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 remodeled "leaving a smooth surface on the skull" by the
ferret's 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 bites "pound per pound" in 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
crests "which mark the origin points of bulky muscles" are 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 accessible " or even visible " in 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 space " the 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 rabbits " and domestication.  Who wouldn't 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 it's 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 teeth " capable of shearing through human flesh
and bone " and 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 ferret's 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
[Posted in FML issue 3487]