Costae

Arranged in pairs and articulate with two successive vertebrae
Bony dorsal part, body of rib, and ventral costal cartilage
Increase in length, curvature and amount of cartilage craniocaudally
Cartilage of last rib may fail to join that of its neighbor: said to be floating
Join ventrally on the midline at the Sternum, which is comprised of three parts
- Manubrium: most cranial, projects beyond the first set of ribs and can be palpated
- Body: segmented sternebrae joined by cartilage in young animals that is later replaced by bone
- Xiphoid Cartilage: caudal end that projects between lower ends of costal arches, providing attachment for the linea alba
Costal Joints:
- Costovertebral joint: head of rib articulates with vertebral column, ball and socket with very restricted mobility
- Costotransverse joint: tubercle articulates with vertebra, sliding joint
- Costosternal joints:
  - Interchondral joints: asternal ribs, elastic syndesmoses
  - Intersternal joints: impermanent synchondroses

Thoracic Musculature

Primarily concerned with respiration
- Inspiratory muscles enlarge the thoracic cavity
- Expiratory muscles diminish the cavity and force air out
The most important thoracic muscle is the Diaphragm, which separates the thoracic and abdominal cavities
- Dome-shaped, convex on its cranial surface
- Central tendon forms the vertex
  - Neutral position (between full inspiration and full expiration): 6th rib behind the olecranon
- Attaches via muscular periphery to the costal arch
- Three openings:
  - Aortic hilus conveying the aorta, azygous vien, and thoracic duct
  - Oesophageal hiatus conveying the oesophagus, vagal trunks and supplying vessels
  - Caval foramen withing central tendon conveying caudal vena cava
- Innervated by the phrenic nerve, which arises from the caudal cervical nerves (C5-C7)
Intercostal muscles
- External fibers run caudoventrally and internal fibers run cranioventrally
- Each is confined to a single intercostal space
Transversus thoracis arises from and covers the dorsal sternum and inserts on sternal ribs close to the costochondral junctions
Rectus thoracis covers the ends of the first four ribs in continuation of the rectus abdominus
Serratus dorsalis overlies the dorsal aspect of the ribs
Innervation of these muscles is supplied by the Intercostal nerves, which are ventral branches of the thoracic spinal nerves