Skeleton femoris: (Synonyms: pelvic limb, femoral appendage, leg skeleton,
crus bones).
 
Attached to the sacrum are the two pelvic bones, the os coxae ( = aitch
bones, innominate bones, hip bones, pelvic bones).  Each ox coxa is
composed of one minor and three major bones, all fused together to support
the weight of the animal while leaping, landing, running, and just plain
getting into trouble.  The ilium (= ilial bone, haunch bone, hook bone, os
ilium) is the more cranial of the four bones, and is attached to the sacrum
at the sacroiliac joint, which is supported by ligaments and rough bony
surfaces, but allows some movement between the sacrum and the os coxae.
The ilium is fused to the ischium (= ischial bone, pin bone, os ischii) in
the acetabular region, which is where you can find the fused remnants of
the acetabular bone (= os acetabulii).  The acetabular bone, only found
in immature individuals, is roughly triangular in shape, and makes up the
central floor of the acetabulum.  =46used to both the ilium and the ischium
is the pubis (= pubic bone, os pubis).  The pubis is roughly "C"-shaped,
and when fused to the other bones forming the os coxa, creates a large
opening, or hole, called the obturator foramen.  The medial aspect of the
pubis is straight and roughened where a cartilaginous joint is formed with
the pubis on the opposite side (in roughly 10% of ferrets or less, the
pubic joint will fuse, forming a hard, immobile joint.  I have found this
most common in older, whole males).  The acetabulum is the cup-shaped
socket which holds the head of the femur in place, and is formed by the
union of the four bones mentioned.  In ferrets, these are delicate bones,
and can display a wide range of pathology.  Fractures are fairly common
and usually occur in the pubis and/or ischium.  Occasionally, one or both
of the os coxae can permanently fuse to the sacrum, which interferes with
mobility and the ferret's ability to tightly curl up to keep warm.  The
acetabulum displays at least some minor pathology in 80% or more of the
ferret skeletons I've studied, usually at least some reactive bone or
arthritis.  Commonly, degenerative joint changes are present within the
acetabular cup, especially opposite the notch.  Minor infections are not
common, but not rare either.  In older female pet ferrets, especially those
who have had long term adrenal disease, the amount of osteoporosis in the
os coxae can be staggering; in several instances, one or both os coxae
have fragmented during normal skeleton preparation.  Most arthritis seems
centered around the acetabulum, but I have seen minor amounts of arthritis
and reactive bone near the sacroiliac joint.  As already mentioned, on
occasion, sometimes the two pubic bones fuse together.  In four
individuals, I have seen false sockets where the head of the femur has
slipped out of the acetabulum and formed a new cup on the ilium; 1 was a
feral ferret from New Zealand, and 3 were pet ferrets.  In one of the pet
ferrets with the false acetabulum, the head of the femur had broken off at
the neck and fused to the ilium, forming the edge of the new cup.  In two
cases, I found bone masses consistent with cancers.  (Synonyms: pelvis hip
bone, pelvic girdle, cingulum membri pelvini).
 
Male ferrets have a heterotrophic bone called the os penis.  It is a
slender, grooved bone with a hook at one end and an inflated base at the
other.  The base is embedded in a tendon which passes under the pelvis and
is attached to the caudal edge at the joint of the two pubic bones.  The
grove on the bone supports the urethra, maintaining patency during coitus.
The hook, about the same shape as the end of a crochet hook, supports the
glans penis and provides a hard substrate used to stimulate the nerve rich
portion of the female ferret's vagina, which helps induce ovulation (odor
and neck biting also help).  The hook also is the cause of many a male
ferret's (and owner's) fright as it is occasionally caught on wire cages
or lips of hard, sharp surfaces, such as bucket rims.  When this happens,
the entire penis can be pulled out of the sheath, and damage to various
tissues, the os penis, or the supporting tendon can occur.  Fractures to
the os penis are more common than you might wish to think; I have seen 5
healed fractures in approximately 350 skeletons.  In each case, the
fracture was well healed, meaning the ferret survived long after the
accident.  The dangers associated with a fractured os penis are damage to
the nerve or blood vessels supplying the penis, damage to the urethra, or
damage to the spongy tissue itself.  The os penis is one of the few bones
that get larger as the ferret gets older, which provides a reliable method
of aging based on the size of the base, but neutering makes the os penis
smaller, so appropriate adjustments have to be made.  Once, I saw reactive
bone tissue on the ventral edge of the os penis.  On rare occasions (less
than 5%), female ferrets have an irregular bony os clitoris.  I have only
seen a few os clitorae and none possessed pathology.  (Synonyms os penis
baculum, os genitale, L'os penien, penis bone, Penisknochen; os clitoris
clitoral bone)
 
The femur in ferrets is fairly straight, has a large head and a huge
trochanter for the insertion of the hip muscles.  There is a lateral
fabella present behind the lateral condyle, and in a few skeletons, I've
found a smaller medial fabella.  The patella is robust, as would be
expected in such a heavily muscled animal.  The femur is one of the most
commonly damaged and pathological bones I've seen in ferrets, having a
12%+ injury rate in the skeletons I've studied.  Fractures are common and
seem to occur most often on one of three areas; the neck, the proximal
shaft, and those involving one or both condyles.  In a third of the
fractures involving the neck of the femur, a fracture to the diaphyseal
shaft was also present.  Condylar fractures tended to occur most often to
the medial condyle.  Arthritis was present in 80% or more of femurs from
ferrets older than 3 years of age, and centered on the articular edges of
the caput (head) and condyles.  In some cases, reactive bone was found on
the head and neck, but was more common in the area of the condyle.  In one
case, the ferret sustained a spiral fracture to the proximal diaphyseal
shaft which never healed properly, forming a functional false joint.  In
this individual, there was an extreme amount of reactive bone tissue
present, suggesting a chronic infection.  In four individuals, the femoral
head slipped out of the acetabulum and formed a false socket on the ilium.
On two femurs, a bone mass as present, suggesting cancer.  (Synonyms:
femur os femoris, roundbone, thigh bone, leg bone; patella = knee cap, os
patellae; fabella = condylar sesamoids, lateral/medial femoral sesamoids,
os sesamoideum m.  gastrocnemii).
 
The tibia is the longest bone in the ferret, with a large, flatish
"head" composed of the medial and lateral condyles and a large, pointed
intercondylar area separating the two.  There is a notch which accepts the
head of the fibula, a long, thin bone which attaches to the tibia at two
points; proximally at the lateral condyle and distally with the lateral
aspect of the medial malleous.  Tibial fractures are common, but not as
common as femoral fractures.  Most fractures are low on the tibal
diaphysis, near the ankle, and might be caused by being stepped on;
however, the same sort of injury may be cause by falls.  In most cases
where the tibia is fractured, the fibula is also.  In two cases, the
fibula fused to the tibia at the site of fracture.  Arthritis is fairly
common in older ferrets, especially along the edges of the condyles.  Some
minor arthritis on the medial malleous has been found in older ferrets.
In one case, the lateral malleous of the fibula was fused to the medial
malleous of the tibia, with lots of reactive bone tissue present.  I
suspected a massive infection, but couldn't rule out a possible old
fracture.  I have seen 3 cases of bone tumors or cancers on the medial
upper diaphysis of the tibia.  (Synonyms: tibia = shin bone, os tibiae;
fibula = brooch bone; pin bone, os fibulae).
 
The ferret has seven tarsal bones, arranged like most other carnivores
(PROXIMAL ROW: ASTRAGALUS = talus, ankle bone, knucklebone, os tali,
tibial tarsal bone [NOTE: talus is historically reserved for primates,
but modern anatomists are moving away from astragalus]; CALCANEUS = heel
bone, calcaneum, os calcis, fibula tarsal bone.  MEDIAL ROW: NAVICULAR os
naviculare, central tarsal, centrale, os tarsi centrale, scaphoid.  DISTAL
ROW: MEDIAL CUNEIFORM = T1, os tarsale I, os cuneiforme mediale, 1st
tarsal bone; INTERMEDIATE CUNEIFORM = T2, os tarsale II, os cuneiforme
intermedium, middle cuneiform, 2nd tarsal bone; LATERAL CUNEIFORM = T3,
os tarsale III, os cuneiforme laterale, 3rd tarsal bone; CUBOID = T4, os
tarsale IV, os cuboideum, 4th tarsal bone).  The cuneiforms are variable in
shape, but the astragalus and calcaneus are distinctive enough for species
identification, with the cuboid and navicular slightly less so.  I have
seen six fractures of the ferret talus; all resulted in moderate to
extensive fusion, with at least minor formation of reactive bone tissue.
Sometimes two cuneiforms are naturally fused together.  Signs of arthritis
on the bones are rare, except in the calcaneus and astragalus, where it is
fairly common in older ferrets.  Sometimes short bone spurs form within
the Achilles tendon at the point of the hock (the caudal part of the tuber
calcanei).  I have never seen a bone mass in the talus.  I suspect the
large number of talus fractures are due to being stepped on, being caught
in doors, or being trapped within narrow crevices.  (Synonyms: tarsal
bones = ossa tarsi, ankle bones, tarsus, hock).
 
Like in the manus (hand), the ferret has five metatarsals in their pes
(foot).  The metatarsals are numbered from 1 to 5, starting at the big
toe (1st metatarsal, MT1, os metatarsale I).  Metatarsals seem to be
injured frequently, based on healed fractures and reactive bone.  In
several cases, I have seen two or more metacarpals fused together midshaft.
In one case, four metacarpals were fused at the proximal end.  A plantar
sesamoid is generally present, although I once found two in a single
individual.  There are two small sesamoids between each metatarsal and
first phalanx, which are rarely fused to the metatarsal or the 1st phalanx.
Signs of infection are uncommon.  I have never seen a cancerous growth.
(Synonyms: metatarsus = foot bones, ossa metatarsalia, pastern bones,
rays).
 
The phalanges of the foot follow those of the hand with no major
difference.  The reader is referred to the description of the thoracic
skeleton, bearing in mind subtle differences exist between the pes and
the manus.
 
Overall, I would say the most common pathologies follow those for the
thoracic skeleton, and are (in order) healed fractures, arthritis,
reactive bone, bone masses.
 
Bob C
[Posted in FML issue 3481]