Message Number: YG5562 | New FHL Archives Search
From: RRC
Date: 2001-07-16 10:55:00 UTC
Subject: Bob C: Ferret Skeleton: Thoracic Skeleton

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