with the stylomastoid foramen and rudimentary styloid immediately behind the tympanic ring. With air cell development, the outer part of the mastoid component grows anteroinferiorly to form the mastoid process, with the styloid and stylomastoid foramen now on the under surface. The descent of the foramen is accompanied by a requisite lengthening of the facial canal. The downward and forward growth of the mastoid process also pushes forward the tympanic part; as a result, its portion that formed the original floor of the meatus, and contained the foramen of Huschke, rotates to become the anterior wall. The fossa subarcuata is nearly effaced. Trauma Temporal bone fractures were historically divided into three main categories, longitudinal, in which the vertical axis of the fracture paralleled the petrous ridge, horizontal, in which the axis of the fracture was perpendicular to the petrous ridge, and oblique, a mixed type with both longitudinal and horizontal components. Horizontal fractures were thought to be associated with injuries to the facial nerve, and longitudinal with injuries to the middle ear ossicles. More recently, delineation based on disruption of the otic capsule has been found as more reliable in predicting complications such as facial nerve injury, sensorineural hearing loss, intracerebral hemorrhage, and cerebrospinal fluid otorrhea. Other animals In many animals some of these parts stay separate through life: Squamosal: the squama including the zygomatic process Tympanic bone: the tympanic part: this is derived from the angular bone of the reptilian lower jaw Periotic bone: the petrous and mastoid parts Two parts of the hyoid arch: the styloid process. In the dog these small bones are called tympanohyal (upper) and stylohyal (lower). In evolutionary terms, the temporal bone is derived from the fusion of many bones that are often separate in non-human mammals: The squamosal bone, which is homologous with the squama, and forms the side of the cranium in many bony fish and tetrapods. Primitively, it is a flattened plate-like bone, but in many animals it is narrower in form, for example, where it forms the boundary between the two temporal fenestrae of diapsid reptiles. The petrous and mastoid parts of the temporal bone, which derive from the periotic bone, formed from the fusion of a number of bones surrounding the ear of reptiles. The delicate structure of the middle ear, unique to mammals, is generally not protected in marsupials, but in placentals, it is usually enclosed within a