Caudal Vertebræ.—The caudal vertebræ are those placed behind the sacrum, and terminating the vertebral column. They vary in number greatly—being reduced to 5, 4, or even 3, in a most rudimentary condition, in Man and in some Apes and Bats, and being numerous and powerfully developed, with strong and complex processes, in many mammals, especially among the Edentata, Cetacea, and Marsupialia. The highest known number, 46, is possessed by the African Long-tailed Pangolin. Connected with the under surface of the caudal vertebræ of many mammals which have the tail well developed are certain bones formed more or less like an inverted arch, called chevron bones, or by the French os en V. These are always situated nearly opposite to an intervertebral space, and are generally articulated both to the vertebra in front and the vertebra behind, but sometimes chiefly or entirely either to one or the other.

In some of the Anomodont Reptiles and Labyrinthodont Amphibians these chevrons are attached to the intercentra—or imperfect disks alternating with the true centra—which suggests that they are primarily intercentral elements which have been transferred to the edges of the centra by the disappearance of the intercentra.

Sternum.—The sternum of mammals is a bone, or generally a series of bones, placed longitudinally in the mesial line, on the inferior or ventral aspect of the thorax, and connected on each side with the vertebral column by a series of more or less ossified bars called “ribs.” It is present in all mammals, but varies much in character in the different groups. It usually consists of a series of distinct segments placed one before the other, the anterior being called the presternum or “manubrium sterni” of human anatomy, and the posterior the xiphisternum, or xiphoid or ensiform process, while the intermediate segments, whatever their number, constitute the mesosternum or “body.” In the Whalebone Whales the presternum alone is developed, and but a single pair of ribs is attached to it.

Ribs.—The ribs form a series of long, narrow, and more or less flattened bones, extending laterally from the sides of the vertebral column, curving downwards towards the median line of the body below, and mostly joining the sides of the sternum. The posterior ribs, however, do not directly articulate with that bone, but are either attached by their extremities to the edges of each rib in front of them, and thus only indirectly join the sternum, or else they are quite free below, meeting no part of the skeleton. These differences have given rise to the division into “true” and “false” ribs (by no means good expressions), signifying those that join the sternum directly and those that do not; and of the latter, those that are free below, are called “floating” ribs. The portion of each rib nearest the vertebral column and that nearest the sternum differ in their characters, the latter being usually but imperfectly ossified, or remaining permanently cartilaginous. These are called “costal cartilages,” or when ossified “sternal ribs.”

In the anterior part of the thorax the vertebral extremity of each rib is divided into two parts, “head” or “capitulum,” and “tubercle”; the former is attached to the side of the body of the vertebra, the latter to its transverse process; the former attachment corresponds to the interspace between the vertebræ, the head of the rib commonly articulating partly with the hinder edge of the body of the vertebra antecedent to that which bears its tubercle. Hence the body of the last cervical vertebra usually supports part of the head of the first rib. In the posterior part of the series the capitular and tubercular attachments commonly coalesce, and the rib is attached solely to its corresponding vertebra. The number of pairs of ribs is of course the same as that of the thoracic vertebræ.

The circumstance that in some of the Anomodont reptiles and Labyrinthodonts the capitula of the ribs articulate with the intercentral elements of the vertebral column has suggested, as in the instance of the chevron bones, that the intercentral capitular articulation of the ribs of mammals is a feature directly inherited from those extinct types by the gradual disappearance of the intercentra.

Appendicular Skeleton.—The appendicular portion of the framework consists, when completely developed, of two pairs of limbs, anterior and posterior (Fig. 15).

Anterior Limb.—The anterior limb is present and fully developed in all mammals, being composed of a shoulder girdle and three segments belonging to the limb proper; viz. the upper arm or brachium, the forearm or antebrachium, and the hand or manus.

Shoulder-girdle.—The shoulder or pectoral girdle in the large majority of mammals is in a rudimentary or rather modified condition, compared with that in which it exists in other vertebrates. In the Monotremata (Ornithorhynchus and Echidna) alone is the ventral portion, or coracoid, complete and articulated with the sternum below, as in the Sauropsida; and in this group alone do we find an anterior ventral element, apparently corresponding with the pre-coracoid of the Anomodont reptiles, although generally known as the epi-coracoid. In all other mammals the coracoid, though ossified from a distinct centre, forms only a process, sometimes a scarcely distinct tubercle, projecting from the anterior border of the glenoid cavity of the scapula. The last-named cavity, which in the Monotremes is formed jointly by the scapula and coracoid, receives the head of the humerus, or arm-bone. The scapula is always well developed, and generally broad and flat (whence its vernacular name “blade bone”), with a ridge called the “spine” on its outer surface, which usually ends in a free curved process, the “acromion.” As the scapula affords attachment to many of the muscles which act upon the anterior limb, its form and the development of its processes are greatly modified according to the uses to which the member is put. Thus it is most reduced and simple in character in those animals whose limbs are mere organs of support, as the Ungulates; and most complex when the limbs are also used for grasping, climbing, or digging. The development or absence of the clavicle or “collar-bone,” an accessory bar which connects the sternum with the scapula and steadies the shoulder-joint, has a somewhat similar relation, though its complete absence in the Bears shows that this is not an invariable rule. A complete clavicle is found in Man and all the Primates, in Chiroptera, all Insectivora (except Potamogale), in many Rodents, in most Edentates, and in all Marsupials, except Perameles. More or less rudimentary clavicles (generally suspended freely in the muscles) are found in the Cat, Dog, and most Carnivora, Myrmecophaga, and some Rodents. Clavicles are altogether absent in most of the Ursidæ, all the Pinnipedia, Manis among Edentates, the Cetacea, Sirenia, Ungulates, and some Rodents.

The Monotremes are peculiar in possessing a T-shaped interclavicle like that of many reptiles, lying upon the sternum, and articulating superiorly with the clavicles.

Brachium and Antebrachium.—The proximal segment of the anterior or pectoral limb proper contains a single bone, the humerus, and the second segment two bones, the radius and the ulna, placed side by side, and articulating with the humerus at their proximal, and with the carpus at their distal extremity (Fig. 15). In their primitive and unmodified condition these bones may be considered as placed one on each border of the limb, the radius being preaxial or anterior, and the ulna postaxial or posterior, when the distal or free end of the limb is directed outwards, or away from the trunk. This is their position in the earliest embryonic condition, and is best illustrated among adult mammals in the Cetacea, where the two bones are fixed side by side and parallel to each other. In the greater number of mammals the bones assume a very modified and adaptive position, usually crossing each other in the forearm, the radius in front of the ulna, so that the preaxial bone (radius), though external (in the ordinary position of the limb) at the upper end, is internal at the lower end; and the hand, being mainly fixed to the radius, also has its preaxial border internal. In the large majority of mammals the bones are fixed in this position, but in some few, as in Man, a free movement of crossing and uncrossing—or pronation and supination, as it is termed—is allowed between them, so that they can be placed in their primitive parallel condition, when the hand (which moves with the radius) is said to be supine, or they may be crossed, when the hand is said to be prone.

The humerus frequently has a foramen piercing the inner border of the distal extremity, known as the entepicondylar foramen, which corresponds with a similar one found in the Anomodont reptiles. The hollow in the head of the ulna for the reception of the head of the humerus is known as the greater sigmoid cavity, and that for the head of the radius as the lesser sigmoid cavity (Fig. 16). The term olecranon is applied to that process of the ulna which forms the prominence of the elbow.

In most mammals walking on four limbs, in which the hand is permanently prone, the ulna is much reduced in size, and the radius increased, especially at the upper end; so that the articular surface of the latter, instead of being confined to the external side of the trochlea of the humerus, extends all across its anterior surface, and the two bones, instead of being external and internal, are anterior and posterior. In many hoofed or Ungulate mammals, and in Bats, the ulna is reduced to little more than its upper articular extremity, and firmly ankylosed to the radius—stability of these parts being more essential than mobility.

Manus.—The terminal segment of the anterior limb is the hand or manus. Its skeleton consists of three divisions: (1) the “carpus,” a group of small, more or less rounded or angular bones with flattened surfaces applied to one another, and, though articulating by synovial joints, having scarcely any motion between them; (2) the “metacarpus,” a series of elongated bones placed side by side, with their proximal ends articulating by almost immovable joints with the carpus; (3) the “phalanges” or bones of the digits, usually three in number to each, articulating to one another by freely movable hinge-joints, the first being connected in like manner to the distal end of the metacarpal bone to which it corresponds.

Carpus.—To understand thoroughly the arrangement of the bones of the carpus in mammals, it is necessary to study their condition in some of the lower vertebrates. Fig. 17 represents the manus in one of its fullest and at the same time most generalised forms, as seen in one of the Water Tortoises (Chelydra serpentina). The carpus consists of two principal rows of bones. The upper or proximal row contains three bones, to which Gegenbaur has applied the terms radiale (r), intermedium (i), and ulnare (u), the first being on the radial or preaxial side of the limb.[11] The lower or distal row contains five bones, called carpale 1, 2, 3, 4, and 5 respectively, commencing on the radial side. Between these two rows, in the middle of the carpus, is a single bone, the centrale (c). In this very symmetrical carpus it will be observed that the radiale supports on its distal side two bones, carpale 1 and 2; the intermedium is in a line with the centrale and carpale 3, which together form a median axis of the hand, while the ulnare has also two bones articulating with its distal end, viz. carpale 4 and 5. Each of the carpals of the distal row supports a metacarpal.

In the carpus of the Mammalia there are usually two additional bones developed in the tendons of the flexor muscles, one on each side of the carpus, which may be called the radial and ulnar sesamoid bones; the latter, which is the more constant and generally larger, is commonly known as the pisiform bone. The fourth and fifth carpals of the distal row are always united into a single bone, and the centrale is very often absent. As a general rule all the other bones are present and distinct, though it not unfrequently happens that two may have coalesced to form a single bone, or one or more may be altogether suppressed.

The following table shows the principal names in use for the various carpal bones,—those in the second column being the terms generally employed by English anatomists:—

The radial and ulnar sesamoids are regarded by Bardeleben[12] as the rudiments of a prepollex and a postminimus digit; the primitive number of digits being thus supposed to have been seven. These bones have been observed in all orders of mammals having five complete digits. Occasionally, as in Pedetes caffer, the so-called prepollex consists of two bones, of which the distal one bears a distinct nail-like horny covering. In Bathyergus maritimus the pisiform, or postminimus, is likewise double; the two elements being regarded by their describer as representing the carpal and metacarpal of the presumed seventh digit.

Similarly in the posterior limb the tibial sesamoid, and a fibular ossification corresponding to the pisiform, are regarded as representing a prehallux and a postminimus.

Metacarpus and Phalanges.—The metacarpal bones, with the digits which they support, are never more than five in number, and are described numerically—first, second, etc., counting from the radial towards the ulnar side. The digits are also sometimes named (1) the pollex, (2) index, (3) medius, (4) annularis, (5) minimus. One or more may be in a rudimentary condition, or altogether suppressed. If one is absent, it is most commonly the first. Excepting the Cetacea, no mammals have more than three phalanges to each digit, but they may occasionally have fewer by suppression or ankylosis. The first or radial digit is an exception to the usual rule, one of its parts being constantly absent, since, while each of the other digits has commonly a metacarpal and three phalanges, it has only three bones altogether; whether the missing one is a metacarpal or one of the phalanges is a subject which has occasioned much discussion, and has not yet been satisfactorily decided. The terminal phalanges of the digits are usually specially modified to support the nail, claw, or hoof, and are called “ungual phalanges.” In walking, some mammals (as the Bears) apply the whole of the lower surface of the carpus, metacarpus, and phalanges to the ground; to these the term “plantigrade” is applied. Many others (as nearly all the existing Ungulata) only rest on the last one or two phalanges of the toes, the first phalanx and the metacarpals being vertical and in a line with the forearm. These are called “digitigrade.” Intermediate conditions exist between these two forms, to which the terms “phalangigrade” (as the Camel) and “subplantigrade” (as in most Carnivora), are applied. When the weight is borne entirely on the distal surface of the ungual phalanx, and the horny structures growing around it, as in the Horse, the mode of progression is called “unguligrade.”

In the Chiroptera the digits are enormously elongated, and support a cutaneous expansion constituting the organ of flight. In the Cetacea the manus is formed into a paddle, being covered by continuous integument, which conceals all trace of division into separate digits, and shows no sign of nails or claws. In the Sloths the manus is long and very narrow, habitually curved, and terminating in two or three pointed curved claws in close apposition with each other, and incapable, in fact, of being divaricated; so that it is reduced to the condition of a hook, by which the animal suspends itself to the boughs of the trees among which it lives. These are only examples of the endless modifications to which the distal extremity of the limb is subjected in adaptation to the various purposes to which it is applied.

Posterior Limb.—The posterior limb is constructed upon a plan very similar to that of the anterior extremity. It consists of a pelvic girdle and three segments belonging to the limb proper, viz. the thigh, the leg, and the foot or pes (Fig. 15).

Pelvic Girdle.—The pelvic girdle is present in some form in all mammals, though in the Cetacea and the Sirenia it is in an exceedingly rudimentary condition. In all mammals except those belonging to the two orders just named, each lateral half of the pelvic girdle consists essentially, like the corresponding part of the anterior limb, of a flattened rod of bone crossing the long axis of the trunk, having an upper or dorsal and a lower or ventral end. The upper end diverges from that of the opposite side, but the lower end approaches, and, in most cases, meets it, forming a symphysis, without the intervention of any bone corresponding to the sternum. The pelvic girdle differs from the shoulder girdle in being firmly articulated to the vertebral column, thus giving greater power to the hinder limb in its function of supporting and propelling the body. Like the shoulder girdle, it bears on its outer side, near the middle, a cup-shaped articular cavity (“acetabulum”), into which the proximal end of the first bone of the limb proper is received. Each lateral half of the girdle is called the “os innominatum,” or innominate bone, and consists originally of three bones which unite at the acetabulum. The “ilium” or upper bone is that which articulates with the sacral vertebræ. Of the two lower bones the anterior or “pubis” unites with its fellow of the other side at the symphysis; the posterior is the “ischium.” These lower elements form two bars of bone, united above and below, but leaving a space between them in the middle, filled only by membrane, and called the “thyroid” or “obturator” foramen. The whole circle of bone formed by the two innominate bones and the sacrum is called the pelvis. In the Monotremata and Marsupialia, a pair of thin, flat, elongated ossifications called epipubic or marsupial bones are attached to the fore part of the pubis, and project forward into the muscular wall of the abdomen.

Thigh and Leg.—The first segment of the limb proper has one bone, the femur, corresponding with the humerus of the anterior limb. The second segment has two bones, the tibia and fibula, corresponding with the radius and ulna. These bones always lie in their primitive unmodified position, parallel to each other, the tibia on the preaxial and the fibula on the postaxial side, and are never either permanently crossed or capable of any considerable amount of rotation, as in the corresponding bones of the fore limb. In the ordinary walking position the tibia is internal, and the fibula external. In many mammals the fibula is in a more or less rudimentary condition, and it often ankyloses with the tibia at one or both extremities. The patella or “knee-cap,” which is found in an ossified condition in all mammals, with the exception of some of the Marsupialia, is a large sesamoid bone developed in the tendon of the extensor muscles of the thigh, where the tendon passes over the front of the knee-joint, to which it serves as a protection. There are frequently smaller ossicles, one or two in number, situated behind the femoral condyles, called “fabellæ.” The processes for the attachment of muscles near the upper end of the femur are termed trochanters; and the third trochanter, found on the hinder aspect of the shaft of this bone in many forms is of considerable taxonomic importance.

Pes.—The terminal segment of the hind limb is the foot or pes. Its skeleton presents in many particulars a close resemblance to that of the manus, being divisible into three parts: (1) a group of short, more or less rounded or square bones, constituting the tarsus; (2) a series of long bones placed side by side, forming the metatarsus; and (3) the phalanges of the digits or toes.

The bones of the tarsus of many of the lower Vertebrata closely resemble both in number and arrangement those of the carpus, as shown in Fig. 17. They have been described in their most generalised condition by Gegenbaur under the names expressed in the first column of the following table. The names in the second column are those by which they are generally known to English anatomists, while in the third column some synonyms occasionally employed are added.

The bones of the tarsus of mammals present fewer diversities of number and arrangement than those of the carpus. The proximal row (see Fig. 18) always consists of two bones, namely the astragalus (a), which probably represents the coalesced scaphoid and lunar of the hand, and the calcaneum (c). The former is placed more to the dorsal side of the foot than the latter, and almost exclusively furnishes the tarsal part of the tibio-tarsal or ankle-joint. The calcaneum, placed more to the ventral or “plantar” side of the foot, is elongated backwards to form a more or less prominent tuberosity, the “tuber calcis,” to which the tendon of the great extensor muscles of the foot is attached. The navicular bone (n) is interposed between the proximal and distal row on the inner or tibial side of the foot, but on the outer side the bones of the two rows come into contact. The distal row, when complete, consists of four bones, which, beginning on the inner side, are the three cuneiform bones, internal (c¹), middle (c²), and external (c³), articulated to the distal surface of the navicular, and the cuboid (cb), articulated with the calcaneum. Of these the middle cuneiform is usually the smallest in animals in which all five digits are developed; but when the hallux is wanting the internal cuneiform may be rudimentary or altogether absent. The three cuneiform bones support respectively the first, second, and third metatarsals, and the cuboid supports the fourth and fifth; they thus exactly correspond with the four bones of the distal row of the carpus.

In addition to these constant tarsal bones, there may be supplemental or sesamoid bones: one situated near the middle of the tibial side of the tarsus, largely developed in many Carnivora and Rodentia; another, less frequent, on the fibular side; and a third, often developed in the tendons of the plantar surface of the tarsus, is especially large in Armadillos. There is also usually a pair of sesamoid bones on the plantar aspect of each metatarso-phalangeal articulation. In the young of the carnivorous genus Crytoprocta there may be a second centrale, which usually coalesces with the ectocuneiform.

The metatarsal bones never exceed five in number, and the phalanges follow the same numerical rule as in the manus, never exceeding three in each digit. Moreover, the first digit, counting from the tibial side, or hallux, resembles the pollex of the hand in always having one segment less than the other digits. As the function of the hind foot is more restricted than that of the hand the modifications of its structure are less striking. In the Cetacea and the Sirenia it is entirely wanting, though in some existing members of the first-named order rudiments of the bones of both the first and second segments of the limb have been detected, and a femur is present in the Miocene Sirenian Halitherium.

IV. THE DIGESTIVE SYSTEM.

General Considerations.—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 generalisations. 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. Thus, 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. That is to say, 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 in its relation to a particular kind of diet, that form would be found in all the animals consuming such diet. But this is far from being the case. Thus the Horse and the Ox, for instance—two animals whose food in the natural state is precisely similar—are 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, 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 versâ? We can only answer that the Seal and Porpoise belong to different natural groups of animals, formed either 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 would appear necessary in the organs of digestion, there is really endless variety in the parts devoted to this purpose.

Mouth.—The digestive apparatus of mammals, as in other vertebrates, consists mainly of a tube with an aperture placed at or near either extremity of the body,—the oral and the anal orifice,—and furnished with muscular walls, the fibres of which are so arranged as by their regular alternate contraction and relaxation to drive onwards the contents of the tube from the first to the second of these apertures. The anterior or commencing portion of this tube and the parts 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 the tube is dilated into a chamber or cavity called the mouth, bordered externally by the lips, which are usually muscular and prehensile, and supported by a movable framework carrying the teeth; 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 the 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 separating 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 papillæ, in which the terminal filaments of the gustatory nerve are distributed.

Salivary Glands.—The fluid known as the saliva is secreted by an extensive and complex system of glands discharging into the cavity of the mouth (buccal cavity), the position and relation of some of which are exhibited in the woodcut on the next page (Fig. 19).

This apparatus consists of small glands embedded in the mucous membrane or submucous tissue lining the cavity of the mouth, 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 nearly identical; and, since the fluids secreted by all become mixed in the month, their functions are, at all events in great part, common. Under the name of salivary glands are commonly included—(1) the “parotid” (p), 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 Stenson’s) which crosses the masseter muscle and opens into the upper and back part of the cheek (Fig. 19); and (2) the “submaxillary” (sm), situated in the neck, near or below the angle of the mandible, and sending a long duct (Wharton’s) forwards to open on the forepart 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 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 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 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, and most fully carried out in the Ruminants, which “chew the cud,”—it is swallowed, and carried along the œsophagus by the action of its muscular coats into the stomach. In the greater number of mammals this organ is a simple saccular dilatation of the alimentary canal, as in Figs. 20, 21, 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, recognisable 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, while the others are 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, but 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 Lophiomys. 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, effected partly by the blood-vessels of the stomach, although 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 position of the ducts of the two latter organs being indicated in Fig. 20.

Intestinal Canal.—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 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 caput cæcum coli, as it was first called, a name generally abbreviated into “cæcum,” 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 whole of the remainder of the alimentary canal. It is only in herbivorous animals that the cæcum is developed to this great extent, and among these there is a curious complementary relationship between the size and complexity of this organ and that of the stomach. Where the latter is simple the cæcum is generally the largest, and vice versâ. Both the cæcum 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 “valvulæ conniventes” of Man form well-known examples.