Mammalia Digestive System
glands animals called liver canal stomach mammals lobe organ food
MAMMALIA DIGESTIVE SYSTEM, the search after the purpose which every modification of structure subserves in the economy is always full of interest, and, if conducted with due caution and sufficient knowledge of all the attendant circumstances, may lead to important generalizations. It must always be borne in mind, however, that adaptation to its special function is not the only cause of the particular form or structure of an organ, but that this form, having in all probability been arrived at by the successive and gradual modification of some other different form from which it is now to a greater or less degree removed, has other factors besides use to be taken into account. In no case is this principle so well seen as in that of the organs of digestion. These may be considered as machines which have to operate upon alimentary substances in very different conditions of mechanical and chemical combination, and to reduce them in every case to the same or precisely similar materials; and we might well imagine that the apparatus required to produce flesh and blood out of coarse fibrous vegetable substances would be different from that which had to produce exactly the same results out of ready-made flesh or blood ; and in a very broad sense we find that this is so. If we take a large number of carnivorous animals, belonging to different fundamental types, and a large number of herbivorous animals, and strike a kind of average of each, we shall find that there is, pervading the first group, a general style, if we may use the expression, of the alimentary organs, different from that of the others. There is a specially carnivorous and a specially herbivorous modification of these parts. But, if function were the only element which has guided such modification, it might be inferred that, as one form must be supposed to be best adapted and most perfect in its relation to a particular kind of diet, that form would be found in all the animals consuming that diet. But this is far from being the case. The Horse and the Ox, for instance, - two animals whose food in the natural state is precisely similar, - are yet most different as regards the structure of their alimentary canal, and the processes involved in the preparation of that food. Again, the Seal and the Porpoise, both purely fish-eaters, which seize and swallow and digest precisely the same kind of prey in precisely the same manner, have a totally different arrangement of the alimentary canal. If the Seal's stomach is adapted in the best conceivable manner for the purpose it has to fulfil, why is not the Porpoise's stomach an exact facsimile of it, and vice versa 1 We can only answer, the Seal and Porpoise belong to different natural groups of animals, formed on different primitive types, or descended from differently constructed ancestors. On this principle only can we account for the fact that, whereas, owing to the comparatively small variety of the different alimentary substances met with in nature, few modifications wouIC ippear necessary in the organs of digestion, there is really mdless variety in the parts devoted to this purpose.
The digestive apparatus of mammals, as in other vertebrates, consists mainly of a tube with an aperture .,)laced at or near either extremity of the body, - the oral and the anal orifice, - with muscular walls, the fibres of which are so arranged as by their regular alternate contraction and relaxation to drive onwards the contents of I.' the tube from the first to the last of these apertures. The rn anterior or commencing portion of this tube and the parts i around it are greatly and variously modified in relation to the functions assigned to them of selecting and seizing the food, and preparing it by various mechanical and chemical processes for the true digestion which it has afterwards to undergo before it can be assimilated into the system. For this end it is dilated into a chamber or cavity called the mouth, bordered externally by the lips, usually muscular and prehensile, and supported by a movable framework which carries the teeth, - organs the structure and modifications of which have been already described. The roof of the mouth is formed by the palate, terminating behind by a muscular, contractile arch, having in Man and some few other species a median projection called uvula, beneath which the mouth communicates with the pharynx. The anterior part of the palate is composed of mucous membrane tightly stretched over the flat or slightly concave bony lamina which separates the mouth from the nasal passages, and is generally raised into a series of transverse ridges, which sometimes, as in Ruminants, attain a considerable development. In the floor of the mouth, between the rami of the mandible, and supported behind by the hyoidean apparatus, lies the tongue, an organ the free surface of which, especially in its posterior part, is devoted to the sense of taste, but which also by its great mobility, being composed almost entirely of muscular fibres, performs important mechanical functions connected with masticating and procuring food. Its modifications of form in different mammals are very numerous. Between the long, extensile, vermiform tongue of the Anteaters, which is essential to the peculiar mode of feeding of those animals, and the short, sessile, and almost functionless tongue of the Porpoise, every intermediate condition is found. Whatever the form, the upper surface is always covered with numerous fine papilla, in which the terminal filaments of the gustatory nerve are distributed.
In connexion with the buccal cavity is an extensive and complex glandular apparatus which pours its secretions into it - secretions which constitute the fluid commonly known as saliva. This apparatus consists of small glands embedded in the mucous membrane or submucous tissue lining the cavity of the mouth, and which are of two kinds (the follicular and the racemose), and of others in which the secreting structure is aggregated in distinct masses removed some distance from the cavity, other tissues besides the lining membrane being usually interposed, and pouring their secretion into the cavity by a distinct tube or duct, which traverses the mucous membrane. To the latter alone the name of " salivary glands " is ordinarily appropriated, although the distinction between them and the smaller racemose glands is only one of convenience for descriptive purposes, their structure being more or less identical ; and, as the fluids secreted by all become mixed in the mouth, their functions are, at all events in great part, common. Under the name of salivary glands are commonly included - (1) the "parotid," situated very superficially on the side of the head, below or around the cartilaginous external auditory meatus, and the secretion of which enters the mouth by a duct (often called Steno's or Stepson's) which crosses the masseter muscle and opens into the upper and back part of the cheek ; and (2) the " submaxdlary," situated 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 " sublingual," closely associated with the last-named, at all events in the locality in which the secretion is poured out ; and (4) the "zygomatic," 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, " bolt " their food entire, - it is swallowed, and is carried along the oesophagus by the k• action of its muscular coats into the stomach. In the greater proportion of mammals thi4 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 " peptic " 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 paitly 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 ] capacity in different animals, and it also offers manifold 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 diverticulum or blind pouch, the capvt =CUM C012, as it was first called, a name generally abbreviated into " cmcum," 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 alimentary canal. It is only in herbivorous animals that the cmcum is developed to this great extent, and among these there is a curious complementary relationship between the size and complexity of the organ and that of the stomach. Where the latter is simple the cmcum is generally the largest, and vice versa. Both cmcurn and colon are often sacculated, 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 "valvular conniventes " 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 " solitary " and " agminated " glands, the latter more commonly known by the name of " Peyer's patches." 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 glands are members. The solitary glands are found scattered irregularly throughout the whole intestinal tract ; the agminated, on the other hand, are always confined to the small intestine, and are most abundant in its lower part. They are subject to great variation in number and in size, and even in different individuals of the same species, and also differ in character at different periods of life, becoming atrophied in old age.
The distinct glands situated outside of the walls of the intestinal canal, but which pour their secretion into it, are the pancreas and the liver. The latter is the most important on account of its size, if not on account of the direct action of its secretion in the digestive process. This large gland, so complex in structure and function, is well developed in all mammals, and its secreting duct, the bile duct, always opens into the duodenum or that portion of the canal which immediately succeeds the stomach. It is situated in the right side of the abdomen in contact with the diaphragm and the stomach, but varies greatly in relative size, and also in form, in different groups of mammals. In most mammals a gall-bladder, consisting of a pyriform diverticulum from the gall duct, is present, but in many it is wanting, and it is difficult to find the rationale of its presence or absence in relation to use or any other circumstance in the animal economy.
The descriptions of the livers of various animals to be-met with in treatises or memoirs on comparative anatomy are very difficult to understand for want of a uniform system of nomenclature. The difficulty usually met with arises from the circumstance that this organ is divided sometimes, as in Man, Ruminants, and the Cetacea, into two main lobes, which have been always called respectively right and left, and in other cases, as in the lower Monkeys, Carnivora, Insectivora, and many other orders, into a larger number of lobes. Among the latter the primary division usually appears at first sight tripartite, the whole organ consisting of a middle, called "cystic " or "suspensory " lobe, and two lateral lobes, called respectively right and left lobes. This introduces confusion in describing livers by the same terms throughout the whole series of mammals, as the right and left lobes of the Monkey or Dog, for instance, do not correspond with parts designated by the same name in Man and the Sheep. There are, moreover, conditions in which neither the bipartite nor the tripartite system of nomenclature will answer, which we should have considerable difficulty in describing without some more general system. In order to arrive at such a system it appears desirable to consider the liver in all cases as primarily divided by the umbilical vein (see fig.14, it) into two segments, right and left. This corresponds with its development and with the condition characteristic of the organ in the inferior classes of vertebrates. The situation of this division can almost always be recognized in adult animals by the persistence of some traces of the umbilical vein in the form of the round ligament, and by the position of the suspensory ligament.
When the two main parts into which the liver is thus divided are entire, as in Man, the Ruminants, and Cetacea, they may be spoken of as the right and left lobes ; when fissured, as the right and left segments of the liver, reserv- - ing the term lobe for the subdivisions. This will involve no ambiguity, for the terms right and left lobe will no longer be used for divisions of the more complex form of liver. In the large majority of mammals each segment is further divided by a fissure, more or less deep, extending from the free towards the attached border, which are called right and left lateral fissures (fig. 14, rlf and llf). When these are more deeply cut than the umbilical fissure (ic), the organ has that tripartite or trefoil-like form just spoken of, but it is easily seen that it is really divided into four regions or lobes, those included between the lateral fissures being the right and left central (rc and lc) separated by the umbilical fissure, and those beyond the lateral fissures on each side being the right and left lateral lobes (rl and II). The essentially bipartite character of the organ and its uniformity of construction throughout the class are thus not lost sight of, even in the most complex forms. The left segment of the liver is rarely complicated to any further extent, except in some cases by minor or secondary fissures marking off small lobules, generally inconstant and irregular, and never worthy of any special designation. On the other hand, the right segment is usually more complex. The gall bladder, when present, is always attached to the under surface of the right central lobe, sometimes merely applied to it, in other cases deeply embedded in its substance. In many cases the fossa in which it is sunk is continued to the free margin of the liver as an indent, or even a tolerably deep fissure (cf). The portal fissure (p), through which the portal vein and hepatic artery enter and the gall duct emerges from the liver, crosses this lobe transversely, near the attached border of the liver. The right lateral lobe always has the great vena cava (ix) either grooving its surface or tunnelling through its substance near the inner or left end of its attached border ; and a prolongation of the lobe to the left, between the vein and the portal fissure, sometimes a mere flat track of hepatic substance, but more often a prominent tongue-shaped process, is the so-called " Spigelian lobe "(s). From the under surface of the right lateral lobe a portion is generally partially detached by a fissure, and called the " caudate lobe " (c). In Man this is almost obsolete, but in most mammals it is of considerable magnitude, and has very constant and characteristic relations. It is connected by an isthmus at the left (narrowest or attached) end to the Spigelian lobe, behind which isthmus the vena cava is always in relation to it, channelling through or grooving its surface. It generally has a pointed apex, and is deeply hollowed to receive the right kidney, to the upper and inner side of which it is applied.
Considerations derived from the comparatively small and simple condition of the liver of the Ungulates, compared with its large size and complex form in the Carnivora, have led to the perhaps too hasty generalization that the first type is related to a herbivorous and the latter to a carnivorous diet. The exceptions to such a proposition are very numerous. The fact of the great difference between the liver of the Cetacea and that of the Seals cannot be accounted for by difference of habits of life, though it perhaps may be by difference of origin, upon the supposition that the former are modifications of the primary branch of mammals from which the Ungulates, and the latter of that from which the Carnivora, are derived.1