Digestive System The digestive system is made up of a series of:-  Tubular organs  Associated glands N.B.: The main function is to break down the ingested food into small units that can be absorbed into the tissues and utilized for the maintenance of the organism.

General structure  Tunica mucosa (mucous membrane): 1-Is the layer next to the lumen 2-The mucosa is composed of an : * Epithelium *Lamina propria (consisting of a thin bed of connective tissue). *Lamina muscularis (muscularis mucosa ), composed of one or more thin layers of smooth muscle. N.B. :In some organs the lamina propria contains glands (mucosal gland

* Tunica submucosa :Is a layer of connective tissue which may contain glandular units (submucosal glands). N.B. : In those organs without a lamina muscularis, the lamina propria and submucosa blend together without a clear line of demarcation.  Tunica muscularis : there are usually two layers muscle in the tubular organs of the digestive system. 1.Inner layer : Oriented circularly 2.Outer layer : Oriented longitudinally.  tunica adventitia: tunica serosa : (serous membrane ): composed of connective tissue with a covering of mesothelium (thoracic, pericardial and abdominal cavities ).  Tunica adventitia : composed of connective tissue but do not have a Mesothelium ( located outside the cavities,cervical portion of the esophagus ).

N.B. : Two types of ganglionic nerve plexuses of the autonomic nervous system are found in the wall of the tubular organs : 1.submucosa plexus. 2.Myenteric plexus. Large accessory digestive glands : 1.Pancreas. 2.Salivary glands. 3.Liver.

Oral cavity of the digestive system  composed of : 1-lips 2- cheeck 3- tongue 4- gingiva 5- teeth 6-palate Lips:-  covered on the outside by skin.  Lined on the inside by a mucous membrane.  The skin is devoid of hair follicles.  The inner side of the lips is covered by stratified squamous epithelium. (keratinized in ruminants and the horse, but nonkeratinized in the carnivores and pig ).  Lamina propria and submucosa blend together without a clear junction.  Labial glands, usually serous or seromucous in type m are found in the propria- submucosa.  Tunica muscularis : striated muscle fibers (orbicularis oris ).

Hard palate:- -Mucosa (lamina epithelialize mucosa ): Keratinized stratified squamous epithelium. -Lamina propria : connective tissue which blend with submucosa. -submucosa : A dense network of collagen and reticular fibers and blends with the adjacent periosteum. N.B. : a dense branched tubuloacinar mucous and seromucous glands are located in the caudal part of the hard palate (except pig ).

 the oral surface is covered by a stratified squamous epithelium.  The nasal surface is covered by a ciliated pseudostratified columnar epithelium.  propria and submucosa is of connective tissue contain branched tubulo acinar mucous and seromucous glands. Tongue: Functions : prehension, mastication, swallowing. -tunica mucosa : stratified squamous with varying degrees of keratinization. N.B.: It may be non-keratinized on the ventral surface. -Dorsal surface of the tongue : Has numerous macroscopic papillae

N.B. : tongue papillae are : 1- mechanical papillae. 2-gustatory papillae Papillae of the tongue 1-filiform papillae 2-conical papillae 3-lentisular papillae 4-fungiform papillae 5-vallate papillae 6-foliate papilla

Filiform papillae: ( mechanical papillae) *slender, sharp-pointed structure *project above the surface of tongue *covered by a keratinized stratified squamous epithelium *supported by a highly vascularized connective tissue core *the connective tissue core has several secondary papillae Conical papillae *occurs on the root of the tongue and on the check. *are not highly keratinized *have both primary and secondary connective tissue papillae.

Lenticular papillae: *Flattened lentil- shaped projections *occur mainly in ruminants *covered by stratified squamous epithelium *have a core of D.I.C.T. Fungiform papillae : *scattered among the filiform papillae *have a dome-shape *covered by a non keratinized stratified squamous epithelium *contains one or more taste buds on the upper surface

Taste Buds *present I the fungiform, circumvallate and foliate papillae *Appear as oval structures *contains small taste pore which provide communication with the external environment *taste buds consist of :- 1- supporting cells 2-neuroepithelial cells Both types of cells have large microvilli (taste hair ) that project into the taste *taste buds are embedded in the stratified squamous epithelium of the fungiform, vallate and foliate papillae and also occurs in the soft palate epiglottis or other area of the mouth and throat.

Vallate (circumvallate ) papillae *surrounded by an epithelium lined cleft or moat. *covered by a stratified squamous epithelium *many taste buds are found on the papillary side of the moat * under the moat, lie groups of gustatory serous glands whose ducts open into the moat. Folliate papillae *Located on the lateral border *covered with stratified squamous epithelium *taste buds are located in the epithelium on the side of the papillae *papillae are separated by gustatory furrows into which the taste buds open *Deep to the furrow lie serous gustatory glands

Tunica muscularis of the tongue: *longitudinal, transvers and, vertical of striated muscle *seromucous glands ( lingual glands) are found a among the muscle fibers and the submucosa of the tongue special structures of the tongue Lyssa: cord- like structure enclosed in a dense collagenous sheath *lyssa of the dog is filled with adipose tissue, striated muscle, B.V. and nerves *lyssa of the horse : contains a mid dorsal fibroelastic cord with hyaline cartilage, striated muscle and fat

Photomicrograph (A) and drawing (B) of a taste bud, showing the taste cells and the taste pore. The drawing also illustrates several cell types (basal, taste, and supporting) and afferent nerve fibers that, upon stimulation, will transmit the sensory information to the central gustatory neurons. A: Hematoxylin and eosin (H&E) stain.