Page:EB1911 - Volume 23.djvu/460

Rh feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw.

In all rodents the upper incisors resemble the lower ones in growing uninterruptedly from persistent pulps, and (except in the hare group, Duplicidentata) agree with them in number. The premolars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series. The orbits are always open behind, never being surrounded by bone. The condyle of the lower jaw is antero-posteriorly elongated. The intestine (except in the dormice or Gliridae) has a large caecum. The testes are inguinal or abdominal. The uterus is two-horned, with the cornua opening separately into the vagina or uniting to form a corpus uteri. The placenta is discoidal and deciduate. And the smooth hemispheres of the brain do not extend backwards so as to cover any part of the cerebellum.

Rodents include by far the greater number of species, and have the widest distribution, of any of the orders of terrestrial mammals, being in fact cosmopolitan, although more abundant in some parts, as in South America, which may be considered their headquarters, than in others, as in Australasia and Madagascar, where they are represented only by members of the mouse-group, or Myoidea.

All rodents are vegetable-feeders, and this uniformity in their food and in the mode of obtaining it, namely by gnawing, has led to that general uniformity in structure observable throughout the group; a feature which renders their classification difficult. Indeed, despite the fact that they present much diversity of habit—some being arboreal, as the squirrels, many of which are provided with expansions of skin or parachutes on which they glide from tree to tree; some cursorial, as the hares; others jumpers, as the jerboas; others fossorial, as the mole-rats; and others aquatic, as the beavers and water-rats—no important structural modifications are correlated with such diversity of habit.

Anatomy.—The rodent skull is characterized by the great size of the premaxillae, which completely separate the nasals from the maxillae; by the presence of zygomatic arches; and by the wide unoccupied space existing between the incisors and the cheek-teeth; and (except in the Duplicidentata) by the antero-posteriorly elongated glenoid cavity for the articulation of the lower jaw. Post-orbital processes of the frontals exist in squirrels, marmots and hares; but in all other genera they are rudimentary or altogether absent; and the zygoma seldom sends upwards a corresponding process, so that the orbit is more or less completely continuous with the temporal fossa. The lachrymal foramen is always within the orbital margin; and in many species the infra-orbital foramen is very large (in some as large as the orbit) and transmits part of the masseter muscle. The zygomatic arch is variously developed, and the position of the jugal is a character for grouping the families. The nasals are, with few exceptions, large, and extend far forwards, the parietals are moderate, and there is generally a distinct interparietal. The palate is narrow from before backwards, this being especially the case in the hares, where it is reduced to a mere bridge between the premolars; in others, as in the rodent-moles (Bathyerginae), it is extremely narrow transversely, its width being less than that of one of the molar teeth. Tympanic bullae are always present and generally large; in some genera, as in the gerbils (Gerbillinae) and jerboas (Jaculidae), there are supplemental mastoid bullae which form great

hemispherical bony swellings at the back of the skull. (fig. 1, Per), in these genera and the hares the meatus auditoriums being tubular and directed upwards and backwards. The lower jaw is characterized by its abruptly narrowed and rounded front part supporting 2.—Skull of Porcupine (Hystrix cristata), with muscle attached. t, temporal muscle; m, masseter; m′, portion of masseter transmitted through the infra-orbital foramen, the superior maxillary nerve passing outwards between it and the maxilla. the pair of large incisors, as well as by the small size of the coronoid process, and the great development of the lower hind, or angular, portion.

The dental formula varies from i. , c. , p. , m. (total 28) in the hares and rabbits to i. , c. , p. , m. (total 12) in the Australian water-rats; but in the great majority of species it presents striking uniformity, and may be set down typically as i. , c. , p. or, m. . In the Duplicidentata only is there more than a single pair of incisors, and in these the additional pair is small and placed behind the middle pair. In this group the enamel extends partially to the back of the incisors, but in all the rest it is restricted to the front surface, so that, by the more rapid wearing-away of the softer structures behind, a chisel-shaped edge is maintained. Both upper and lower incisors are regularly curved, the upper ones slightly more so than the lower; and, their growth being continuous, should anything prevent the normal wear by which their length is regulated—as by the loss of one of them, or by displacement owing to a broken jaw or other cause—the unopposed incisor may gradually curve upon itself until a complete circle or more has been formed, the tooth sometimes passing through some part of the animal's head. The cheek-teeth may be either rooted or rootless, and either cusped or formed of parallel plates, this diversity of structure often occurring in the same family. When there are more than three cheek-teeth, those which precede the last three have succeeded milk-teeth, and are premolars. In some species, as in the agoutis (Dasyproctidae), the milk-teeth are long retained, while in the allied cavies (Caviidae) they are shed before birth.

3.—Vertical and Longitudinal Section through the Skull of the Beaver (Castor fiber), showing the brain-cavity, the greatly developed plates of bone in the nose-cavity, the mode of implantation of the ever-growing chisel-edged incisor, and the curved rootless cheek-teeth.

The tongue presents little variability in length, being short and compressed, with a blunt tip, which is never protruded beyond the incisors. In most species there are three circumvallate papillae at the base, and the apical portion is generally covered with small thread-like papillae, some of which in the porcupines become greatly enlarged, forming toothed spines. The stomach varies in form from the simple oval bag of the squirrels to the complex ruminant-like organ of the lemmings. In the water-rat and agoutis it is constricted between the oesophagus and pylorus; while in the dormouse the oesophagus immediately before entering the stomach is much dilated, forming a large egg-shaped bag with thickened glandular walls; and in certain other species, as in Lophiomys and the beaver, glandular masses are attached to and open into the cardiac or pyloric pouches. All rodents, with the sole exception of the dormice, have a caecum, often of great length and sacculated, as in hares, the water-rat and porcupines; and the long colon in some, as the hamster and water-rat, is spirally twisted upon itself near the commencement. The liver is divided in the typical manner in all, but the lobes are variously subdivided in different species (in Capromys they are divided into minute lobules); and the gall-bladder, though present in most, is absent in a few. In most species the penis (which is generally provided with a bone) may be more or less completely retracted within the fold of integument surrounding the vent, and lie curved backwards upon itself under cover of the integument, or it may be carried forward some distance in front of the anal orifice, from which, as in voles and marmots,