5 Minute Reminder: The Thoracic Skeleton

The canine skeleton is composed of three hundred and twenty one bones

Posted by Stephen on August 26, 2012

The canine skeleton is composed of three hundred and twenty one bones, of which there are five types: long, short, sesamoid, irregular and flat bones. The latter two are present as part of the vertebral column and skull, while the rest make up the limbs.

Thoracic vertebrae via Wikipedia

Thorax

The thorax is the region of the body between the abdomen and head. The apex of the thorax is located cranial and the thoracic inlet caudal. The first segment of the sternum starts at the thoracic inlet, the manubrium sterni. The thoracic cavity occupies only the cranial part of the thoracic skeleton. The caudal part of the bony thorax is actually the intrathoracic portion of the abdominal cavity. The thorax acts as a sling for abdominal viscera, offers protection to the spinal cord, and acts as a lever point for certain muscles to act.

Vertebral column

The vertebral column contains and protects the spinal cord. It is composed of cervical, thoracic, lumbar, sacral and caudal (coccygeal) vertebrae. The general formula for each type is 7 cervical, 13 thoracic , 7 lumbar, 3 sacral and 30 caudal.

In humans there are 12 thoracic vertebrae, in domestic animals 13 and in the horse 18. In large animals, such as horses and cattle, the typical number of lumbar vertebrae is around 6 while that for sacral is 5. For medium sized animals, such as pigs and sheep, the number of lumbar increase to 6-7 Lumbar and for sacral decrease to 4 . In small animals (dog and cat) the number increases again to 7 Lumbar and decreases to 3 sacral vertebrae. The number of caudal vertebrae can be quite variable between species.

Typical vertebrae consist of a body, arch or foramen that has both left and right pedicles and laminae. Transverse processes and spines can act as levers for muscles and ligaments. Each vertebrae has four articular surfaces, which have articular facets containing hyaline cartilage. Vertebrae have 4 articular surfaces that form synovial joints with other vertebrae. These are called interneural or facet joints.

Cervical vertebrae

Cervical vertebrae via Wikipedia

In cervical vertebrae articular processes are horizontal, in the thoraco-lumbar region they lie vertically. The spinal nerve and arteries pass through the intervertebral foramen of the cervical vertebrae. Nerves and veins exit and arteries enter through these foramina. A typical cervical vertebrae (such as the 5th) has four sides (a quadrangular shape) and has a transverse foramen on each transverse process. This allows the passage of the vertebral artery, vein and nerve trunk. Anatomical features include horizontal articular surfaces, the cranial of which face dorsally and the caudal ventrally, and a small spinous process.

Thoracic vertebrae

Typical thoracic vertebrae have a long spinous process, a short body and short transverse processes. The former thoracic vertebrae have spines that face caudally and gradually change angulation to a more perpendicular direction; this usually occurs at the 11th vertebrae. From the 11th vertebrae the spines point cranial and are described as being anticlinal from this point. Costal articular surfaces for articulation with ribs. Only vertebrae to have this feature. 8 articular surfaces.

Thoracic Vertebrae via Wikipedia

Lumbar vertebrae

The lumbar vertebrae have a longer body than the thoracic with long transverse processes and a shorter height. A synovial joint is present between the sixth lumbar vertebrae and the wings of the sacrum. In equidae the transverse processes of the fifth and sixth lumbar vertebrae articulate via a synovial joint.

Lumbar Vertebrae via Wikipedia

Sacrum

The sacrum is created through the fusion of the sacral vertebrae. Due to obliteration of intervertebral discs. This attaches to the ileum forming the sacro-iliac joint. The canine sacrum consists of 3 vertebrae, equine 5 and bovine 5 but fusion involves spines as well as bodies; forming the sacral crest. The main anatomical features include the sacral wings/ala, median sacral crest, dorsal sacral foramina and pelvic sacral formina. These latter two carry the dorsal branches and ventral branches of the first two sacral spinal nerves respectively.

Atypical vertebrae

There are approximately 20 coccygeal (caudal vertebrae). The first few are lumbar in appearance whereas the latter are simply rods.

Atypical vertebrae are also present; the first and second cervical vertebrae, the atlas, axis and the sacrum. The atlas does not have a spinous process and has a reduced body. Its modified articular processes allows flexion and extension forming the ‘yes’ joint. Transverse processes are increased in size and called wings. The axis has an elongated spinous process and it has a projecting forward process known as a dens. It forms a rotatory joint called the atlanto-axial joint that allows a ‘no’ movement. Nodding can occur due the articulation at the atlanto-occipital joint.

Embryology

The joint between vertebrae offers an anticoncussive function; this is a fibrocartilage joint. This fibrocartilage forms the intervertebral disc. The outer fibrous capsule consists of collagen fibres. The fibrous connective tissue portion is known as the anulus fibrosus. Inside this anulus fibrosis lurks the nucleus pulposus. This nucleus pulposus is a remnant of the embryonic notochord. If the pulpy nucleus herniates through the anulus fibrosus then this results in a condition known as slipped disc. This can occur dorsally, ventrally or sideways. If this occurs sideways then pressure is placed upon the sciatic nerve; causing sciatica.

Paraxial mesoderm of the embryo is seperated into somites. The sternum, ribs and vertebrae are formed from medial divisions of somites, known as sclerotomes. The muscles of the vertebral column are derived from the lateral division of somites, the myotome. The vertebral bodies develop from cells originating from sclerotomal division of somites. These cells are of mesenchymal origin; and they completely surround the notochord. The denseness of these developing cells decides which structure they will form; the vertebral body forms from a less dense accumulation of cells and the annuli fibrosi from an accumulation of increased density.

Ribs

Ribs consists of a head, tubercle,body and cartilagenous area. The cartilagenous joint meets the former at the costochondral junction. The head of each rib articulates with two vertebrae, which is known as the costo-central joint. The tubercle of the rib articulates with the articular surface of vertebrae forming a costo-transverse joint. Sternal ribs are ribs that connect to the sternum and floating ribs are those which connect neither to the sternum or sternal cartilage. The last sternabra is the xiphoid process, which continues as the xyphoid cartilage. The intercapital ligament passes through the intervertebral foramen to attach to the head of the rib at the opposite side.

Anterior surface of sternum and costa cartilages via Gray’s anatomy

The dorsal and ventral longitudinal ligament run above and below the vertebral canal respectively. The supraspinous ligament crosses above the summits of the spinous processes. In the cervical region this is modified to form the nuchal ligament. The nuchal ligament is more elastic and is a more complicated structure in larger animals due to the increased weight of the head. The interspinous ligament connects adjacent spinous processes and the interarcuate ligament is present between vertebral arches. The intertransverse ligament runs between the long transverse processes of the lumbar vertebrae.


References:

Dyce, K.M., Sack, W.O., Wensing, C.J.G., 2002. Textbook of Veterinary Anatomy. 3rd ed. Philadelphia, WB Saunders.
Pasquini, C., Spurgeon, T., Pasquini, S., 2007. Anatomy of Domestic Animals. 11th ed. Pilot Point, Tx: Sudz Publishing.