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

 
     
 

This large group of Mammalia is most definitely characterized by its dentition. There are no canines, and the mandible never contains more than two incisors, which are placed one on each side of the symphysis, and continue to grow throughout life. They are coated with enamel much more thickly upon their front surfaces than elsewhere; so that by attrition they acquire and retain a chisel-shaped edge, the enamel in front wearing away less rapidly than the rest of the tooth.

With the exception of one group of Rodents, there are only two teeth in the premaxillae; and these have the same characters as the incisors of the mandible. The Lagomorpha, or Hares and Rabbits, however, have a second pair of incisors of small size, behind the first, in the upper jaw. The molars are from two to six in number, in each half of the upper jaw, and two to five, in the lower jaw. They consist of enamel, dentine, and cement, and their crowns may be tuberculate or laminate in pattern. Sometimes they form roots, but, in other cases, they grow throughout life. Where there are more than three grinding-teeth, the one which precedes the three hindermost has displaced a milk-tooth; but, where the grindingteeth are fewer than three, or only three, none of them displace a milk-tooth. Even when milk-teeth exist they may be shed before birth, as in the Guinea-pig.

The premaxillary bones are always large, and the orbits are never shut off by bone from the temporal fossa. Very generally, the condyle of the mandible is elongated from before backward.

With the exception of one group, the Dormice (Myoxinae), all Rodents have a large caecum.

The cerebral hemispheres leave the cerebellum largely uncovered, when the brain is viewed from above. They are either smooth externally, or very moderately convoluted. The corpus callosum is well developed.

With the exceptions noted, the foregoing characters are universal among the Rodentia. There are other peculiarities which are generally present, and, when they exist, are very characteristic, though they are not universal.

Thus the dorso-lurabar vertebrae are usually nineteen in number. There is a large interparietal ossification. The jugal bone is comparatively short, and occupies only the middle of the zygomatic arch.

The clavicles are very generally present; though wholly absent in some genera, as, for example, the Guinea-pig, (Cavia). The acromion commonly sends a process backward over the infra-spinous fossa. There is a ninth bone in the carpus intercalated between the proximal and the distal series. The digits are five, ungulate, and provided with small claws.

There is a bone in the penis. The testes do not leave the abdomen, but come down into the groin in the breeding-season. Vesiculce seminales and prostatic glands are present. In the female the uterus is, in many genera, completely divided into two cornua, each of which opens separately into the vagina; but, in the rest, the cornua unite into a corpus uteri.

Some genera depart widely from the rest in particular points; for example, in the Porcupines, the hairs on the dorsal region of the body are very much enlarged, acquire a peculiar structure, and formed the so-called "quills." Some of the Porcupines have prehensile tails.

In Cavia and Hydrochoerus the toes are reduced to three, and the nails have almost put on the character of hoofs.

The Squirrels have the short pollex almost opposable. The femur in some Rodents has a well-developed third trochanter; and in Dipus the Jerboa, the long metatarsals become anchylosed together into a cannon-bone.

In the Porcupines, the suborbital foramen is enormous, and an anterior fasciculus of the masseter muscle arises from the maxilla, and traverses the foramen to its insertion.

The Hamster (Cricetus) has great cheek-pouches, provided with special retractor muscles connected with the spines of two lumbar vertebrae.

In some genera, the stomach, which is usually simple, tends to become complex. Thus the cardiac division of the stomach of the Beaver is provided with a special glandular mass. The cardiac end of the oesophagus of the Dormouse is glandular and dilated like the proventiculus of a bird. And, in Arvicola, the stomach becomes deeply constricted, and a groove leads from, the oesophagus toward the pyloric end, reminding one of certain Artiodactyla.

In some few genera, the ureters open into the fundus of the bladder, or near it.

Although the genera and species of the Rodentia are more numerous than those of any other mammalian order; and although they are adapted to very different modes of life- some, like the"Flying Squirrels," floating through the air by means of a parachute-like expansion of the integument between the fore-and hind-limbs; others being arboreal, like the ordinary Squirrels; or among the swiftest of runners, as the Hares; or strong burrowers, as the mole-like Bathyeryus; or aquatic, like the Water-vole-their structural differences are comparatively insignificant, and the subdivision of the order into large groups is proportionately difficult.

Brandt has divided the Rodents according to their cranial characters into Sciuromorpha, Myomorpha, Hystricomorpha, and Lagomorpha; or, Squirrels, Rats, Porcupines, and Conies, if we use these English names in a broad and tribal sense.

The student will find the Rabbit, one of the Lagomorpha, to be a conveniently-sized and easily-obtained subject for study. The following are the most important points to be noted in its structure: The hairy covering of the body extends over the palmar and plantar regions of the feet, and into the interior of the mouth, so that there is a band of hair on the inside of each cheek. There are five digits on the fore-foot, or manus; but the pollex is smaller than the others. The pes has only four digits, and the hind-limb is longer than the fore-limb. The upper lip is large, flexible, and cleft in the middle line the large eyes are provided with a third eyelid, and the pinnae of the ears are very long and mobile. The tail is short and recurved. The male has a recurved penis, and on each side of it a scrotal sac. The female has five pair of abdominal teats. In both sexes perineal glands are present, consisting of a saccular involution of the integument with rugose walls, into which the duct of a special gland lodged at the side of the penis, or of the clitoris, opens.

There are nineteen dorso-lumbar vertebrae, of which twelve are dorsal. Of the four sacral vertebrae only the first unites with the ilia. The dorsal vertebrae have well-developed spinous and transverse processes. At about the eighth, a mammillary process, or metapophysis, becomes obvious; and in the succeeding vertebrae this increases in length and strength, till in the lumbar region it becomes as long as the spinous process. In the last lumbar, it is short, and in the sacrum it is obsolete, but it is traceable through the series of the anterior caudal vertebrae. Accessory processes, or anapophyses, are observable in the last dorsal and four or five anterior lumbar vertebrae. The transverse processes of the lumbar vertebrae are exceedingly long, and that of the first lumbar is bifurcated at its extremity. These transverse processes give attachment above, to the sacro-lumbalis, and below, to the psoas major, both which muscles are very large ; while the heads of the longissimus dorsi are attached to the long metapophyses. The great mass of these extensor and flexor muscles of the spine, and the leverage afforded by the mode of their attachment to the long processes of the vertebrae, would seem to be related to the leaping and scratching movements of the Rabbit. Strong median processes are developed from the ventral faces of the centra of the three anterior lumbar vertebrae; these give attachment to the crura of the diaphragm.

The tubercles of the second to the eighth ribs inclusively are prolonged into spiniform processes, which give attachment to the tendons of the longissimus dorsi. There are five sternebrae and a long xiphoid process. The manubrium is long, narrow, deep, and keeled inferiorly.

In the skull, the great supra-orbital processes of the frontal are to be noted. The presphenoid is high and greatly compressed from side to side, so as to form a thin septum between the orbits, and the optic foramina run into one, as in some Seals. The tympanic and the periotic are anchylosed together, but remain distinct from the adjacent bones, and are merely held in position by abutting against the basi-sphenoid on the inner side and by the post-tympanic hook of the squamosal on the outside. The tympanic is prolonged upward and outward into a tubular meatus. The glenoid cavity is elongated from before backward. The suture between the jugal and the maxillary becomes obliterated, and there is no orbital process given off from the zygoma. A considerable extent of the outer wall of the maxilla remains incompletely ossified. The premaxilla is extremely large and trifurcated.

The ascending portion of the ramus of the mandible is long, and the coronoid process well developed. The long axis of the condyle is antero-posterior, and the angular process has a slight inward projection. In the palate, the prepalatine, or incisive foramina are enormous; and partly in consequence of this, partly by the posterior excavation of the palatal plate of the palatine, the roof of the palate is reduced to little more than a transverse bar of bone.

The scapula is long and narrow, and the backward process of the acromion, to which reference has already been made, gives attachment to a slip of the trapezius. A bony clavicle is present, but it is incomplete at both ends. There is a supracondyloid foramen in the humerus. The radius and ulna are complete, but are fixed in the attitude of pronation.

The femur has a small third trochanter. The tibia and fibula are anchylosed. The internal cuneiform bone is wanting, and the plantar surface of the naviculare gives off a large process. The inner side of the base of the second metatarsal sends a process along the inner face of the meso-cuneiform to articulate with the naviculare. This may represent a rudiment of the hallux with the ento-cuneiform.

In the myology of the Rabbit the vast size of the flexors and extensors of the back has already been noted. The muscles moving the fore-and especially the hind-limbs, and the masseter, are not less remarkable for their dimensions. In the fore-limb, the supinator longus is absent. The extensor indicts and secundi internodii pollicus from one muscle. The extensor minimi digiti goes to the fourth and fifth digits. The flexor perforans and the flexor pollicis longus unite in a common tendon which divides into five slips, one for each digit. There are three lurabricales from the radial sides of the tendons for the third, fourth, and fifth digits. The flexor sublimis, or perforatus, for digits ii., iii., and iv., arises from the inner condyle as usual; but that for the fifth digit springs from the pisiform bone-thus simulating the ordinary arrangement of the perforated flexor in the pes. There is no pronator quadratus; but the palmaris longus is distinct, and its slender tendon expands into the palmar aponeurosis. Each digit, except the pollex, has a pair of flexores breves, or interossei, which lie on the palmar faces of the metacarpal bones.

In the hind-limb, the soleus has only a fibular origin. The plantaris is very large and ensheathed in the gastrocnemius; it ends in a tendon nearly as large as the tendo Achillis, which passes over the end of the calcaneum, being connected with this and the tendo Achillis by a strong fascia laterally, but being otherwise separated from it by a synovial sac. In he sole of the foot it divides into four tendons, which become the perforated tendons of the four digits. The flexor perforans and flexor hallucis are fused into one muscle, the tendon of which divides in the sole into the four perforating tendons. There are three lumbricales, and four pair of interossei (flexores breves). There is no proper tibialis posticus, but a muscle arises from the upper part of the inner face of the tibia, internal to, and in front of, the insertion of the poplitaesus, becomes tendinous about the middle of the leg, passes behind the inner malleolus, and runs along the inner and dorsal aspect of the second metatarsal to be inserted into the extensor tendons. It seems to stand in the same relation to the second digit as the peronaeus quinti, on the opposite side of the pes, to the fifth digit. The peronaeus longus is inserted into the base of the second metatarsal: a peronaeus brevis, p. quarti, and. quinti digiti, are present. There is no extensor hallucis longus, nor any extensor brevis digitorum.

The principal characters of the brain of the Rabbit have already been described (see p. 60, and Figs. 21 and 22). There is a single large corpus mammillare. Of the corpora quadrigemina, the nates are larger than the testes. There is a very large and completely-exposed flocculus, and the vermis is large in proportion to the lateral lobes of the cerebellum. The corpora trapezoidea are well marked.

The membrana nictitans is very large, has a convex free edge, and contains a triangular cartilage. There are no puncta lachrymalia, but a cresoentic aperture leads into the lachrymal canal. The large lachrymal gland lies above and external to the eyeball, and there is a well-developed Harderian gland on its lower and inner side.

The dental formula is i. 2-2/1-1 c. 0-0/0-0 p.m. 3.3/2.2 m. 3.3/3.3=28.

The lower, and the inner upper, incisors are very large and long; they grow continuously from persistent pulps, and they are coated with enamel only in front, so that wear keeps them constantly sharp. The second pair of small incisors exists only in the upper jaw. A great diastema separates the incisors from the first premolar above and below. The grinding teeth all grow from persistent pulps, and do not form fangs; they have transversely-ridged crowns, the patterns of which are very similar throughout, the first and the last only presenting some differences. The young Rabbit has three incisors and three milk-molars on each side, in the upper jaw. In the lower jaw, there are only two milk-molars on each side.

The stomach is simple, and there is a large caecum. Special glands pour their secretions at the side of the anus.

The pancreas is very large, and its duct enters the intestine nearly a foot from the pylorus, and far distant from the biliary duct.

There are two anterior cavae; and the external jugular vein is very much larger than the internal.

In the male, the inguinal canal remains permanently open, and there is a large uterus masculinus. In the female, the uteri are quite separate, and each opens by a distinct os tincae into the vagina.

The distribution of the Rodentia is almost world-wide, Madagascar being the only considerable island in which indigenous Rodents are unknown. The Austro-Columbian province may be regarded as the headquarters of the group.

Remains of Rodents have been found, in the fossil state, as far back as the eocene formation.

 
     
 
 
     



     
 
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