Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/384

Rh 362 MAMMALIA [DIGESTIVE ated in the neck, near or below the angle of the mandible, and sending a long duct (Wharton s) forwards to open in 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 &quot; sublingual,&quot; closely associated with the last-named, at all events in the locality in which the secretion is poured out; and (4) the &quot;zygo- matic,&quot; 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 introduction 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 experiment 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 sub- maxillary glands ; while, on the other hand, the parotids are of great size in those animals which habitually masticate dry and fibrous food. After the preparation which the aliment has undergone 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, &quot;bolt&quot; their food entire, it is swallowed, and is carried along the oesophagus by the Stomach, action of its muscular coats into the stomach. In the greater proportion of mammals this&quot; organ is a simple saccular dilatation of the alimentary canal, but in others it undergoes remarkable modifications and complexities. The lining of the stomach is thickly beset with tubular glands, which are generally considered to belong to two different forms, recognizable by their structure, and different in their function the most numerous and important secreting the gastric juice (the active agent in stomachic digestion), and hence called &quot; peptic &quot; glands, the others concerned only in the elaboration of mucus. The relative distribution of these glands in different regions of the walls of the stomach varies greatly in different animals, and in many species there are large tracts of the mucous membrane which do not secrete a fluid having the properties of gastric juice, and often constitute more or less distinct cavities devoted to storing and perhaps softening or otherwise preparing the food for digestion. Sometimes there is a great aggregation of glands forming distinct thickened patches of the stomach wall, as in the Beaver and Koala, or even collected in pyriform pouches with a common narrow opening into the cavity, as in the Manatee and the curious African Rodent Lophyomys. The action of the gastric fluid is mainly exerted upon the nitrogenous elements of the food, which it dissolves and modifies so as to render them capable of undergoing absorption, which is partly effected by the blood-vessels of the stomach, though the greater part passes through the pylorus, an aperture surrounded by a circular muscular valve, into the intestinal canal. Here it comes in contact with the secretion of a vast number of small glands called the crypts of Lieberkuhn, somewhat similar to those of the stomach, and also of several special glands of a different character, namely, the small racemose, duodenal, or Brunner s glands, the pancreas, and the liver. The intestinal canal varies greatly in relative length and Intest capacity in different animals, and it also offers manifold canal, peculiarities of form, being sometimes a simple cylindrical tube of nearly uniform calibre throughout, but more often subject to alterations of form and capacity in different portions of its course, the most characteristic and constant being the division into an upper and narrower and a lower and wider portion, called respectively the small and the large intestine, the former being divided quite arbitrarily and artificially into duodenum, jejunum, and ileum, and the latter into colon and rectum. One of the most striking peculiarities of this part of the alimentary canal is the frequent presence of a diverticu- lum or blind pouch, the caput ceecum coli, as it was first called, a name generally abbreviated into &quot;caecum,&quot; situated at the junction of the large and the small intestine, a structure presenting an immense variety of development, from the smallest bulging of a portion of the side wall of the tube to a huge and complex sac, greatly exceeding in capacity the remainder of the ali mentary canal. It is only in herbi vorous animals that the caecum is developed to this great extent, and among these there is a curious com plementary relationship between the size and complexity of the organ and that of the stomach. Where the latter is simple the caecum is generally the largest, and vice versa. Both caecum and colon are often saccu- lated, a disposition caused by the arrangement of the longitudinal bands of muscular tissue in their walls ; but the small intestine is always smooth and simple-walled externally, though its lining membrane often exhibits various contrivances for increasing the absorbing surface without adding to the general bulk of the organ, such as the numerous small villi by which it is everywhere beset, and the more obvious transverse, longitudinal, or reticulating folds projecting into the interior, met with in many animals, of which the &quot; valvulaa conniventes &quot; of Man form well-known examples. Besides the crypts of Lieberkuhn found throughout the intestinal canal, and the glands of Brunner confined to the duodenum, there are other structures in the mucous membrane, about the nature of which there is still much uncertainty, called &quot; solitary &quot; and &quot; agminated &quot; glands, the latter more commonly known by the name of &quot; Peyer s patches.&quot; These were formerly supposed to be secretory organs, which discharged some kind of fluid into the intestine, but are now more generally considered to belong to the group of structures of somewhat mysterious function of which the lymphatic and lacteal of the Alimentary Canal in a typical Mammal, o, oeso phagus ; st, stomach ; p, pylorus ; ss, small intestine (abbreviated) ; c, caecum ; II, large intestine or colon, end ing in r, the rectum.