From:
RRC
Date: 2001-07-20 09:51:00 UTC
Subject: Bob C: Ferret Skeleton: Head Skeleton
Skeleton cranii: (Synonyms: Skull, cranium and mandible, cranial bones,
cranial skeleton, head skeleton, craniofacial bones, craniomandibular bones).
If you were to imagine the ferret's skull to be composed of two parts,
one made from a sphere and the other from a tube, you would have a good
idea of the basic subdivisions of the skull. The sphere would represent
the cranial skeleton and the tube would represent the facial skeleton,
which includes the mandible and hyoid apparatus. Cranium can mean the
entire skull, the entire skull less the mandible, or just the cranial
skeleton. Calvaria (= calvarium, skull cap) means the roof of the
skull and is composed of the upper portions of the frontal,
interparietal, occipital, parietal and temporal bones (in a human, it is
the part you would find under your hat, at about eyebrow level).
The bones in the adult ferret cranium fuse so completely and the sutures
remodel so quickly that it is all but impossible to determine the exact
margins for any particular cranial bone. This makes identification of
the skull bones rather problematic, forcing the use of anatomical
position to recognize specific elements. In other words, while you can
identify the frontal or parietal region easily, you cannot easily
determine the exact margins of the frontal or parietal bone. Cranial
fusion begins soon after the skull reaches adult size, which is at about
five or six months of age, and is more or less complete by the time a
ferret is a year old. During this time the sutures, which mark the fused
bony joints between skull bones, begin to remodel. Remodeling means the
older bone is etched away and replaced with new bone, which effectively
erases the suture lines and hides joint margins. Because remodeling is a
continuous process at a constant rate (averaged over a year), it can be
used to estimate the age of the ferret. However, the technique is
limited because most sutures are fused and remodeledleaving a smooth
surface on the skullby the ferrets second or third year. Such complete
skull fusion and smoothing is very common among the Mustelidae, which
includes the weasels and polecats, because it gives so much strength to
the skull. This strength is necessary to reduce serious injury from
combative prey or predators, but mostly it provides support for
burrowing, the dental arcade and the muscles of mastication, which,
together with skull-mandible biomechanics, provide the ferret with one
of the strongest bitespound per poundin the Mammalia.
The cranial skeleton in the ferret is composed of seventeen bones in
total. Five bones are single and twelve are composed of six bilaterally
symmetrical pairs. The five single bones are the ethmoid (= os
ethmoidale), interparietal (= inca bone, os interparietale), occipital
(= os occipitale), basisphenoid (= os basisphenoidale) and presphenoid
(= os presphenoidale, os praesphenoidale). The paired bones are the
frontal (= os frontale), parietal (= os parietale), and temporal (=
squamosa, os temporale, os squamosum) bones, as well as the six auditory
ossicles. A common supernumerary (or extra) bone found in many mammals
within the sutures of the calvaria is the sutural bone (= suture bone,
ossa suturarum), but it cannot be seen in the cranium of the adult
ferret. However, they can be identified in very young ferrets prior to
fusion of the skull. The cranial bones form a strong vault which houses
and protects the brain, provides support for the face and anchors the
muscles of mastication. In the ferret, the cranium is narrow, long and
flat, which serves two important purposes. First, the shape is superbly
adapted for a subterranean lifeway, allowing the animal to pursue prey
down burrows, the ability to use the bridge of the nose to push dirt or
objects aside, and the placement of the all-important nose at the front
of the face. Second, it provides a large base of support for a massive
jaw and neck musculature, giving the ferret a tremendous bite as well as
the ability to carry large prey from deep within the bowels of the
earth. The large crestswhich mark the origin points of bulky
musclesare evidence of this massive musculature in ferrets. Paramount
among these are the sagittal and nuchal crests, which are the large
ridges of bone that are respectively found along the midline and back
edges of the skull.
The auditory ossicles (= ear ossicles, acoustic bones, ear bones,
ossicula auditus, bony middle ear) are composed of the incus (= anvil),
malleus (= hammer) and stapes (= stirrup), and are a characteristic of
the Mammalia (reptiles and birds only have a stapes, which in these
species is usually called a columella). These three bones are found
inside the tympanic bulla (= auditory bulla, bulla tympanica, pars
tympanica ossis temporalis) and are not accessibleor even visiblein
most skeletons. The tympanic bullae are thin-walled egg-like extensions
of bone, which project away from the middle ear region like a bubble,
and are part of an ingenious and elegant hearing system. The external
auditory meatus (= bony ear canal, external acoustic meatus, bony meatus
acusticus, meatus acusticus externus, externus meatus auditorius) faces
towards the ferret's nose, angled at about 15 to 20 degrees laterally.
At the end of the external auditory meatus, where it becomes part of the
tympanic bulla, are the eardrum and the attached malleus, which is
articulated with the other ossicles at an angle away from the rear of
the tympanic bulla. The elegant part of the design is that behind the
eardrum is open spacethe enlarged tympanic cavity inside the tympanic
bulla. The wall of the tympanic bulla opposite the tympanic membrane is
curved and acts like a sort of parabolic dish, which not only redirects
sounds that pass through the eardrum back to its surface, but also
focuses it, increasing tympanic membrane vibration and significantly
boosting hearing sensitivity. This is somewhat analogous to the
reflective layer in the ferret's eye, which bounces light back through
the retina, stimulating each rod twice, greatly increasing night vision.
The result is the ferret has greatly enhanced forward hearing, necessary
for locating prey in dark, sound-deadening burrows (there is also the
possibility that the ferret can rest their chin on the ground and
vibrations can be transmitted along the mandible to vibrate portions of
the middle ear; sort of a biological sound detector). Additionally,
because the right and left external auditory meatus are not placed on
the skull in a perfectly symmetrical fashion, sound in one ear arrives a
fraction of a second later in the other, which allows the ferret to use
the Doppler effect to better locate the source of sound. This is an
elegant, impressive system, and one that made the ferret perfect for
hunting rats and rabbitsand domestication. Who wouldnt want a
carnivore that could hunt in darkened burrows because besides their
searchlight nose, they owned a built-in auditory amplifier? Most
deafness in ferrets does not seem to be related to the structure of the
middle ear bones. I have specifically removed the auditory ossicles from
the skulls of ferrets known to have been deaf, and see no significant
difference in comparison to those from ferrets known to have had normal
hearing. I have found the malleus is occasionally fused to the incus.
Interestingly enough, the size and shape of these bones in the ferret is
not so different from those found in the human, despite obvious
differences in body and skull size. Because the tympanic membrane in the
ferret is at the end of a bony tube, the external auditory meatus, it is
usually not in danger of injury when cleaning wax from the ears,
providing earwax is not impacted into the ear canal. Ferrets with
histories of chronic ear infections or ear mite infestation have been
reported in the literature to have pathological changes to the auditory
ossicles, but I have never seen an instance. Aside from the occasional
fused malleus and incus, I have never seen a pathological condition
preserved in the bones, but I know they occasionally exist.
One of the more striking features of the cranial skeleton is the cranial
capacity; a measure of the cranial space that is more or less equal to
brain volume. The ferret has a cranial capacity far larger than would be
expected in a mammal of their size, and proportionately larger than many
other carnivores. The brain to body size ratio has long been used to
estimate intelligence (no one really agrees on a definition, so just
consider intelligence the ability to problem solve, memory, and
association skills). Intelligence in the ferret has been compared to
that of primates of the same size. All carnivores are intelligent, but
ferrets are particularly so. This makes intellectual stimulation a
particularly important aspect of ferret care.
(Synonyms cranial skeleton = braincase, cranial capsule, cranium, ossa
cranii, neurocranium; neurocranial bones).
The facial skeleton is composed of thirty-two bones in total. Two are
single and thirty are bilaterally symmetrical pairs. These include the
dorsal nasal conchae (= dorsal turbinate, superior turbinal, concha
nasalis dorsalis, endoturbinate I, nasal turbinate), middle nasal
conchae (= medial turbinate, medial turbinal, os concha nasalis media,
endoturbinate II) and ventral nasal conchae (= ventral turbinate,
inferior turbinal, os conchae nasalis ventralis, maxilloturbinate), the
hyoid apparatus (= hyoid bone, apparatus hyoideus, os hyoideum), the
incisive (= premaxilla, premaxillary, os incisivum) and lacrimal bones
(= os lacrimale), the right and left mandibles (= mandibula, lower
jawbone, dentary), the maxillae (= maxillary, os maxillare, upper jaw
bone), the nasal (= os nasale), palatine (= os palatinum, os
pterygopalatinum), and pterygoid bones (= os pterygoideum), the vomer
(= ploughshare bone, share bone), and the zygomatic bones (= os
zygomaticum, cheekbone, malar, os jugale, jugal). While most facial
bones do not fuse as rapidly as the bones of the cranial skeleton, they
are still pretty well fused by the end of the first year. Some of the
facial bones never fuse, such as the mandibles, which allow some
flexibility when biting hard objects, or during periods of extreme
biomechanical stress during the killing bite. The most obvious feature
of the facial bones is the pronounced nasal region, which houses and
protects the various nasal conchae (including some originating from the
ethmoid). The conchae support the mucus membranes richly embedded with
olfactory (smell) receptors, and represent approximately half the volume
of the facial skeleton. Relating this to the size of the olfactory
region in the ferret brain, the obvious conclusion is that smell is a
more important sense to these mustelids than vision. This is obvious to
any ferret owner who notices the first thing a ferret will do when
encountering a new object in their environment is to put their nose on it.
The primary purpose for the strength of both the cranial and facial
skeletons is to support the dental arcade. Like other members of the
weasel group, the polecat progenitor is a small mammal that hunts
animals as large or larger than its own size. Many prey have impressive
defensive mechanisms; ratters a century ago estimated that in a fight
between a ferret and a rat (Rattus norvegicus), the ferret was only
expected to win about half the time. Ratters used ferrets to chase the
rats into the teeth of terriers. European rabbits (Oryctolagus
cuniculus) might seem like soft bundles of fur on Easter morning, but
they possess powerful legs terminated with sharp claws that can
disembowel a ferret in a second. Both rats and rabbits have hard, sharp
teethcapable of shearing through human flesh and boneand crushing a
ferret skull would be quite simple. To counter this, the ferret has a
skull and hunting strategy designed to end the conflict as soon as
possible; they rush in, pin the prey with powerful legs connected to a
muscular body, then bite at the base of the skull with powerful jaws and
sharp teeth. Still, a great deal of struggle can occur, which is why it
is important for the ferret to have sharp teeth embedded in a strong
jaw. Because such large prey can twist or attempt to run, it places huge
biomechanical forces on the ferrets teeth and jaws. To counter this,
the ferret skull is designed to be stiff and strong in some areas
(cranial skeleton), yet flexible and giving in others (facial skeleton).
The result is a skull perfectly designed to maximize predation and
minimize injury.
This is best seen in the design of the ferret mandible. Normally, the
two bones are connected at the chin with a thin sheet of tough
connective cartilage. This holds the jaws together, but allows some
minor rotation. This is important when the ferret bites something hard
on one side of their mouth. If the two jawbones were fused, then as the
muscles of the jaw contract (both sides contract simultaneously), the
side impacting the hard object would stop moving, but the other side
would continue to try and close. This causes a twisting motion in the
jaw, which wastes energy and leverage, but more importantly, causes
great stress to the teeth and temporomandibular joint, where the
jawbones articulate to the skull. The design of the ferret skull has an
elegant solution. The temporomandibular joint is designed in such a way
that the mandibular condylar process (= articular condyle, mandibular
condyle, condylar process, condyloid process, processus condylaris) of
the mandible sits deep in the transverse, slot-like mandibular fossa (=
articular fossa, mandibular fossa, temporomandibular fossa, fossa
mandibularis) of the temporal bone. The mandibular fossa has an
extension, the postarticular process, which locks the mandibular
condylar process in position, preventing disarticulation while biting
hard objects, when shear forces are being applied as prey attempts to
escape, or when the jaws are widely opened, such as when yawning or
applying a killing bite. Still, biting on something hard, such as a bone
or bit of gravel, is tough on the jaw and teeth. When one of the
mandibles hit the object and stop, the other is allowed to rotate a tiny
bit at the mandibular symphysis, reducing mechanical stress and injury.
The hyoid apparatus is composed of nine bones; a single basihyoid (=
corpus, basihyal, basihyoideum, corpus ossis hyoidei, body of the
hyoid), and paired thyrohyoids (= thyrohyal, thyrohyoideum,
thyreohyoideum, cornu majus, major horn), ceratohyoids (= ceratohyal,
ceratohyoideum, cornu minus, lesser horn; sometimes misspelled
'keratohyoid'), epihyoids (= epihyal, epihyoideum), and stylohyoids (=
stylohyal, stylohyoideum) bones, and a short tympanohyoid (=
tympanohyal, tympanohyoideum) made of a rigid connective tissue which is
sometimes ossified. The base of the hyoid apparatus is deeply embedded
in muscles in the base of the tongue, and connecting it to both the base
of the skull at one end, as well as the larnyx at the other. Each hyoid
bone is articulated to the other within a small synovial joint, which
allows the hyoid apparatus great flexibility in movement, important for
swallowing and other throat actions. The stylohyals are attached to the
skull along the lateral aspect of the tympanic bullae with connective
tissue and joined to the skull by the cartilaginous tympanohyals. The
thyrohyals are connected to the larnyx at the thyroid cartilages. The
overall effect is that the linked bones of the hyoid apparatus suspend
the tongue and the larynx from the bottom of the skull. On occasion, the
thyroid cartilage or individual tracheal rings will partially ossify,
forming a semi-hard, spongy bone. I have never seen a healed fracture of
the hyoid, probably because the energy required to fracture them would
cause such harm to the rest of the upper throat that it would be fatal.
In a single individual that had suffered from a chronic throat
infection, I found signs of reactive bone on the basihyal. I have never
seen a bone mass affecting the hyoid apparatus, but suspect since these
bones are important for normal respiration and swallowing, bone masses
would probably result in serious dysfunction. (Synonyms hyoid apparatus
= apparatus hyoideus, os hyoideum, hyoid bones, tongue bones, throat bones).
(Synonyms facial skeleton = facies, ossa faciei; facial bones,
splanchnocranium, splanchnocranial bones).
Skull pathology is common, consisting mostly of reactive bone from
infections and inflammations, minor fractures, and on rare occasion,
bone masses. On a New Zealand feral ferret skull, there is a series of
small nodule-like raised areas near the nuchal crest, suggesting a
tubercular infection, but unconfirmed. I have found bone masses on four
pet ferret skulls; in once case, a veterinarian had removed the mass and
the bone was in the process of growing back at the time of death. Healed
fractures are uncommon, which may reflect bone remodeling hiding the
evidence, or it may simply suggest rarity. In two ferrets, arthritis was
present in the temporomandibular joint. In six skulls, healed fractures
were found in the mandibles, sometimes associated with loss of teeth. By
far the most common pathology is reactive bone, especially along the
maxillary and mandibular gum lines. New Zealand feral ferrets and wild
polecats also have minor amounts of reactive bone at that location, but
none to the same degree as found in pet ferrets. There is little doubt
the reactive bone found along pet ferret gum lines is due to
inflammatory gum disease (gingivitis) or infection (periodontitis). A
comparison between feral ferret and pet ferret mandibles and maxillae
demonstrate a significant difference in degree and extent of reactive
bone, with pet ferrets displaying a far greater degree of pathology. It
is probable the diet, or more specifically, the presence of processed
grains in the diet, is to blame.
Overall, I would say the most common pathologies are reactive bone,
fractures and bone masses.
Bob C