The anterior abdominal aponeuroses are located just superficial to the rectus abdominis muscle. The rectus abdominis is an important postural muscle. The rectus abdominis muscles are long and flat. It inserts into the linea alba behind the rectus abdominis. The rectus abdominis is similar in most vertebrates. It is located in the lower abdomen in front of the rectus abdominis. At the lower end of the rectus abdominis, the pyramidalis tenses the "linea alba". The rectus abdominis has many sources of arterial blood supply. Horizontal vergence involves mainly the medial and lateral rectus. For others, such as the rectus femoris or deltoid muscle, it becomes more complicated. Branches of the infraorbital artery supply the inferior rectus and inferior oblique muscles. It supplies the superior rectus and levator palpebrae superioris. The characteristic ocular abnormalities are nystagmus and lateral rectus weakness. Thus, the medial rectus is the muscle closest to the nose. Normally the tibia sits vertically above the calcaneus ("pes rectus").