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

 
     
 

The number of the dorso-lumbar vertebrae in this group is always fewer than twenty-two, and rarely exceeds nineteen.

The third digit of each foot is asymmetrical in itself, and usually forms a symmetrical pair with the fourth digit; and the functional toes of the hind-foot are even in number-that is to say, either two or four.

The femur is devoid of any third trochanter; the facets upon the distal face of the astragalus are subequal, that for the cuboid being nearly as large as that for the navicular bone. The tympanic is large, and the pterygoid process of the sphenoid is not perforated.

The posterior premolar teeth usually differ a good deal from the succeeding molars, being simpler in pattern. The last milk-molar in the lower jaw is trilobed; but this is also the case in some Perissodactyla.

The stomach is more or less complex. The caecum, though well developed, is smaller than in the Perissodactyla.

The mammae are inguinal or abdominal. When horns are present, they are double, supported, wholly or partly, by the frontal bone and provided with an osseous core, which is almost always an outgrowth from that bone.

The Artiodactyla are divisible into the Non-Ruminantia and the Ruminantia.

A. The Non-Ruminantia usually have more than one pair of incisors in the upper jaw. The molar teeth have either a mammillate, a transversely-ridged, or a rhinocerotic pattern. In only one genus, Dicotyles, are any of the metacarpal or metatarsal bones anchylosed together. They are devoid of horns, and the stomach has rarely more than two divisions.

The Non-Ruminantia are divisible into three families:
The Suidae, the Hippopotamidae, and the Anoplotheridae; but more or fewer of the members of this last group may have ruminated.

a. The Suidae have the skin of moderate thickness and hairy; the limbs slender, and the third and fourth toes considerably longer than the second and fifth. The teats are abdominal, and there is a scrotum. The dental formula varies considerably, but the molars have a multituberculate or transversely-ridged grinding surface.

In the genus Sus, the dental formula is i. 3.3/3.3 c. 1.1/1.1 p.m. 4.4/4.4 m. 3.3/3.3.

By way of contrast with the Horse, I add some more detailed statements regarding the anatomy of the Pig as a common and very good example of an Artiodactyla. The Pig has seven cervical vertebrae, nineteen (Exceptionally, the number may be increased to twenty-two.) dorso-lumbar, of which fourteen are dorsal, four sacral, and twenty to twenty-three caudal. The atlas has wide oblique alae, as in the Horse. The centra of the other cervical vertebrae are short, with nearly flat articular surfaces, and this flatness is retained in the dorso-lumbar region. The cervical and dorsal vertebrae are provided with long spines, that of the first dorsal vertebra being the longest of all. Up to the twelfth dorsal the spines all slope backward; beyond it they slope forward, if at all.

In the ninth dorsal vertebra the postzygapophysis presents an articular surface on its dorsal side, and the prezygapophysis of the tenth vertebra bends round so as to overlap this surface. This character is continued in the succeeding vertebrae as far as the first sacral. The transverse processes of the penultimate and last lumbar vertebra are tolerably long, but they are inclined forward as well as outward, and do not articulate with one another, or with the first sacral.

In the skull the supraoccipital is inclined upward and forward into a great transverse crest, to which the parietals contribute but little. The parietals are early anchylosed. The temporal ridges remain widely separated in the middle of the roof of the skull.

The frontal bone has a post-orbital process, and so has the jugal, but the two do not meet so as to bound the orbit. The lachrymal is very large, and its two canals open on the face. The nasals are very long, and the premaxillae unite with them for a great distance. There is a praenasal bone, or ossification of the cartilaginous septum of the nose. The bony palate extends back beyond the level of the last molar. The base of the external pterygoid process is not perforated. The surface for the articulation of the lower jaw is transversely elongated, convex from before backward, and bounded behind and internally by a post-glenoidal ridge.

The tympanic bulla is very large, and the exceedingly long bony meatus curves upward and outward, between the squamosal and the mastoid, with both of which it is anchylosed, to the root of the zygoma, where its aperture looks almost directly upward. The post-tympanic is closely appressed to the postglenoidal process, so as, with the latter, to encircle the meatus. The proper mastoid is distinct, though short, but there is a very long paramastoid developed from the exoccipital and extending behind and below the mastoid.

The rami of the mandible are completely anchylosed at the symphysis. There is a long perpendicular portion of the ramus. The condyle is transversely elongated and. convex, antero-posteriorly; the coronoid process ascends hardly higher than it. In a longitudinal section, the cavity of the cerebral hemispheres is more rounded than in the Horse, and lies above, as well as in front of, that for the cerebellum.

The scapula is long and narrow. It is devoid of acromion, and has but a small coracoid process.

The radius and ulna are complete, but are anchylosed together in the prone position. The distal end of the ulna articulates with the cuneiform bone.

The carpus contains eight bones, but the radial bone in the distal series may be either the trapezium, or a rudiment of the pollex. The lunare and the axis of the third metacarpal have the same relation as in the Horse. The third and fourth digits are larger than the other two, and form a symmetrical pair. There are sesamoid bones on the ventral face of the articulations between the metacarpal and the basal phalanx, and of that between the middle and the distal phalanges. Each distal phalanx is incased in a small hoof. The femur has a round ligament. There is no third trochanter. The fibula is complete, and its distal end articulates with the calcaneum. There are the usual seven tarsal bones. The tibial end of the astragalus has the form of a deeply-grooved pulley, the direction of the groove corresponding nearly with the length of the foot. The distal end presents a convex subcylindrical surface divided by a ridge into two facets, of which one is somewhat less than the other, and articulates with the cuboid.

The metatarsus and phalanges of the pes are disposed like the corresponding bones in the miinus.

The fore-part of the body is supported upon the anterior extremities by a muscular sling composed of the serratus, levator anguli scapulae, and sternoscapularis, much as in the Horse, with which the Pig exhibits a general correspondence in its myology. The muscles which move the digits, however, have undergone less modification. Each digit of the manus, for example, has its proper extensors, and there is an extensor ossis metacarpi pollicis which ends on the basal phalanx of the second digit. A pronator teres is inserted into the lower half of the radius. The flexor perforatus has only two tendons, which go to the third and fourth digits. The flexor perforans sends two large tendons to the third and fourth, and two small ones to the second and fifth digits. There is a large interosseus muscle on the radial side of the third digit, and another on the ulnar side of the fourth; but the interossei of the interspace between these digits are represented only by fibrous tissue. The second and fifth digits have each two interossei. There is no soleus. The strong and fleshy plantaris arises from the outer condyle, beneath the gastrocnemius; and, enclosed between the two heads of the latter, passes to the inner side of the tendo Achillis; its tendon curves round this tendon, passes over the end of the calcaneum as over a pulley, enters the sole, and finally divides into the two perforated tendons of the third and fourth digits. The inner and outer digits, of the pes, like those of the manus, have no perforated tendons.

A large and fleshy flexor hallucis longus arises from the fibula and the interosseous ligament, and its broad tendon passes into the sole and coalesces with the tendon of the smaller flexor longus digitorum. The conjoined tendons divide into four slips-two large, median, and two small, inner and outer. These go to the distal phalanges and sesamoids of the respective digits.

The tibialis posticus is absent, but there is a small tibialis anticus,

A very complicated muscle represents the extensor longus digitorum and the peronaeus tertius. It arises by (a) a strong round tendon from the outer condyle of the femur, just in front of the external lateral ligament. From this tendon proceed two fleshy bellies, one of which supplies tendons to the third, fourth, and fifth digits, while the other ends in a broad band of tendinous fibres, which is inserted into the third metatarsal and the ectocuneiform. Into this band is inserted (b) the second fleshy head which arises from the upper part of the tibia; and it is traversed by the tendon of (c) the third head, which is slender, arises from the fibula, and sends its long and delicate tendon to the dorsum of the second digit.

The peronaeus longus is present, and its tendon is inserted into the entocuneiform and the second metatarsal. There is no peronaeus brevis. A peronaeus 4ti et 5ti digiti arises from the upper part of the fibula, behind the peronoeus longus, and ends in a tendon which passes behind, and on the inner side of, that of the latter muscle, to the dorsum of the foot, where it divides into two branches which join the extensor sheaths of the fourth and fifth digits.

The extensor brevis goes to the two middle digits, and is connected with the middle tendon of the extensor longus.

The interossei are similar to those of the manus.

The formula of the milk dentition of the Pig (which is complete at the third month after birth) is d.i. 3.3/3.2 d.c. 1-1/1-1 d.m. 4.4/4.4.

The outer upper incisors are directed obliquely outward and backward. In the upper jaw, the anterior two molars present sharp longitudinal edges, while the posterior two have broad crowns with two transverse ridges. In the mandible the anterior three molars have sharp longitudinal edges, while the hindermost has a broad, three-ridged crown.

The first permanent molar is the first tooth of the permanent set which comes into place (at about six months after birth), and the permanent dentition is completed in the third year, at which time the first deciduous molar, which is not replaced, falls out. Hence the formula of the permanent dentition is i. 3.3/3.3 c. 1-1/1-1 p.m. 3.3/3.3 m. 3.3/3.3=40.

The permanent incisors in the upper jaw have short, broad, vertically-disposed crowns, and lie in a longitudinal series, the external being separated by an interval from the others. The elongated inferior incisors lie side by side, are greatly inclined forward and upward, and are grooved upon their upper or inner faces. The strong, angulated crowns of the canines are bent upward and outward in both jaws. They work against one another, in such a manner that the upper wears on its anterior and external face, the lower on the posterior aspect of its apex. The crowns of the premolars are all brought to a cutting longitudinal edge, while the molars have broad crowns with transverse ridges subdivided into tubercles. Of these ridges there are two in the anterior two molars of each jaw, while the posterior molar is more complex, having at fewest three distinct ridges. The molar teeth all develop roots; but the canines continue to grow for so long a time, in the Boar, that they might be said to be rootless.

The alimentary canal is ten or twelve times as long as the body.
The stomach is less simple in structure than it appears to be at first sight. The cardiac end presents a small caecum, in which is a spiral fold of the mucous membrane; and, at the entrance of the oesophagus, the epithelial lining is folded so as to form a sort of valve. Folds of the mucous membrane, between which there lies a groove, extend from the cardia toward the pylorus, and foreshadow the more developed structure observable in Ruminants.

The caecum has not above one-sixth the capacity of the stomach, and the ilium projects into it, so as to form a very efficient illocaecal valve. The liver is provided with a gallbladder. The heart is devoid of a Eustachian valve, and sometimes, but not always, possesses a septal ossification.

There is only one anterior cava. The aorta gives off an innominata, whence the right subclavian and the two carotids arise, and a left subclavian. This is an arrangement midway between that observed in the Horse and that in Man.

The trachea, before it divides, gives off a third bronchus, which passes to the right lung; and the lungs are deeply lobed.
In the brain the cerebral hemispheres rise above the cerebellum much more than they do in the Horse.

In the male, the penis is contained in a long prepuce, and, like that of the Horse, is devoid of a bone and provided with retractor muscles. The prostate is lobed. There is a large uterus masculinus and well-developed vesiculae seminales. The ducts of Cowper's glands open into a caecal cavity contained in the muscular bulb. The testes descend into a scrotum. In the Sow, a pair of Gaertner's canals, or persistent Wolffian ducts, open into the vestibule beside the urinary meatus. The uterine cornua are very long, and the ovaries are lobulated. The period of gestation is sixteen to twenty weeks. The ovum, at first spherical, retains that form until it attains a diameter of nearly half an inch. It then rapidly elongates into a coiled filiform body, as much as twenty inches long. Both the allantois and the umbilical vesicle at the same time assume a spindle-shape.

The allantois soon becomes divided into an internal epithelial and an external vascular layer; the latter becoming united with the chorion, through the extremities of which the allantois eventually passes. The villi are very numerous, minute, and spread over the whole surface of the ovum.

The Suidae exhibit great variations in their dentition and in the structure of the stomach.

In Porcus (the Babyrussa) the dental formula is i. 3.2/3.3 c. 1.1/1.1 p.m.m. 5.5/5.5; the canines are enormously elongated and recurved, and the pharynx is provided with peculiar air-sacs.

The stomach is divided into three chambers, and the groove leading from the oesophagus toward the pylorus is more distinctly marked than in the Sus.

In Dicotyles (the Peccaries) the upper incisors are also reduced to two on each side, and the molar teeth present transverse ridges, which are more distinct and less tuberculated than in Sus.

The stomach is divided into three sacs, and is provided with an oesophageal groove as in the preceding genus.

The middle metatarsals and metacarpals coalesce into a cannon-bone, and the fifth digit of the pes is represented only by its metatarsal.

In Phacochaerus (the Wart-hog) the upper incisors are reduced to one pair, and the hindermost molars, which are the only ones which are not shed in the old animal, are of great size, and possess a complicated, tuberculated structure.

The Suidae are represented by one genus or another in all the great distributional provinces except the Australian (The Papuan pig may have been introduced from the weatward.) and Novo-Zelanian. Porcus is peculiar to part of the Malay Archipelago, Dicotyles to South America, and Phacochaerus to South Africa.

A great variety of swine-like Ungulata existed during the deposition of the older tertiary strata, and are the earliest known members of the group.

b. The Hippopotamidae are represented at present only by the genera Hippopotamus and Chaeropus. These animals have a huge head, a heavy body, covered with a thick integument, provided with scanty hairs, and short, stout, tetradactyle limbs, all the four toes of which rest on the ground. The female has inguinal teats, and the male is devoid of a scrotum.

The dental formula of the adult Hippopotamus is i. 2.2/2.2 c. 1.1/1.1 p.m. 3.3/3.3 m. 3.3/3.3, while Chaeropus has only two incisors in the lower jaw. The tubercles of the molar teeth, when ground down by mastication, present a double trefoil pattern, and the hindermost inferior molar is trilobed. The incisors are straight and tusk-like. The very large and curved canines are directed downward in the upper jaw, upward in the lower. Their mutual attrition wears the anterior face of the extremity of the upper, and the posterior face of that of the lower, flat.

The milk dentition consist of d.i. 3.3/3.3 d.c. 1.1/1.1 d.m. 4.4/4.4 The last lower deciduous molar is trilobed, and the first deciduous molar persists a long time, and seems not to be replaced.

The stomach is divided into three or four compartments, and there is no caecum. The liver has a gall-bladder, and the kidneys are lobulated.

The skeleton is very pig-like, but in some respects approaches the Ruminants. The centra are slightly convex in front, and concave behind, in the cervical region, but not elsewhere. The prezygapophyses overlap the postzygapophyses in the posterior dorso-Iumbar vetebrae. On the other hand, the transverse processes of the last lumbar vertebrae articulate with those of the preceding and succeeding vertebrae, as in the Horse and other Perissodactyles.

In the skull the orbits are nearly complete posteriorly, and they become almost tubular by the outward production of the frontal and lachrymal bones.

The nasals and premaxillae unite for a great extent. The osseous palate is long; the large tympanic bone is anchylosed with the approximated post-gienoidal and post-tympanic processes.

The mandible is extremely massive, and has a backwardly produced angle.

The scapula has a short acromion. The radius and ulna are complete and anchylosed, and there are eight bones in the carpus. The fibula is complete, and the tarsus, which has seven bones, much resembles that of the Pig.

The Hippopotamidae are at present confined to Africa; but a species abounded in the rivers of Europe in the later tertiary times.

Merycopotamus of the miocene Fauna of the Sewalik Hills appears to have been a Hippopotamid, with upper molars having a quadri-crescentic, ruminant-like pattern, and lower molars bi-crescentic and rhinocerotic in character.

In the Suidae and Hippopotamidae, it is interesting to remark the tendency to the coalescence of the metacarpals and metatarsals in Dicotyles; the disappearance of the upper incisors by pairs in Dicotyles, Parcus, and Phacochaerus; and the great complexity of the stomach in Dicotyles and Hippopotamus; as they are so many approximations toward the structure of the Ruminant Artiodactyla. And the transition from the non-Ruminant to the Ruminant groups, or rather the common stem of both, is furnished by the Anoplotheridae.

c. The family of the Anoplotheridae exclusively contains extinct Mammals belonging to the eocene and miocene epochs. They are most conspieuously distinguished by the circumstance that the teeth, of which there are eleven on each side, above and below, in the adult dentition, are not interrupted by any gap in front of and behind the canine, as they are in the preceding genera, but form an uninterrupted and even series, as in Man.

The dental formula of the adult Anoplotherium is i. 3.3/3.3 c. 1.1/1.1 p.m. 4.4/4.4 m. 3.3/3.3, supposing that the first premolar is really such, and not a persistent milk-molar.

The skeleton of an Ox (Bos)
Fig. 101. The skeleton of an Ox (Bos)
The upper and lower molars have the general structure of those of the Rhinoceros; but the laminae of the upper are Lent more backward into parallelism with the outer wall, and a strong conical pillar is developed on the inner side of the anterior lamina. The skull resembles that of the Ruminant Tragulidae in structure, but the orbit is incomplete behind The rest of the skeleton partly resembles that of the Pigs, and partly that of the Ruminants.(It Anoplotherium secundauium the digit ii. is developed in each foot, though not nearly so long as iii., which is nearly symmetrical in itself. Then is an approach to the same structure in the manus of Cainotherium.)

In Xiphodon and Cainotherium, which are ordinarily comprised among the Anoplotheridae (though, in all probability, they are true Ruminants of the Traguline group), the orbit is complete, and both upper and lower molars put on the Ruminant characteristics. In dentition, Cainotherium differs from a Ruminant only in possessing all the upper incisors, while no existing adult Ruminant has more than the outer upper incisors. We are of course unacquainted with the structure of the stomach in these animals, but they so closely resemble Ruminant Artiodactyla that it is highly probable they may have possessed the faculty of rumination in a more or less perfect degree.


 
     
 
 
     



     
 
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