COMPOSITION OF SALIVA. 137 



The mouth glands are divided into two sets, which produce 

 different kinds of secretion : (1) Mucous glands, which secrete 

 mucus, and (2) Salivary glands, which produce watery saliva. 

 The functional distinction is seldom absolute, for most salivary 

 glands have a mixed secretion, and -various gradations of the 

 transition from purely salivary to purely mucous glands can be 

 made out. 



The proper mucous glands are small, varying in size from a 

 pin's head to a pea. They are found in groups under the mucous 

 membrane in various parts of the mouth, and from their positions 

 are called labial, buccal, etc. Their cells contain a clear muci- 

 laginous substance. 



The great salivary glands are the three large glands which are 

 known as the parotid, sub-maxillary and sublingual. On account 

 of their great size they form striking anatomical objects, being 

 large masses of irregularly arranged glandular packets, which 

 might be spoken of as lobes to distinguish them from the smaller 

 packets or lobules. Their ducts are of considerable size, and 

 have strong walls made of dense fibrous tissue, containing many 

 elastic fibres, and in one of them, the sub-maxillary, smooth mus- 

 cle tissue has been demonstrated. 



The parotid duct (Steno's) opens into the mouth about the 

 middle of the cheek just opposite the second molar tooth. The 

 sub-maxillary has also a single duct (Wharton's), which opens 

 beneath the tongue beside the fnenum. The sublingual gland 

 has several ducts, some of which open into that of the sub-maxil- 

 lary, and others unite to enter the mouth beside Wharton's duct. 



In different animals and in different glands of the same animal 

 a variable amount of mucus is secreted by these glands, which, 

 however, are all called salivary, though the parotid alone de- 

 serves the name. 



THE CHARACTERS OF MIXED SALIVA. 



The liquid in the mouth is a mixture of the secretion of the 

 salivary glands as well as of the small, purely mucous glands. 



It is a slightly turbid, tasteless fluid of a distinctly alkaline 

 reaction, of 1004-1008 specific gravity, and so tenacious that it 

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