COMPANA LEC LE3 (Digestive)

Functions of digestive system

- Take in food

- Store food

- break up food physically and chemically

- Absorb nutrients

- Process and remove solid waste

Ingestion

act of eating or feeding

Digestion

process of breaking food down

Absorption

uptake of nutrients by body cells

Elimination

passage of undigested material out of the digestive system

1. Type of food eaten

2. Level of activity and metabolism

3. Body size

Morphological variations

• Oral cavity

• Pharynx

• Esophagus

Embryonic origin of foregut 

• Stomach

• Small intestine

Embryonic origin of Midgut 

• Large intestine

• Cloaca

Embryonic origin of Hindgut

oral cavity 

begins at the mouth and ends at the pharynx 

fishes

have a very short oral cavity

tetrapods

typically have longer oral cavities

Mammalian 

mouth is specialized to serve as suckling and masticatory organ (muscular cheeks) 

Palate

roof of the oral cavity

Secondary palate

• Separates the oral passageway from the nasal passageway

• Breathing air influenced the evolution of the secondary palate

• Primary palate only

• Have complete roof to the mouth

Palate in fishes and amphibian

• evolution of a secondary palate

• Turtles - development of maxilla, premaxillae, and a palatine

• Alligators - complete bony secondary palate 

Palate in reptiles 

• Simple tonged, crescent-shaped elevation in the floor of the oral cavity caused by the underlying hyoid skeleton

Tongues for bony fishes and primitive amphibians

• Primary tongue with glandular field and is stuffed with hypobranchial musculature

Tongues for most amphibians

• Primary tongue with glandular fields and lateral lingual swellings stuffed with hypobranchial musculature 

Tongues for reptiles and mammals

• Lateral lingual swellings are suppressed and intrinsic muscle is usually lacking 

Tongues for birds

tongue is largely immobilized in the floor of the oral cavity and cannot be extended

Tongue mobility for turtles, crocodilians, some birds and whales 

• Tongue sometimes long and may move in and out of the oral cavity 

Tongue mobility for snakes, insectivorous llzards, amphibians and some birds 

• Tongue is attached to the floor of the oral cavity but can still be extended out of the oral cavity 

Tongue mobility for mammals

Saliva

used to moisten food and secrete digestive enzymes

few and found on roof of mouth and tongue

Oral glands in amphibians

numerous and developed

Oral glands in reptiles

water-feeders lack glands while some with oral glands secrete mucus for nest-building 

Oral glands in birds 

• Salivary glands

• Usually 3 pairs (parotid, mandibular, and sublingual)

• Saliva: mucus, salts, proteins & enzymes (amylase)

Oral glands in mammals

Contains poison

Oral glands in lizards, snakes, and mammals

Anticoagulants

Oral glands in vampire bats

teeth 

• derivations of dermal armor

• Vary among vertebrates in number, distribution in the oral cavity, degree of permanence, mode of attachment, and shape

placoid scales

show gradual transition to teeth at the edge of the jaw

Composition of teeth

primarily dentin surrounded by enamel

wear patterns of teeth 

are functionally important 

enamel

is >95% inorganic matter; hardest substance in vertebrate bodies

enamel

is harder than dentine

Dentine

is harder then cement

• Agnathans

• Sturgeons

• Some toads

• Turtles

• birds

• Baleen whales 

toothless vertebrates 

• Numerous and distributed in the oral cavity and pharynx

teeth for fish

widely distributed on the palate 

Teeth for early tetrapods 

Replace teeth in “waves” 

Teeth for most vertebrates (except mammals) 

have two sets of teeth: deciduous and permanent teeth

Teeth for mammals

• Aglyphous

• Solenoglyphous

• Proteroglyphous

• Opisthoglypous

morphological variations in teeth for reptiles

Aglyphous

no modifications for venom delivery 

Solenoglyphous

retractable teeth, fangs

Proteroglyphous

fangs in front of mouth 

Opisthoglypous

fangs in back of mouth 

• Incisors

• Canines

• Premolars

• molars

morphological variations in teeth for mammals

Incisors 

securing food and grooming, single roots, conical spikes for holding and blades for cutting 

canines

simple spike-like teeth with single roots for tearing

premolars

for grinding

molars

larger than premolars & have more cusps and roots

• Acrodont

• Pleurodont

• Thecodont 

type of teeth based on attachment

Acrodont

• no root

• attached to rim of jawbone (most fishes, some lizards)

Pleurodont 

• no root

• attached to lingual side of jawbone (some lizards & snakes)

Thecodont

• Rooted in sockets

• Mammals, crocodilians, and birds

• Polyphyodont

• Diphyodont

• Monophyodont  

Types of teeth based on replacement 

Polyphyodonty

continuous replacement throughout life (most vertebrates)

Diphyodont

two sets of teeth (most mammals)

Monophyodont 

single set of teeth (cetaceans) 

• Homodont

• Heterdont

Type of teeth based on shape

Homodont

teeth of similar shape along jaw

Heterodont

teeth of different shape along the jaw

• 4th premolar and 1st molar contact one another during mastication

• Effective at slicing flesh 

• Protocone of P4, metaconid and taloned of M1 is typically reduced 

Ecomorphology of teeth (Canivory) 

• Tribosphenic molar

• Dilambdodont molar

Ecomorphology of teeth (Insectivory)

Tribosphenic molar

evolved to crunch, mash and shear insect exoskeletons

Dilambdodont molar

marked by W-shaped ectoloph ridge

• heavy wear on occlusal surfaces

• has tail crowns, or continuously growing to replace worn tissue

• Occlusal ridges and crests

Ecomorphology of teeth (Herbivory) 

• Prepare food by pounding

• Hypocone is added to the upper cheek teeth

• Paraconid is lost from the lower cheek teeth

• Bunodont dentition is typical (bears, pigs, raccoons and primates)

Ecomorphology of teeth (Omnivory) 

Pharynx

• Region of the foregut between oral cavity and the esophagus

• exhibits pharyngeal pouches that may give rise to slits

Pharynx is the respiratory organ 

In fishes 

• site of openings of auditory tube

• Crossroad between food and respiratory passages

In tetrapods

1. Nasopharynx

2. Oropharynx

3. Laryngopharynx

tetrapods pharynx

Nasopharynx

opening of internal nares and the opening of eustachian tubes

Oropharynx 

common passage for food and air 

• Two openings

1. Glottis - leading into the trachea

2. gullet - leading into the esophagus

Laryngopharynx

• Short digestive tracts that produce fast-acting digestive enzymes

length of the tract for carnivores

• Long intestine that have large colonies of bacteria for cellulose digestion

length of the tract for Herbivores

Mucosa

layer of epithelial cells in direct contact with the lumen 

Submucosa 

loose connective tissue containing blood vessels and nerves 

Muscularis externa

smooth muscular layer

Inner circular

lengthening and constriction

Outer longitudinal

shortening and peristalsis

Visceral serosa/peritoneum

outer layer in contact with mesenteries

Esophagus

connects the pharynx to the stomach

striated muscle fibers throughout the entire esophagus (voluntary control)

Esophagus for herbivores 

lined with numerous folds; simple structure serving as extension of the pharynx

Esophagus for Agnathans 

bears longitudinal folds to allow distention

Esophagus for fishes

Teleosts

have very short esophagus

Elasmobranchs (sharks)

longer esophagus lined by papillae

extremely short 

Esophagus for amphibians

generally longer, longitudinal folds

Esophagus for reptiles

Snake

capable of swallowing large objects

Marine turtles

lined with cornified papillae pointing posteriorly

• longer esophagus

• lined with horny papillae

• Has a crop (large sac for storing food) 

Esophagus for birds