In addition to animating the skin, the position of some important facial structures is partly determined by underlying muscle tension. At rest, our muscles are not completely flaccid, but rather maintain a resting tension that enables our body to maintain position. For example, contracture of the frontalis pulls the eyebrows up, but at rest normal muscle tension holds the eyebrows up at a normal position. If the function of the frontalis is completely lost, such as is seen with severing of the temporal branch of the seventh cranial nerve, the eyebrow will droop downward (eyebrow ptosis) ( Fig. 2.2 ). Often, normal position results from the dynamic exchange of opposing muscle groups. Careful use of BoNTA to alter such relationships can be used to alter the position of structures such as the eyebrows and corners of the mouth.