Digestive System
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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
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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
Tongue
Procurement 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