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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
 
 
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The Ruminantia

 
     
 

In the commonly-recognized members of this division of the Artiodactyla there is never more than one pair of incisors, and that the outermost, in the upper jaw of the adult. Canines may or may not exist in the upper jaw; they are always present in the lower jaw, and are generally inclined forward and closely approximated to the incisors, which they usually resemble in form. It consequently happens that they are often reckoned as incisors, and Rumiaants are said to possess eight cutting teeth in the lower jaw.

With one exception (Hyoemoschus), the metacarpal and metatarsal bones of the third and fourth digits early become anchylosed together into a single, so-called cannon-bone. There is a peculiar bone called malleolar, which takes the place of the distal end of the fibula, articulating below with the calcaneum and above with the astragalus.

The great majority of the Ruminantia possess horns, the bony supports, or cores, of which are developed on each side of the middle line; and, except in the Giraffe, are outgrowths of the frontal bones.

The stomach has, at fewest, three divisions; and, in the majority of the Ruminantia, it has four compartments.

If the stomach of a typical Ruminant, such as a Sheep or an Ox, be examined, it will be found to be divisible into two principal moieties, the one cardiac and the other pyloric, while each of these is again subdivided into two others. Thus the extreme cardiac end of the cardiac moiety is dilated into an enormous sac of irregular form, the mucous membrane of which is raised up into a vast number of close-set papillae. This chamber is the Rumen, or Paunch. It communicates, by a wide aperture, with a much smaller chamber, which constitutes the second subdivision of the cardiac moiety. This is called the Reticulum, or Honeycomb stomach, from the fact that its mucous membrane is raised up into a great number of folds, which cross one another at right angles, and, in this way, enclose a multitude of hexagonal-sided cells. The reticulum communicates by a narrow aperture with the first subdivision of the pyloric moiety, which is somewhat more elongated in form. The mucous membrane of this subdivision ia produced into a vast number of longitudinal folds of various heights, but the majority of them are sufficiently large to extend almost completely across the cavity of the chamber; they thus reduce that cavity to a series of narrow radiating clefts interposed between the lamellae. When this portion of the stomach is slit open, longitudinally, the lamellae fall apart like the leaves of a book, whence it has received the fanciful name of the Psalterium from anatomists, while butchers give it that of Manyplies.
A, the stomach of a Sheep. B, that of a Musk-deer (Tragulus). oe., aesophagus; Rn., rumen; Ret., reticulum; Ps., psalterium; A., Ab, abomasum; Du,. duodenum; Py, pylorus
Fig. 102. - A, the stomach of a Sheep. B, that of a Musk-deer (Tragulus). oe., aesophagus; Rn., rumen; Ret., reticulum; Ps., psalterium; A., Ab, abomasum; Du,. duodenum; Py, pylorus.
The fourth segment of the stomach, or second subdivision of the pyloric moiety, is termed the Abomasum, or Rennet stomach. This portion is comparatively slender and elonguted, and its mucous membrane has a totally different character from that of the other three segments, being soft, highly vascular, and glandular, and raised into only a few longitudinal ridges.

It will be observed that the psalterium is so constructed as to play the part of a very efficient strainer between the reticulum and the abomasum; nothing but very finely-divided, or semi-fluid matter, being capable of traversing the interstices of its lamellae.

The gastric aperture of the oesophagus is situated at the junction of the paunch and the reticulum; the margins of its opening are raised into muscular folds, and are produced, parallel with one another, along the roof of the reticulum to the opening which leads into the psalterium. When the lips of this groove are approximated together, a canal is formed, which conducts directly from the oesophagus to the psalterium.

A Ruminant, when feeding, crops the grass rapidly and greedily, seizing it with its tongue and biting off the bundle of blades thus collected, by pressing the lower incisors against the callous pad formed by the gum which covers the premaxillae. The bunches of grass are then hastily swallowed, accompanied by abundant saliva. After grazing until its appetite is satisfied, the Ruminant lies down, usually inclining the body to one side, and remains quiescent for a certain space of time. A sudden movement of the flanks is then observed, very similar to that which might be produced by a hiccough; and careful watching of the long neck will show that something is, at the same time, quickly forced up the gullet into the cavity of the mouth. This is a bolus of grass, which has been sodden in the fluids contained in the stomach, and is returned, saturated with them, to be masticated. In an ordinary Ruminant this operation of mastication is always performed in the same way. The lower jaw makes a first stroke, say in the direction from left to right, while the second stroke, and all those which follow it until the bolus is sufficiently masticated, take place from right to left, or in the opposite direction to that of the first. While the mastication is going on, fresh quantities of saliva are poured into the mouth, and, when the grass is thoroughly ground up, the semifluid product is passed back into the pharynx and swallowed once more. These actions are repeated until the greater portion of the grass which has been cropped is pulpified.

The precise nature of the operation, the external features of which have now been described, has been the subject of much investigation and discussion. The following points appear to have been clearly established:

1. Rumination is altogether prevented by paralysis of the abdominal muscles, and it is a good deal impeded by any interference with the free action of the diaphragm.

2. Neither the paunch, nor the reticulum, ever becomes completely emptied by the process of regurgitation. The paunch is found half full of sodden fodder, even in animals which have perished by starvation.

3. When solid substances are swallowed, they pass indifferently into the rumen, or reticulum, and are constantly driven backward and forward, from the one into the other, by peristaltic actions of the walls of the stomach.

4. Fluids may pass either into the paunch and the reticulum; or into the psalterium, and thence at once into the fourth stomach, according to circumstances.

5. Rumination is perfectly well effected after the lips of the oesophageal groove have been closely united by wire sutures.

It would appear, therefore, that the cropped grass passes into the reticulum and rumen, and is macerated in them. But there is no reason to believe that the reticulum takes any special share in modelling the boluses which have to be returned into the mouth. More probably, a sudden and simultaneous contraction of the diaphragm and of the abdominal muscles compresses the contents of the rumen and reticulum, and drives the sodden fodder against the cardiac aperture of the stomach. This opens, and then the cardiac end of the oesophagus, becoming passively dilated, receives as much of the fodder as it will contain. The cardiac aperture now becoming closed, the bolus, thus shut off, is propelled, by the reversed peristaltic action of the muscular walls of the oesophagus, into the mouth, where it undergoes the thorough mastication which has been described.

The sodden fodder is prevented from passing out of the psalterial aperture of the reticulum, in part by the narrowness of that aperture, and in part by the fine grating formed by the edges of the psalterial lamina. But when the semifluid matter, returned after mastication, once more reaches the cardia, it is compelled to pass toward the psalterial end of the reticulum (even apart from the guidance afforded by the lips of the oesophageal groove) on account of the direction of the oesophagus and the bounding of the cardiac aperture, on the side of the rumen, by a raised ridge. The chewed matter thus flowing over the surface of the more solid contents of the reticulum reaches the psalterium; and, in consequence of the fine state of division of its solids, readily traverses the interspaces of the lamellae of that organ, and passes into the fourth stomach, there to be submitted to the action of the gastric juice and to undergo the digestion of the protein compounds, which have remained unaffected by the previous mastication and insalivation.

The Ruminantia are divided into three groups: a. the Tragulidae, b. the Cotylophora, and c. the Camelidae.

a. The Tragulidae are a remarkable family, formerly united with the genus Moschus, and still commonly known under the name of Musk Deer, though they are devoid of the musk-sac and, in other respects, are totally different from Moschus. They are at present restricted to Southern Asia and Africa; and they are particularly interesting, as affording, in many respects, a connecting link between the typical Ruminants and the other Artiodactyla, especially the Anoplotheridae. Thus, the second and fifth digits are complete in both fore-and hindfeet, and the metacarpals and metatarsals of the third and fourth digits unite very late, or, as in one genus, Hyoemoschus, not at all. The canines are well developed in both jaws, and the premolar teeth are sharp and cutting.

The oesophagus opens at the junction of the rumen with the reticulum, the communication between the two being very wide (Fig. 102 B). The epithelium of the rumen is papillate, and there are two oesophageal folds, as in ordinary Ruminants, but the psalterium is represented only by a very short and narrow tube, the lining membrane of which is devoid of folds.

The surface of the hemispheres of the brain has fewer convolutions than in any other Ruminants, though this may very possibly be connected with the small size of the animal; as it is a general rule that, within the same group, the brain is less convoluted in small than in large animals.

The blood-corpuscles, small in all Ruminantia, are remarkably minute in the Tragulidae, not exceeding 1/10000 of an inch in diameter. They have circular contours.

The placenta is very nearly diffuse, the foetal villi being scattered over the chorion in bands, not collected into cotyledons.

As further remarkable peculiarities of this group may be mentioned the anchylosis of the malleolar bone with the tibia, and the tendency to ossification in the pelvic ligaments and of the aponeurosis of the, muscles of the back, in adult males. Finally, the navicular, cuboid, and ectocuneiform bones in the tarsus are all anchylosed together. If, as is probable, Xiphodon is one of the Tragulidae, the group has existed since the eocene epoch.

b. The Cotylophora are, like the preceding group, unguligrade, but the outer metacarpals and metatarsals are incomplete at their proximal ends, and the middle ones are early anchylosed into a cannon-bone. The malleolar bone is always distinct. The navicular and the cuboid bones of the tarsus are anchylosed together, but rarely with any other tarsal bone. The premaxilla is devoid of teeth in the adult. The stomach has the structure which has been described as typical.

The blood-corpuscles are circular, and may have a diameter of as little as 1/5000 of an inch.

The foetal villi are gathered together into bunches or cotyledons, which may present either a convex or a concave face toward the uterus. They are received into persistent elevations of the mucous membrane of the uterus, the surfaces of which present a reverse curvature.

All the Cotylophora except Moschus, the true Musk Deer, are provided with horns, but these horns are of two kinds. The bony core, in the one case, is ensheathed in a strong horny epidermic case; while, in the other, the epidermis of the integument which covers the core does not become so modified. In the former kind of horn, the core becomes excavated by the extension into it of the frontal sinuses, whence the Ruminants which possess such horns are not unfrequently called Cavicornia (Antelopes, Sheep, Goats, Oxen). As a general rule, the horny sheath persists throughout life, growing with the growth of the core. But in the remarkable Prong-horned Antelope of North America (Antilocapra), the horny sheath is annually shed and replaced by a newly-formed one.

Of the second kind of horn, or that which acquires no homy sheath, there are also two kinds. In the Giraffe, the horncores are attached over the coronal suture, at the junction of the frontal and parietal bones, with which they are not anchylosed; they persist throughout life, and are always covered by a soft and hairy integument.

In the Deer, on the other hand, the frontal bones grow out into solid processes, which are, at first, covered by soft and hairy integument; generally they are developed in the male sex only, but both sexes have them in the Reindeer. The horns attain their full size very rapidly, and tben a circular ridge, which makes its appearance at a short distance from the root of the horn and is called the "burr," divides the horn into the "pedicel" on the skull-side of the burr, and the "beam" on the opposite side. The circulation in the vessels of the beam now gradually languishes, its integument dies and peels off, and the dead bony substance beneath is exposed. Absorption and sloughing next occur at the extremity of the pedicel, just as might happen in any other case of necrosis. The beam and burr are shed, and the end of the pedicel scabbing over, fresh integument gradually grows up under the scab, and eventually restores to the extremity of the pedicel its pristine smooth and hairy covering.

The rapidity with which the development of bony matter into Deer-horn takes place is wonderful, horns weighing seventy-two lbs. having been produced in ten weeks.

The Cotylophora are represented in all parts of the world except the Australian and Novo-Zelanian provinces. They have not yet been traced back farther than the miocene epoch.

c. The Camelidae or (Tylopoda) are devoid of horns; and, unlike the other Ruminants, they walk upon the palmar and plantar surfaces of the phalanges of the third and fourth toes, which are alone developed. Broad integumentary cushions form a sole to the foot; while the nails are flattened and can hardly be called hoofs.

The arches of the cervical vertebrae, and not their transverse processes, are perforated by the canal of the vertebral artery; a character which the camels share with the Macrauchenidae.

The metacarpals are separated by a deep cleft, and the distal phalanges of the digits are nearly symmetrical in themselves. The distal facets of the astragalus are more unequal than in the other Ruminantia, and the navicular and cuboid bones are not anchylosed together.

The premaxillae have a single strong outer incisor on each side. Large curved and pointed canines are developed in each jaw, and are quite distinct from the series of the incisors in the mandible. There are not more than five grinding teeth in a continuous series above and below.

The stomach is unlike that of the typical Ruminants. The oesophagus opens directly into the paunch, which is lined by a smooth, not papillose, epithelial coat. From its walls, at least two sets of diverticula, with comparatively narrow mouths, are developed. These, the so-called "water-cells." serve to strain off from the contents of the paunch, and to retain in store, a considerable quantity of water. The reticulum is sharply defined from the rumen, and communicates with it by a comparatively small aperture. The oesophageal groove is bounded by only one ridge, which lies upon its left side. The psalterium is reduced to a mere tubular passage, without laminae; and the abomasum is large, and has the ordinary structure. The pyloric end of the duodenum is considerably dilated, and has been taken for a division of the stomach. The caecum is short and simple. By a remarkable exception among the Mammalia, the red blood-corpuscles are elliptical. The foetal villi are scattered evenly over the chorion, so that the placenta is diffuse.

While the Tragulidae connect the typical Ruminants with the non-ruminant Artiodaotyles, the Camelidae, on the other hand, link them with Macrauchenia and the Perissodactyles.

The Camelidae are at present represented by two very distinct groups-the Camels of the Old World and the Llamas of the New. They occur in the fossil state as far back as the miocene epoch.
 
     
 
 
     



     
 
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