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 and Condylura 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 t
o 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.
The following e-book gives detailed description and colour diagrams. Buy from Amazon.
