From:
RRC
Date: 2001-07-13 12:41:00 UTC
Subject: Bob C: Ferret Skeleton: Thoracic Limb
Q: "Your post on ferret vertebra was wodnerful! Would you do it for the
rest of the skeleton?"
You are asking a guy who collects, cleans, identifies (element, species,
sex, age, taphonomy, pathology), studies, measures, draws, photographs,
labels, and curates bones to TALK about them? Are you crazy? Do you
realize what you have caused??!! How about I do it over several days to
minimize the trauma for those not so interested?
For ease of study, the ferret skeleton can be divided into two major
categories: the axial and the appendicular skeleton. The axial skeleton
is composed of all the bones found along the body's axis, which include
the skull, mandibles, auditory ossicles, teeth, hyoid apparatus,
vertebrae, ribs and sternebrae. The appendicular skeleton is basically
everything else, which includes the scapula and clavicle, the os coxae,
the bones of the thoracic and pelvic limbs, sesamoids, and genital
bones. Sometimes dermal bones are present, such as in the armadillo, but
since ferrets never evolved them, they won't be discussed. The sesamoids
and genital bones are actually part of the heterotrophic skeleton, which
are bones that are not associated with either the axial or appendicular
bones, and include the os cordis (a heart bone common in horses, cattle
and other large ungulates), the os rostrale (a rostral bone common in
pigs, occasionally cows, and other species), sesamoids (found inside
muscle tendons and include the patella and the fabella) and the os penis
(baculum) and os clitoris. However, for THIS discussion, I shall include
the heterotrophic bones within the appendicular skeleton (just be aware
they are heterotrophic, not appendicular).
Skeleton brachii: (Synonyms: thoracic limb, pectoral appendage, arm skeleton).
Contrary to popular opinion, the ferret has a clavicle! However, it is
usually cartilage or only partially ossified. It is a rather small (4-8
mm) "J"-shaped bone or cartilage rod, and is rarely present in curated
skeletons (it is usually lost during preparation). If completely or
partially ossified, then it is clearly visible on X-ray. While it has
little function in supporting the pelvic girdle (as does the clavicle in
humans), it does help support muscles in the neck and shoulder (which is
probably why it has been retained; it acts like a sort of sesamoid
bone). Clavicles are absent or diminished in carnivores to increase
thoracic limb mobility. This is very important because it makes it
easier for ferrets to turn around in tight spaces, allows greater
mobility in using the hands and arms to obtain food, and increases the
arc of movement in the front limb. This last reason is very important in
a short-limbed creature like the ferret because it allows for a greater
stride in running. I am unaware of pathologies in the ferret clavicle,
and have never heard of an instance where it has been fractured.
(Synonyms: clavicle = clavicula, collarbone).
The scapula supports the thoracic limb, but unlike the pelvis, it is not
directly connected to the spine. Instead, it is held in place with a
series of muscles, tendons and ligaments, which allow the scapula to
freely move with the appendage. This increases stride and arc of
movement, both very important in short-limbed ferrets. The blade of the
scapula is roughly shaped like a bee's wing, with a smooth oval area at
the small end where it connects to the humerus. Scapular bone is very
thin in ferrets, and sometimes not all areas within the blade are
completely ossified. Even though the scapula is quite thin, it is
typically protected from most injury because it is heavily muscled on
the top, which protects the bone from most fractures. I have only seen a
single healed injury on a scapula; a healed fracture of the acromion
process. However, I often find injuries to the scapula in ferrets which
died of trauma. I suspect most injuries which could seriously damage the
scapula are usually terminal for ferrets. It is possible cancerous
growths could occur in the scapula, but I have never seen one.
(Synonyms: scapula = shoulder blade, shovel bone).
The humerus in ferrets is fairly stout as a result of heavy neck,
shoulder and upper arm muscles. I have seen large deltoid tuberosities
in whole males, but both the male and female ferret humerus show
pronounced muscle insertion scars. Healed fractures of the humerus seem
to be fairly rare in ferrets; most I have seen are found in terminal
trauma cases. This is probably because the energy required to fracture
the humerus is sufficient to damage the contents of the thoracic cavity
and few ferrets survive. In the three healed humeri fractures I have
studied, all were distal, near the condyle. All were displaced spiral
fractures which healed well, but resulted in some degree of shortening
or misalignment of the skeletal element. In one, the trochlea was
heavily furrowed where the fracture had passed through the joint,
resulting in massive arthritis and reactive bone. In a single case, a
bone mass near the deltoid tuberosity was present, which was
approximately a centimeter in diameter. Quite often, arthritic lipping
or the formation of bony projections can be found along the edges of the
head and trochlea in older ferrets, and occasionally you can see scoring
or furrowing on the articular surfaces of the trochlea. (Synonyms:
humerus = arm bone, clod bone).
In ferrets, the ulna is large, with a massive olecranon process, which
make sense considering the amount of digging done by polecats. It has a
proximal and distal articulation with the radius, and both make up the
bones of the forearm. Both bones slightly bowed, and are heavily
muscled, which gives the ferret Popeye forearms. In ferrets, the radius
and ulna can easily cross over the other, which allows the ferret to
turn their hand palm-side up. This allows the ferret great flexibility
in manipulating prey, digging, climbing, and making mischief, and makes
their wrist and hand bones structurally similar to bears, raccoons and
humans. I have found a large number of healed fractures of the radius,
ulna or both in pet ferrets, which are probably a result of being
stepped on or having their arm caught in a door. I have hypothesized
that vets can identify radioulnar injuries caused by a ferret being
stepped on as opposed to those from a swinging door by the following
criteria: Stepping injuries = ulna and/or radius broken midarm to distal
with force progressing from cranial to caudal; door injuries = ulna
and/or radius broken randomly with mediolateral force. In three cases,
the fractured radius and ulna fused together. In another case, the
proximal radius healed as it if was amputated and the distal portion
fused to the ulna. Arthritic lipping is common in older ferrets along
the proximal articular surfaces of both bones. Sometimes furrowing or
scoring is present on the proximal articular surfaces. On rare occasion,
reactive bone is seen at the elbow joint, usually on the olecranon. I
have not seen a cancerous growth in the ulna or radius, but suppose it
is possible. (Synonyms: ulna = elbow bone, funny bone; radius = radial bone)
The ferret has seven carpal bones and one sesamoid bone in the wrist,
and they are arranged like most other carnivores (PROXIMAL ROW:
SCAPHOLUNATE = os scapholunatum, os carpi intermedioradiale,
radiointermediate, radial carpal; TRIQUETRUM = os triquetrum, ulnar
carpal, os carpi ulnare, triangular, cuneiform; PISIFORM = accessory
carpal, os carpi accessorium, os pisiforme. DISTAL ROW: TRAPEZIUM = C1,
1st carpal, os trapezium, os carpale I; TRAPEZOID = C2, 2nd carpal, os
trapezoideum, os carpale II; CAPITATE = C3, 3rd carpal, os capitatum, os
carpale III, magnum; HAMATE = C4, 4th carpal, os hamatum, os carpale IV.
Medial Sesamoid = os sesamoideum medialis). I have seen two fractures of
the ferret carpus; both resulted in extensive fusion and formation of
reactive bone tissue. The bones form distinctive shapes and can be
identified from each other, BUT, those shapes have a moderate amount of
variability, so identification from position is preferable (especially
of the distal carpals). Sometimes two (or on rare occasions, three) are
naturally fused together. Signs of arthritis on the bones are rare, even
in older ferrets. Again, I have never seen a cancerous growth in the
wrist bones, but it is possible I suppose. (Synonyms: carpal bones =
ossa carpi, wrist bones).
The ferret has five metacarpals in their palm, with two (rarely 3)
palmar sesamoids. The metacarpals are numbered from 1 to 5, starting at
the thumb (1st metacarpal, MC1, os metacarpale I). I have seen a lot of
healed fractures of the metacarpals, as well as a lot of reactive bone.
In several cases, two metacarpals have fused together midshaft. In
another case, three metacarpals have fused at the proximal end. There
are two small sesamoids between each metacarpal and first phalanx, which
are on rare occasion fused to the metacarpal (or the 1st phalanx). Signs
of infection are not common, but not rare either. I have never seen a
cancerous growth, but assume it can occur. (Synonyms: metacarpus = hand
bones, ossa metacarpalia, pastern bones, rays).
The ferret has the same number of finger bones as humans; 2 on the thumb
and 3 on each finger; the third one is compressed mediolaterally to form
the support for the claw. The phalanx closest to the metacarpal is the
proximal phalanx (= P1, 1st phalanx, phalanx proximalis, os compedale).
The middle bone is the medial phalanx (= P2, 2nd phalanx, phalanx media,
os coronale, middle phalanx). The third bone, the one with the claw, is
the distal phalanx (= P3, 3rd phalanx, phalanx distalis, os unguiculare,
terminal phalanx, ungual bone). "First phalanx" means the first bone
past the metacarpal, but which one? There are various ways to
distinguish this, but I prefer "first phalanx third metacarpal." (=
P1MC3, first phalanx third ray). Ray is commonly used as a synonym for
metacarpal (or metatarsal), but I find the use of metacarpal allows
immediate knowledge of the exact location (if you say "right 1st
phalanx, third ray", is it the hand or foot?). The proximal and medial
phalanges are very similar in form to humans, bear and raccoon, because
the ferret, unlike the dog or cat, walks on the entire foot
(plantigrade). The distal phalanges support the claws, and are modified
with a large ungual crest. You can see this part of the bone when the
claw is torn off; it is the pink "quick" you see from the side, through
the claw (bone is VERY vascular, and tiny blood vessels run under the
bottom of the crest, which is why toe clipping can be used to get blood
samples). Technically, the claw is the horny "fingernail" and the bone
is the distal phalanx; the phalanx is NOT the claw, but supports it. A
pair of sesamoids are found between each metacarpal and the proximal
phalanx, and on occasion, some tiny ones can be found between the
proximal and medial phalanx. I have seen a lot of broken and healed
toes, lots of reactive bone, and lots of minor arthritis. I have never
seen a bone tumor. (Synonyms: phalanges = finger bones).
Overall, I would say the most common pathologies (in order) are healed
fractures, arthritis, and reactive bone (generally results from
infections or inflammations). I have seen very few bone masses
(tumors/cancers) in the thoracic limb skeleton.
Bob C