Each thoracic vertebra articulates with a rib on either side. Therefore there are 18 pairs of ribs; these may be classified as sternal (true), asternal (false) or floating. The ribs form a bony cage to protect the lungs and heart; they are also an essential part of the breathing apparatus. The ribs contain spongy red bone marrow and are an important source of blood cells for the horse. In young horses, the ribs are quite soft, but as the horse gets older they become calcified and harden.
There are eight pairs of sternal or ‘true’ ribs. Each rib head and tubercule forms a joint with each thoracic vertebra. The springiness of the rib is a property of its shaft or bony body. Each rib ends in a costal cartilage which is attached to the sternum.
Asternal or false ribs
The costal cartilages of ribs 9–18 are united by elastic tissue to form the costal arch. In turn, this is connected by fibrous tissue to the costal cartilage of rib eight.
These do not reach the costal arch, ending freely in the musculature.
Sternum (Fig. 5.3)
The floor of the chest is formed by the sternum, which is held in position by the first eight pairs of ribs. The sternum is made up of a number of bones (stenebrae) held together by cartilage with which the sternal ends of the true ribs articulate, except for the first pair which articulate with the first sternebra (manubrium).
The rear end of the sternum is a flattened and heart shaped cartilagenous structure known as the xiphoid process to which muscle fibres of the diaphragm are attached.
Between the curved ribs lie the intercostal muscles which move them in a forward and outward direction helping the horse to breathe in.
The diaphragm is a sheet of muscle and tendon separating the chest and abdominal cavities. Peripherally it is attached to the ventral surface of the lumbar vertebrae, the ribs and the sternum: the muscle fibres arise on these skeletal parts and radiate towards the tendinous centre. It projects forward into the chest cavity like a dome as far as the level of the seventh rib; this is almost opposite the olecranon in the standing horse.