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]