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
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
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,
). 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
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.
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
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,
; 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,
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
which muscles are very large ; while the heads of the longissimus
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
goes to the fourth and fifth digits. The flexor
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,
for digits ii., iii.,
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,
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
). 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
into the base of the second metatarsal: a peronaeus brevis, p.
and. quinti digiti
, are present. There is no extensor
, 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
, 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.
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
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.