ABSTRACT
This chapter provides evolutionary theoretical background for variations and defects of head and neck muscles; and summarizes each variation and defect found in every skeletal muscle. The occipital and frontal components of occipitofrontalis may be referred to as two separate muscles: occipitalis and frontalis. Occipitofrontalis consists of the occipitalis and frontalis muscles, connected by the epicranial aponeurosis. Occipitofrontalis is innervated by the facial nerve, the occipital portion via the posterior auricular branch and the frontal portion via the temporal branches. The muscular portions of occipitofrontalis may fuse or decussate across the midline. Occipitalis may join with auricularis posterior or receive fibers from sternocleidomastoid. Understanding variations in frontalis decussation is helpful for administering botulinum toxin type A therapies. Understanding variations in the presence and presentation of malaris is helpful for planning and administering periorbital botulinum toxin type A.
