Digestive System

• Ingestion

• Processing

• Absorption

• Elimination

Ingestion

• Primitive chordates were filter feeders.

• Jaw development allowed more aggressive methods of feeding

• However, some of the largest mammals retain filter feeding (baleen whales).

Jaws

• Branchiomeric muscles serve the pharyngeal arches

• Basic pattern seen in shark remains in tetrapods. Posterior muscles, however, now move head and neck

• Two important arches - mandibular and hyoid. 3-6 remain important for other reasons

Mandibular Arch (I)

Muscles that operate jaw

• Adductor mandibulae gives rise to massseter, temporalis and pterygoideus.

• Intermandibular - myohyoideus, digastricus and tensor tympani

Hyoid Arch (II)

Constrictors of pharynx

• Posterior belly of digastricus

• Platysma

• Stapedius

Remaining Arches (III-VI)

• Trapezius

• Extrinsic laryngeal muscles

• Neck muscles

Innervation of Branchiomeric Muscles

Cranial nerves:

• V - Trigeminal

• VII - Facial

• IX - Glossopharyngeal

• X - Vagus

• XI - Accessory

Overview of Digestive System

• Alimentary canal - tube running from mouth to cloaca or anus

• Smooth muscle moves food by peristalsis

• Divisions - oropharynx, esophagus, stomach and intestines

• Associated structures - tongue, teeth, oral glands, pancreas, liver, gallbladder and ceca

• Digestive organs lie in pleuroperitoneal cavity (except animals with a diaphragm

• Parietal peritoneum and skeletal muscle from somatic mesoderm; visceral peritoneum and smooth muscle from splanchnic mesoderm

• Visceral and parietal peritonea are continuous via dorsal and ventral mesentaries

Formation

• Endoderm forms digestive tract

• Three regions: – Foregut - post. oral cavity thru most of small

intestine– Midgut - attachment of yolk sac– Hindgut - large intestine and cloaca

Oropharynx

• Mouth and oral cavity - entrance to digestive system

• Oral vestibule -separates alveolar ridges from cheeks

• Structures include teeth, salivary glands, taste buds and the tongue

• Several other important structures form in oropharynx

Teeth

• Began as dermal plates in ostracoderms• Morphology - composed of enamel, dentin

and cementum. Articulation is a gomphosis. Teeth anchored by periodontal ligament

• Attachment:– Acrodont -outer surface or top of jaw– Pleurodont - inner surface of jaw– Thecodont - bony sockets

• Replacement– Polyphydont: continuous replacement

throughout life– Diphydont: 2 sets - deciduous and permenant– Monophydont: one set only

If all teeth are similar dentition is homodontic.

Variation in morphology produces heterodontic dention

TongueProcurement and manipulation of food; sensory

Primary Tongue: mesenchyme of hyoid arch

• present in sharks, bony fish and amphibians

• cannot move independently

• An extension of the glandular field of the pharyngeal floor is used by terrestrial amphibians to capture prey

Secondary Tongue: reptiles, birds and mammals

• develops from primary tongue and tuberculum impar (old glandular field): I

• lateral lingual swellings develop anterior to tuberculum impar: I

• copula develops as a median swelling (II, III and IV)

• epiglottis forms another median swelling

(IV)

• Anterior 2/3’s of tongue (lat. Lingual swellings + tuberculum impar form body of tongue innervated by mandibular branch of trigeminal n.

• Terminal sulcus separates body from posterior 1/3, the root of the tongue. This is innervated by the vagus and glossopharyngeal nerves

• A depression in the middle of the terminal sulcus, the foramen caecum gives rise to the thyroglossal duct and forms the thyroid

Oral Glands• Multicellular• Secrete serous or mucus fluid into oral cavity

to lubricate food. Enzymes may facilitate digestion.

• Modified glands by produce venom.• Glands named for location: parotid,

submandibular and sublingual• Glands may be serous, mucous or mixed

Alimentary Canal

Morphology• mucosa: endothelium and connective tissue limited by

muscularis mucosa• submucosa: connective tissue containing glands and rich

vascular supply• muscularis: smooth muscle - inner circular, outer

longitudinal• serosa: mesothelium and connective tissue of visceral

peritoneum (retroperitoneal structures covered by adventitia)

Esophagus:• muscular tube between oropharynx and stomach• variable in length• specialized mainly in birds where diverticula form crop

Stomach:• primary site of digestion requiring acid environment• modifications depend on diet

(stomach cont.)

• 3 regions: cardiac, fundus, pylorus

• glands may secrete mucus, proteolytic enzymes and acid

• birds and crocodiles have proventriculus and gizzard

• herbivores have stomachs with multiple chambers

• flexion of stomach during development produces greater and lessor curvatures

Small intestine:

• morphology varies with diet

• duodenum is proximal; receives pancreatic and bile ducts

• length and modifications such as spiral valves, plica and villi increase surface area

• in mammals, jejunum and lieum are distal to duodenum

Colon:

• begins at ileocaecal valve; terminates in rectum

• caeca are present

• absorption of water

• vitamin synthesis by bacteria

Liver• Midventral diverticulum of foregut

• coronary ligament anchors to transverse septum/diapragm

• falciform ligament to ventral body wall

• Lobes are drained by hepatic ducts; cystic duct drains gallbladder while common bile duct empties into duodenum (ampulla of Vater

Pancreas• Consists of both exocrine and endocrine

tissue

• may be compact or diffuse

• ventral pancreatic buds form body; dorsal bud forms tail

• usually a single pancreatic duct empties into duodenum, although an accessory duct by persist