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]