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SALIVARY GLAND AND ITS SECRETION
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Salivon is the functional salivary unit.
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It begins most proximally as several Acini which converge into a intercalated duct.
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Numerous intercalated ducts unite to form a striated duct. A group of these ducts directly drain their content into glandular tubule which pour their content into fewer excretory ducts.
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Finally, the excretory ducts converge to form a single excretory duct, which leads to oral cavity.
Salivary gland and secretion
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Saliva is the mixed secretion of three pair of main salivary glands, namely parotid, sub maxillary or sub mandibular and sublingual and also many small glands found in the mucous membrane of the mouth.
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Glands in general are divided into serous, mucous and mixed types.
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Serous glands give rise to thin, watery secretion containing protein without mucin, mucous glands produce a secretion containing the glycoprotein in mucin ; mixed glands produce both serous and mucous secretions.
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Cells that gives rise to a serous secretion also secrete enzymes. Zymogens granules are the precursors of enzyme stored in the serous cells.
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Mucous gland is collected in a system of ducts which discharge their content through larger ducts into the mouth.
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The absorptive and secretary activities of the cells lining the ducts affect the composition of the saliva.
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In most of the mammals, parotid glands are serous. In some animals the secretion of this glands is devoid of enzymes .
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The sub maxillary glands is mixed in ungulates, dogs and cats but it is serous in rodents.
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The sublingual gland of the horse, ox, pig, dog and cat is a mixed gland and that of rodents is mucous.
Salivary glands of ruminants
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Parotid, submaxillary and sublingual are the major salivary glands of ruminants.
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In addition, sheep and cattle have two inferior molar glands, small and numerous buccal and labial glands in cheek and lips, palatine glands in hard and soft palate and pharyngeal glands in the pharynx and roof of the tongue.
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Inferior molar is a serous gland
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Buccal, Pharyngeal and Palatine are mucous glands
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Labial is a mixed gland
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Pattern of secretion
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The parotid glands secrete spontaneously and continuously. Its secretion is rapid during feeding and rumination on the side of bolus chewing. Its flow is about 2ml/min. at rest and 30 to 50ml/min. during rumination. Mechanical stimulation of mouth, cardia, reticulo-omasal orifice, lips of oesophageal groove and walls of reticulum reflexly stimulate parotid secretion.
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Secretion of inferior molar occur only during feeding, whereas buccal and palatine glands shows slow resting rate of secretion. Flow of saliva in cow is 60 to 160litres/day; in sheep 6.0 to 16litres/day.
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In dogs, submaxillary and sublingual glands show free flow of saliva during chewing of normal meat (no secretion by parotid), whereas dry meat powder excites abundant secretion from the parotid.
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In horse, parotid secretion occurs only during feed intake, whereas in ruminants, parotid secretion is continuous.
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Sheep show slight increase in parotid secretion as a result of feeding.
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Numerous small glands like inferior molar, buccal, labial, palntine and pharyngeal do contribute to the quantity of ruminat saliva. Volume of saliva produced by adult cattle is 90-190 litre per day.
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Salivary secretion is continuous.
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Flow of saliva varies with activity and increase with feeding and rumination to aid deglutition.
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Type of ration influences rate of salivation.
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Saliva is predominantly of two types i.e. serous and mucus.
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Serous saliva is rich in carbonate ions and is secreted continuously.
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Mucus saliva helps to reduce foaming by increasing surface tension and is secreted when animal is feeding and is rich in mucopolysaccharides.
Methods of studying salivary secretion
- Mixed saliva may be obtained directly from the mouth.
- By means of cannulation of the appropriate salivary ducts.
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