known as mucus gland.	These are lymph nodules. They produce lymphocytes. Lymphocytes are phagocyte in nature which destroy harmful bacteria.	Known as intestinal gland. Found in duodenum and ileum only. Secrete succus entericus i.e. intestinal juice. Formed by folding of lamina propia.

Function : Digestion and absorption of food.

Large intestine

The name large intestine is due to large diameter (4-6 cm).

(1) Structure : Endodermal, approximately 

1.5−1.75

  metre long.

(2) Parts : They are following –

(i) Caecum : Spirally coiled 6 cm long in human and 45 cm long in rabbit. Its posterior end is present as a blind sac in abdominal cavity called vermiform appendix. Vermiform appendix is vestigeal but contains lymphatic tissue. Caecum in human is concerned with passage of food whereas in rabbit it is concerned with cellulose digestion and conduction of food.

(ii) Colon : Single endodermal approximately 1.3m long in human distinguished into four limbs as ascending, transverse, descending and pelvic or sigmoid limb. Colon posses two specialized structures as Taeniae coli (present in the middle of colon) and Haustra, (dilated sac-like or pockets like structures surrounding taeniae). Colon is concerned with absorption of water of undigested food, 5%, salts, vitamins etc. hence concerned with faeces formation. Colon bacteria also synthesized vit. 

B12

  and K.

(iii) Rectum : Single small dilated sac like in human whereas large beaded in rabbit. It is concerned with storage of faeces. Rectum has strong sphincter muscle in its wall. The sphincter keeps the canal as well as anus, closed when not used for defecation.

(iv) Function : Absorption of water from undigested food.          

Anal canal and anus : Anal canal connects rectum with anus and it is about 3 cm. long. Anus is the terminal inferior opening of alimentary canal, which is guarded by an internal involuntary sphincter and an external voluntary sphincter.

Digestive glands

The various types of digestive glands present in mammals are salivary glands, gastric glands, intestinal glands, pancreas and liver. The digestive glands secrete digestive juices. Parasympathetic nervous system increases the secretion of digestive juice whereas sympathetic nervous system decreases it.

(a) Salivary glands : The three pairs of salivary glands present in humans are as follows –

(1) Parotid : One-pair, largest salivary gland present below pinna. A stenson’s duct arises from each gland, opening in vestibule between the 2nd molar teeth of upper jaw and checks. Parotid glands secrete enzymes. Viral infection of parotid glands causes “Mumps” (by paramyxo virus).

(2) Sub-mandibular / sub-maxillary : One-pair, present at the junction of upper and lower jaw in cheek region.  A wharton’s duct arises from each gland and opens on lower jaw. These are seromucous glands.

 (3) Sub-lingual : One-pair, present in the floor of buccopharyngeal cavity. These are mucous glands 6-8 ducts, called ducts of rivinus or Bartholin's duct arises from these glands and opens below tongue on the floor of buccopharyngeal cavity.

Saliva / salivary juice : The secretion of salivary glands is called saliva or salivary juice. Some of the characteristics are as follows –

(1) Amount : 1.0-1.5 litre/day    

(2) Chemical nature : Slightly acidic.

(3) 

pH: 6.36.8

(4) Control of secretion : Autonomic reflex (parasympathetic nervous system increases salivation while sympathetic nervous system inhibit secretion.)

(5) Chemical composition : Water (99.5%), mucous (acts as lubricant), salts (

NaCl, NaHCO3

 etc.), enzymes (ptyalin, lysozyme) etc.

Functions : Salivary juice and its enzymes –

(1) Makes the medium slightly acidic for the action of its enzyme.

(2) Help in taste detection, deglutition, speaking etc.

(3) Starch 

−→−−−−−−−−Ptyalin/Diastase(Salivaryamylase)

  Maltose + Isomaltose + Limit dextrin.

(4) Bacteria (living) 

−→−−−−Lysozyme

 Bacteria killed.

Gastric glands : There are approximately 35 million of gastric glands present in human stomach and grouped into three categories as already described along with stomach. The gastric gland secretes gastric juice.

Gastric juice      

(1) Amount : 2-3 liters/day.

(2) Chemical nature : Highly acidic

(3) 

pH: 1.03.5

 (due to presence of 

HCl

 )

(4) Control of secretion : By gastrin hormone.

(5) Chemical composition : Water (99%), mucous, inorganic salts, castle’s intrinsic factor, 

HCl

  ( 0.5%, conc.) and enzymes prorennin and pepsinogen and gastrin lipase.

Functions of gastric juice and its enzymes

(1) Inactivates the action of ptyalin.

(2) Makes the medium acidic for the action of gastric enzymes.

(3) 

HCl

  kills micro organisms.

(4) 

HCl

  kills the living organism (prey etc.) if ingested.

(5) Pepsinogen (inactive) 

−→−HCl

  Pepsin (active).

(6) Prorennin (inactive) 

−→−HCl

  Rennin (active).

(7) Proteins + Peptones   

−→−−−PepsinpH−1−3

  Polypeptides + Oligopeptides.

(8)

Casein(milk protein)−→−−−RenninCa2+

  Paracasienate Above phenomenon is called “curdling of milk”.

(9) Lipids 

−→−−−−−−−−−−−−−−−−−−−−−−−GastricLipasenegligibleinhumanstomachactsatpH4−6

 Triglycerides + Monoglycerides.

(10) 

HCl

 is antiseptic.

(11) It act as preservative.

Lactose intolerance : Among mammals, man alone takes milk even after becoming adult. In some humans, secretion of lactase decreases or ceases with age. This condition is called lactose intolerance. Lactose intolerant persons fail to digest lactose of milk. In their large intestine, lactose fermented by bacteria, producing gases and acids.

Intestinal glands : Intestinal glands in mammals is a collective name for crypts of Lieberkuhn (secretes alkaline enzymatic juice) and Brunner’s glands (secretes mucous). Intestinal glands secrete intestinal juice.

Succus entericus (intestinal juice)

(1) Amount : 1.5 – 2.0 l/day.

(2) Chemical nature : Alkaline.

(3) 

pH: 7.6−8.3

              

(4) Control of secretion : Nervous and hormonal (Enterocrinin Duocrinin etc.)

(5) Chemical composition : Water (99%), mucous, inorganic salts, enzymes etc.

Function of Intestinal juice and its enzymes.

(1) Inhibits the action of gastric enzymes.

(2) Makes the medium alkaline for the action of it’s enzymes.

(3) Starch 

−→−−−−Amylase

  Maltose + Isomaltose + limit dextrin.

(4) Maltose 

−→−−−−−−Maltase(α-glucosidase)

  Glucose + Glucose.

(5) Isomaltose  

−→−−−−−Isomaltase

  Glucose + Glucose.

(6) Lactose (milk sugar) 

−→−−−−−−−Lactase(β-galactosidase)

  Glucose +Galactose.

(7) Sucrose (cane sugar) 

−→−−−−−−−−−Sucrase / Invertase(β-fructosidase)

 Glucose + Fructose.

(8) Polypeptides + Oligopeptides 

−→−−−−−−−−−Erepsin(Amino−peptidase)

  Amino acids.           

(9) Trypsinogen (inactive) 

−→−−−−−−Enterokinase

  Trypsin (active).

(10) Lipids 

−→−−Lipase

  Fatty acids + Glycerol + Monoglycerides.

(11) Phospholipids  

−→−−−−−−−Phospholipase

  phosphorous + Fatty acids + Glycerol + Monoglycerides.

(12) Organic phosphate  

−→−−−−−−Phosphatase

  Free phosphate.

(13) Nucleic acid 

−→−−−−−−−−Polynucleotidase

  Nucleotides.

(14) Nucleosides 

−→−−−−−−Nucleosidase

  Nitrogenous bases.

Pancreas : Single, endodermal, flat, leaf-like yellowish, heterocrine (mixed) gland, present between the ascending and descending limb of duodenum and opens into duodenum through pancreatic duct. It’s can devided into following parts-   

Exocrine : It is the major part (about 99%) of pancreas. The exocrine tissue of the pancreas consists of rounded lobules (acini) that secrete an alkaline pancreatic juice. The juice is carried by the main pancreatic duct, also called duct of Wirsung, into the duodenum through the hepatopancreatic ampulla (ampulla of vator). An accessory pancreatic duct, also named duct of Santorini, may sometimes lead directly into the duodenum.

Endocrine : Minor part (1% only) also called as islets of Langerhans scattered in the exocrine part. It consist of four various type of cells, as 

α(A)

  cells, 

β(B)

  cells, 

δ(D)

  cells and F or PP cells. 

α−

 cells secretes glucagon hormone, 

β−

 cells secretes insulin hormone and 

δ

  cells secrets somatostatin. The PP or 

F−

 cells secrete pancreatic polypeptid hormone to control somatostatin. The secretion passes directly into blood.

Pancreatic juice

(1) Amount : 1-1.5 l/day               

(2) Chemical nature : alkaline

(3) 

pH: 7.1−8.2

(4) Control of secretion : Hormonal and normal mechanism.

Secretin hormones stimulate the production of more alkaline pancreatic juice but low in enzyme content. Pancreozymin or Cholecystokinin stimulates the production of enzyme rich pancreatic juice.

(5) Chemical composition : Water (99%), enzymes and salts.

Functions of pancreas and its enzymes

(1) The islets of Langerhans secrete insulin and glucagon hormones.

(2) The exocrine part of pancreas secretes pancreatic juice.

(3) Elastase : It act upon elastin protein.

(4) Trypsinogen 

−→−−−−−−−−EnterokinaseofIntestinaljuice

  Trypsin.

(5) Trypsinogen 

−→−−−−−−Trypsin(Autocatalysis)

  Trypsin.

(6) Chymotrypsinogen 

−→−−−−−TrypsinAutocatalysis

 chymotrypsin.

(7) Polypeptides + peptones  

−→−−−−−−−−−−Trypsin(Pancreaticprotease)

 Tripeptides + Dipeptides + Oligopeptides.          

(8) Starch  

−→−−−−−−−−−−Amylopsin(Pancreaticamylase)

  Maltose + Isomaltose + limit dextrin.

(9) Emulsified Lipids 

−→−−−−−−−−−(Pancreaticlipase)Steapsin

  Fatty acids + Glycerol + Monoglycerides.

(10) Nucleic acid 

−→−−−−Nuclease

  Nucleotides + Nucleosides.

(11) Nucleic acid 

−→−−−−−−Nucleasidase

  Purines + Pyrimidines.

(12) Polypeptides  

−→−−−−−−−Chymotrypsin

  Oligopeptides.

Liver

(1) Structure : The liver is largest and heaviest  gland in the body. Its upper and anterior surfaces are smooth and curved to fit the under surface of the diaphragm; the posterior surface is irregular in outline. It consists of three lobes in frog: right, left and median; five lobes in rabbit: left lateral, left central, spigelian, right central and caudate; four lobes in man: right, left, quadrates and caudate lobe. It is divided into two main lobes : right and left lobes separated by the falciform ligament.

A pear-shaped sac, the gall bladder is attached to the posterior surface of the liver by connective tissue. The right and left hepatic ducts join to form the common hepatic duct. The latter joins the cystic duct, which arises from the gall bladder. The cystic duct and common hepatic duct join to form common bile duct or ductus cholidochus which passes downwards posteriorly to join the main pancreatic duct to form the hepatopancreatic ampulla (ampulla of Vater). The ampulla opens into the duodenum. The opening is guarded by the sphincter of Oddi. The sphincter of Boyden surrounds the opening of the bile duct before it is joined with the pancreatic duct. The basic structural and functional unit of the liver is the hepatic lobule.

Each lobule is composed of plates of polyhedral, glycogen-rich cells, the hepatocytes, arranged radially around a central vein. Between the plates are radial blood sinusoids. At the periphery of the lobules, the branches of portal vein, hepatic artery, bile ducts, and lymphatics course together. A network of tubular spaces between the hepatocytes represents the bile canaliculi. At the periphery of the lobule the bile canaliculi empty into small hering’s canals walled by cuboidal epithelium. These canals lead into bile ducts walled by columnar epithelium. The sinusoids are lined by incomplete endothelium with scattered phagocytic Kupffer cells, that eat bacteria and foreign substances

Gall bladder : The gall bladder is a slate-blue, pear-shaped sac connected with an supported from liver by a small omentum or ligament. Its distal part is called fundus, while the narrow part, continued as cystic duct, is called the neck.

Functions of liver : Liver, the largest gland of vertebrate body, is an essential organ, which performs many functions –

(1) It secretes bile which is a complex watery fluid containing bile salts (Na taurocholate and Na glycocholate), bile pigments (biliverdin and bilirubin), cholesterol, mucin, lecithin and fats etc. It breaks and emulsifies the fat.

(2) In the liver, haemoglobin of the worn out erythrocytes breaks down to bile pigments bilirubin and biliverdin. The bile pigments are also converted in the bowel into stercobilin which colours the faeces.

(3) Excess quantities of carbohydrates (glucose) are converted to glycogen (Glycogenesis) in the presence of insulin in the liver cells, and stored therein.

(4) Glycogen is a reserve food material, which is changed into glucose (Glycogenolysis) and released into the blood at concentrations maintained constant by the liver. In this way, blood–sugar level is maintained under diverse dietary conditions.

(5) Under abnormal conditions, liver can convert proteins and fats into glucose by complex chemical reactions. Formation of this “new sugar” i.e. from non–carbohydrate sources, is called gluconeogenesis.

(6) If the level of blood–glucose rises beyond normal even after glycogenesis and catabolism, the excess glucose is converted into fat and stored in the liver. The process is termed lipogenesis.

(7) Amino acids resulting from protein digestion finally come into the liver from the intestine. They are partly released into the blood for distribution and protein synthesis, partly transaminated into other amino-acids and deaminated.

(8) In the embryo, red blood cells are manufactured by the liver. In the adult, liver stores inorganic salts of iron, copper and vitamin

B12

 (anti–anaemic factor) and thus helps in the formation of red blood cells and haemoglobin.

(9) Liver functions as a store–house for blood and regulates blood–volume.

(10) Fibrinogen, prothrombin and certain other blood coagulation factors are formed in the liver. Heparin is an intravascular anticoagulant that is stored in the liver.

(11) The plasma proteins serum albumin and serum globulin are synthesized by the liver from the amino acids.

(12) Liver synthesizes vitamin A from the provitamins A (carotenoid pigments). Liver cells also store fat–soluble vitamins A, D, E and K. Besides, it is the principal storage organ for vitamin 

B12.

(13) The liver is the site of detoxification of different toxic substances either produced in the body or taken along with food.

(14) It is the main heat producing organ of the body.

(15) Kupffer cells in the liver sinusoids phagocytose and remove bacteria, worn-out blood elements and foreign particles.

(16) Liver is an important site of lymph formation.

Bile/chole

(1) Amount : 

800−1000ml

 daily. On the average about 

700ml.

(2) Source : Secreted by hepatic cells

(3) Storage site : Gall bladder                                     

(4) Colour : Greenish-blue

(5) Chemical nature : Alkaline                                    

(6) 

pH: 7.68.6

Functions of bile          

(1) Emulsification of fats.

(2) Helps in absorption of fat-soluble vitamins.

(3) Increases alkalinity to make the medium suitable for enzymatic action.

(4) Elimination of heavy metals such as 

Cu, Hg, Zn

  etc.

(5) Elimination of excess of bile pigments.

(6) Stercobilin and urobilin (urobilin found in urine) is formed by bilirubin and biliverdin is responsible for colouration of faeces..