Digestion is the mechanical and chemical
breakdown of food into forms that cells can absorb.
• Mechanical Digestion – breaks down food into smaller pieces
Chemical Digestion – decomposes food into smaller molecules
The digestive system consists of:
• The alimentary canal – extends from mouth to anus
Accessory glands –secrete chemicals into the alimentary canal.
THE ALIMENTARY CANAL
• If stretched out, the alimentary canal is
about 8 meters long. That’s 26 feet!
• The alimentary canal (A.C.) wall has four
distinct layers that differ from region to region.
STRUCTURE OF THE WALL
• Mucosa – mucous membrane
• Epithelial tissues on a bed of connective tissue (lamina propria).
• Makes direct contact with the lumen
• This layer absorb nutrients, secrete chemicals, and protect the
• Submucosa – Beneath the mucosa
• Loose connective tissue
• Blood vessels, nerves, and lymphatic vessels
• This layer nourishes the surrounding tissues, and carries away
STRUCTURE OF THE WALL
• Muscular – provides movements within the A.C.
• Inner coat = transverse muscles: decrease the diameter of the A.C.
• Outer coat = longitudinal muscles: shortens length of the A.C.
• Serosa – serous membrane
• Composed of visceral peritoneum in most places
• Secretes serous fluid, which lubricates the A.C.
Figure 17.3 The wall of the small intestine, as in other portions of the
alimentary canal, consists of four layers: An inner mucosa, a
submucosa, a muscular layer, and an outer serous layer.
MOVEMENTS OF THE ALIMENTARY CANAL
Muscles of the A.C. provide two basic movements:
• Segmentation – mixing movement
• Smooth muscles contract and
relax, mixing foods with digestive
•Peristalsis – propelling movement
• Smooth muscles contract in a wave-like motion
pushing food through the alimentary canal.
Figure 17.4 (b) segmentation mixes the contents of the small
intestine. (c) Peristaltic waves move the contents along the canal
INNERVATION OF THE ALIMENTARY CANAL
• Enteric Nervous System –
subdivision of the ANS that innervates the alimentary canal.
•Submucosal plexus – controls secretions
•Myenteric plexus – controls gastrointestinal motility
• Parasympathetic impulses – increase activities of digestive system
• Sympathetic impulses – inhibit certain digestive actions
• The mouth:
• Receives food
• Mechanically breaks up solid particles using saliva
• Prepares food for chemical digestion
• This action is called mastication
• The mouth also functions as an organ of
speech, and sensory reception.
BOARDERS OF THE MOUTH
• The cheeks form the lateral walls of the mouth
• Buccinator muscle + buccal fat pad (gives form to cheeks)
• The lips form the anterior boarder
•Highly mobile structures that surround the mouth opening
•Orbicularis oris skeletal muscle
•Semitransparent epithelium, and a good blood supply makes the lips appear red.
•Highly innervated by sensory receptors
• The tongue is a thick, muscular organ that occupies the floor of
the mouth and nearly fills the oral cavity when the mouth is closed
Mucous Membrane – covers the surface of the tongue
Lingual Frenulum – membranous fold, anchors the tongue to the floor of the mouth.
Body – largely composed of skeletal muscle
• Manipulate foods and aids in swallowing
Root – posterior portion of the tongue
• Anchored to the hyoid bone.
• Covered with lingual tonsils
Papillae – projections on the surface of the tongue.
• Some provide friction, others house taste buds.
• The palate forms the roof of the oral cavity
Hard Palate - bony roof of the mouth
• Formed by the palatine bones and portions of the maxilla
Soft Palate - Muscular arch
Uvula – cone-shaped projection
During swallowing, muscles draw the soft palate and the uvula upward
preventing food from entering the nasal cavity.
Figure 17.7 Sagittal section of the
mouth, nasal cavity, and pharynx.
Teeth are the hardest structures in the body
•primary (deciduous) teeth numbering 20
•Usually erupt through the gums from age of 6 months to 2 years
•secondary (permanent) teeth numbering 32
•Usually begin to erupt at 6 years
• 3rd molars = wisdom teeth, which may erupt between 17-25 years of age
Figure 17.8 This partially dissected child’s skull
reveals primary and developing secondary teeth in
the maxilla and the mandible.
TYPES OF TEETH
Incisors- blade shaped teeth that bite or cut off large pieces of food
Adult – 8 incisors
Child – 8 incisors
Canines- cone shaped teeth that grasp and tear food
Adult – 4 canines
Child – 4 canines
Premolars – flattened surface for grinding food
Adult – 8 premolars
Child – 0 premolars
Molars – flattened surface for grinding food
Adult – 12 molars
Child – 8 molars
Enamel- Calcium salts; hardest structure in the body.
Dentin- Living cellular tissue similar to bone.
Pulp cavity- Filled with loose connective tissue, blood vessels, and nerves
Crown- portion of tooth above the gums (gingiva)
Root- portion of tooth below the gums (gingiva)
Root canal – entrance into the pulp cavity
Tooth attachments to the jaw
Cementum – bone-like layer surrounding the root
Periodontal ligaments – attaches the tooth to the jaw
Microbes on the surface of teeth metabolize carbohydrates from foods left in the mouth.
• Their acidic wastes dissolves and the destroys the enamel and dentin.
• Dental caries form when the damage spreads to the underlying dentin.
Figure 17A Actinomyces bacteria (falsely
colored) clinging to teeth release acids that
decay tooth enamel (1,250X)
• Salivary glands secrete saliva
•Saliva moistens the food, and begins the digestion of carbohydrates
Three pairs of major salivary glands, include:
Parotid glands – located anterior to the ear
• A parotid duct enters the mouth opposite the second upper molars
Submandibular glands – located on the floor of the mouth inside the jaw.
Sublingual glands – located on the floor of the mouth, inferior to the tongue.
•Many minor glands are also scattered throughout
the mucosa of the tongue, palate, and cheeks
Figure 17.11 Locations of the major salivary glands.
There are two types of secretory cells within the salivary glands:
• Serous cells produce a watery fluid with digestive enzymes (salivary amylase)
• Mucous cells secrete mucous
• Parotid glands
• Secrete clear watery, serous fluid
• Rich in salivary amylase – begins the chemical digestion of carbohydrates
• Secretes a mixed saliva with both serous fluid and mucus
• Secrete primarily mucus