Skeleton thoracis: (Synonyms: chest bones, bony thorax, thoracic skeleton,
rib cage, pectoral skeleton, thoracic girdle, thoracic cage).
 
The thoracic skeleton contains parts of both the appendicular and axial
skeletons, and consists of the costae, thoracic vertebrae, clavicle,
scapula, and sternibrae.  In mammals, the thoracic skeleton has
multipurpose functions: it makes negative pressure respiration possible,
it protects the heart and lungs, and it provides locomotor support, except
in those species where the appendages are no longer used for movement, in
which case it just supports the arms.  I have already discussed the
thoracic vertebrae, clavicle and scapula, and refer the reader to earlier
posts on those subjects.
 
In ferrets, the sternum is composed of eight bones, called sternibrae
(sternal bones, chest bones, sternal units, ossa sternibrae).  The most
cranial bone (that is, closest to the skull) is called the manubrium, and
it is usually fused to the second sternibrae to create a long, arrow-shaped
bone, which starts at the base of the throat and extends caudally (that
is, towards the tail) perhaps an inch in large male ferrets (maybe half
that in smaller female ferrets).  The other six sternibrae are separated
by an intersternal cartilage, which are also connected to the ends of
associated ribs.  On occasion one or more sternibrae can be found fused
(this appears to be an ossification of the intersternal cartilage).  The
xiphoid process in the ferret usually remains a flattened cartilage rod,
but in very rare instances, it is partially or completely ossified.  The
flexibility of the sternum is important to ferrets, allowing the greatest
chest volume possible, while remaining flexible enough to allow the ferret
into spaces typically only large enough to admit the head.  This can be
best noticed from a side view of an articulated ferret skeleton, which has
a thoracic cavity possessing a very small cranial aspect (thoracic inlet),
but a very large, wide caudal aspect.  If the head can get in, so can the
upper chest because of the flexibility of the ferret sternum, as well as
the small size of the 1st ribs, and the rest of the chest can compress to
make it through.  I have never seen a healed fracture of a sternibrae, but
I have seen reactive bone tissue around one or more of the sternibrae
indicating some sort of inflammation or infection.  I suspect the
sternibrae would separate at the intersternal cartilage prior to fracturing
if sufficient impact energy was applied, which would probably be fatal in
any case.  I have seen three bone masses attached to or associated with
the sternum (one sternibrae had a tiny stalk of bone tissue attached to a
larger partially ossified mass which was probably within the mediastinum,
but because of the nature of skeletal preparation, not confirmed).  In
older ferrets, there is frequent partial ossification of the intersternal
and costal cartilages, as well as some bone growth and spurs on the
articular surfaces of the sternibrae.  (Synonyms sternum = breastbone,
chestbone, os sternales).
 
Ferrets typically have 15 pairs of ribs, although I have found that number
to range from 14-16 pairs in about 17% of the population I've studied.
Not all ribs are found on the thoracic vertebrae; supernumerary ribs are
sometimes found on the 7th cervical and 1st lumbar vertebrae (rarely found
on the 2nd Lumbar).  Sometimes a ferret will have 14 ribs on one side and
15 on the other.  One ferret skeleton I examined had 18.5 pairs of ribs;
one pair of cervical ribs, 16 pairs of thoracic ribs, and a pair of lumbar
ribs on the 1st Lumbar, with a single, short rib on the 2nd Lumbar.  Even
though there is a wide variation in costal number, the vast majority of
ferrets have 15 pairs of ribs.  True ribs are those which are directly
attach to the sternum, which in the ferret are the first 10 pairs
(sometimes only 9 pairs).  False ribs are those which attach to each other
by way of costal cartilages, forming the costal arch, and in the ferret
are generally the last 4 or 5 pairs.  Floating ribs are not attached to
each other with cartilage, and sometimes the last pair of ribs in ferrets
float, lacking distal attachments.  The supernumerary ribs found on lumbars
generally float.  The occasional cervical rib can float, but generally will
act like an extra set of sternal ribs, either attaching directly to the
manubrium, or to the same costal cartilage as the 1st ribs.  Ferrets have
a rapid growth spurt at about 3 months of age, which usually results in a
swelling on the end of the rib where it fuses to the costal cartilage.
This is common, and sometimes the swelling can be felt on either side of
the sternum as a series of small bumps (this type of swelling is so common
in rapid-growth mammals that the presence can be used to age a single,
isolated rib as coming from a juvenile).  This is normal in growing
ferrets, but abnormal in adults, where distal costal swelling can be an
uncommon sign of chronic anemia (ribs are filled with red bone marrow, and
under the stress of long term anemia, the ends can enlarge on rare occasion
to increase blood cell production.  I have only noticed this twice, both
times in ferrets suffering from long term, moderate anemia).  Ribs are
commonly injured, and multiple fractures on one side are not uncommon (I
once found a series of 7 broken ribs on the right side of a single ferret
skeleton).  In three cases, I found one or more ribs possessed a large,
inflated area filled with debris; probably due to an infection with in a
fracture.  In a New Zealand feral ferret skeleton, I found bone changes in
the rib cage consistent with tuberculosis.  I have seen several ribs with
the types of bone masses typical of cancers.  Arthritic lipping and spurs
are commonly found in older ferrets, especially in the distal sternal ribs.
Sometimes the head of a rib is injured, and reactive bone can cause it to
fuse to the associated vertebrae.  In one case, I found a 4th rib broken at
the neck, and fused to the thoracic vertebra approximately 2-3 mm down from
the cranial costal facet.  (Synonyms ribs = costae, costals, os costae,
thoracic ribs.  True ribs = costae sternales, costae verae, sternal ribs;
false ribs = costae spuriae; floating ribs = costae fluctuantes).
 
Overall, I would say the most common pathologies follow those for the rest
of the skeleton skeleton, and are (in order) healed fractures, arthritis,
reactive bone, and bone masses.
 
Bob C
[Posted in FML issue 3481]