A human skull has many areas that have distinct names and significance. The bony area atop the upper ear canal is known as the porion. It is located interiorally behind the tragus, the small inner portion of the ear that protrudes from the head. Scientists use this structure as a reference point when studying the skull and skull measurements.
Reference points and planes are designations that are used in cephalometrics or craniometrics, which are methods of skull measurement. This field makes use of cephalograms, which are pictoral representations of the skull and its components. Porions and other reference points are visible on these films, and physicians use them to help map the skull and catalog any changes or abnormalities in individual skulls. The porion might appear as a bony region around either ear canal area on films. On occasion, the word "porion" might refer to the earpost of a head positioning machine called a cephalostat, and in these cases, it is known as a machine porion.
Each porion comprises part of the Franfurt plane. This largely abstract designation is intended to help fix the location of the skull in anatomical terms. It is an approximation of how a person's head is carried on the body. In addition to the skull’s two porions, a portion of the skull area that contains the eye sockets — the left orbit — is also used to determine the Frankfurt plane.
Anthropologists often use the porion to compare the anatomy and physiology of humans with those of other living organisms. It also can be a comparative tool for distinctions between humans, such as giving insight into human ancestors and into how the human skull might have evolved over time. For example, scientists often use skulls to distinguish the current human species Homo sapiens from earlier, similar species such as Homo erectus. Several anthrometrical methods can be used, like cataloging the amount of space between the porion and a structure near the back of the skull called the asterion. This particular measurement constitutes the Mastoid Index.
Porion-related skull measurements and designs have more practical uses as well. People who are involved in forensic studies can use distinct characteristics to identify human remains, for example. Specific traits such as age, size and race can often be inferred from these considerations. Surgeons or other medical professionals might use porions as reference points when conducting medical procedures. More controversial applications also have occurred throughout history, including the use of skull measurements for intelligence testing or for determining personality traits.