Costal arch

The costal arch is the curved ventral and lower margin of thoracic cavity formed by the cartilages of the false ribs (last sternal and asternal ribs.). These are typically the last few ribs (in carnivores, usually the 10th to 12th or 13th, depending on the species) that do not directly attach to the sternum but instead connect to each other to form a cartilaginous arch. This structure helps define the caudal boundary of the thoracic cavity and the cranial boundary of the abdominal cavity. The exact shape and rib composition of the costal arch vary among animal species (e.g., dogs, horses, cattle).