The serratus posterior inferior muscle is one of the back's two intermediate muscles (the other is called the serratus posterior superior muscle). The serratus posterior inferior muscle is located in the spine at the back, in the upper portion of the thorax. It branches off of the ventral primary rami, a section of spinal nerves (T9 to T12). It originates in the thoracolumbar fascia, an area made up of spinal processes featuring the vertebrae ligaments T11 to T12 and L1 to L2. The serratus posterior inferior muscle is thin and inclines downward, where it becomes more muscular. It is supplied blood by the lowest posterior intercostal artery, the subcostal artery, and the first two lumbar arteries. The serratus posterior inferior muscle is a respiratory muscle that inserts in ribs nine to twelve at lateral angles. The function of the serratus posterior inferior muscle is to pull down the lower ribs, assisting with forced expiration. This muscle receives ventral ramus innervation that is related embryonically to the frontal portions of the intercostal muscles.