THE DIGESTIVE SYSTEM 57 



(Wharton's) forwards to open on the fore-part of the floor of the 

 cavity of the mouth, below the apex of the tongue. These are the 

 most largely developed and constant of the salivary glands, being 

 met with in various degrees of development in almost all animals 

 of the class. Next in constancy are (3) "the sublingual," closely 

 associated with the last-named, at all events in the locality in which 

 the secretion is poured out ; and (4) the " zygomatic " (z.gl), found 

 only in some animals in the cheek, just under cover of the anterior 

 part of the zygomatic arch, its duct entering the buccal cavity near 

 that of the parotid. 



The most obvious function common to the secretion of these 

 various glands, and to that of the smaller ones placed in the mucous 

 membrane of the lips, the cheeks, the tongue, the palate, and fauces, 

 is the mechanical one of moistening and softening the food, to 

 enable it the more readily to be tasted, masticated, and swallowed, 

 though each kind of gland may contribute in different manner 

 and different degree to perform this function. The saliva is, 

 moreover, of the greatest importance in the first stage or introduc- 

 tion to the digestive process, as it dissolves or makes a watery 

 extract of all soluble substances in the food, and so prepares them 

 to be further acted on by the more potent digestive fluids met with 

 subsequently in their progress through the alimentary canal. In 

 addition to these functions it seems now well established by experi- 

 ment that saliva serves in Man and many animals to aid directly 

 in the digestive process, particularly by its power of inducing the 

 saccharine transformation of amylaceous substances. As a general 

 rule, in mammals the parotid saliva is more watery in its 

 composition, while that of the submaxillaries, and still more the 

 sublingual, contains more solid elements and is more viscid ; so 

 much so that some anatomists consider the latter, together with the 

 small racemose glands of the cheeks, lips, and tongue, as mucous 

 glands, retaining the name of salivary only for the parotid. These 

 peculiar properties are sometimes illustrated in a remarkable 

 degree, as, for example, the great secretion of excessively viscid 

 saliva which lubricates the tongue of the Anteaters and Armadillos, 

 associated with enormously developed submaxillary glands ; while, 

 on the other hand, the parotids are of great size in those animals 

 which habitually masticate dry and fibrous food. 



Stomach. After the preparation which the aliment has under- 

 gone in the mouth, the extent of which varies immensely in 

 different forms, being reduced almost to nothing in such animals as 

 the Seals and Cetaceans, which, to use the familiar expression, 

 " bolt " their food entire, and most fully carried out in the Rumin- 

 ants, which " chew the cud," it is swallowed, and carried along 

 the O3sophagus by the action of its muscular coats into the stomach. 

 In the greater number of mammals this organ is a simple saccular 



