Pinna, derived from the Latin word for feather, are what a casual observer usually refers to when they are pointing to your ears. The pinna, or auricles, as they are scientifically known, are the visible parts of the ears located on the outside of the head. The biological structure of the outer ear is composed mainly of cartilage, which provides the pinna with a great deal of flexibility and ability to be positioned for optimal listening.
The anatomy of the human pinna is split up into several major components: the helix, the antihelix, the concha, the tragus, and the lobe. The helix is the outer edge of the ear, which usually folds down and inward from the top. The antihelix is the Y-shape that is located just below the helix, and is the second-highest part of the ear. The concha is the hollow part of the ear located right next to the ear canal, and serves as the entrance to the inner ear. It is usually slightly covered by the tragus, the small flap of cartilage that faces backward. When listening to music through earbuds, the tragus is the protrusion that holds the earbuds in place. The final component, the lobe, is located at the bottom of the ear, and is the only part of the ear that is not cartilaginous, being made up primarily of fatty tissues. It serves no known biological function, and is the most common location for ear piercings.
The entire purpose of the outer ear is to collect sonic waves, redirecting them into the aural canal so they can be interpreted and sent to the brain. This is where the pinna's unique shape comes into play, causing them to act as funnels that amplify sonic waves and redirect them straight into the ear canal. In collecting and filtering these sonic waves, the pinna also perform several important secondary functions. The most important of these secondary functions is sound localization, which is the ability to pinpoint the origin or location of a sound after hearing it. The biological architecture of the pinna allows the listener to determine the direction that a sound came from, as well as the sound's distance from the ear.
The concept of sound localization is strongly linked to the idea of head-related transfer function (HRTF) because it allows human beings to locate sounds in three dimensions. Because of HRTF, sounds can be located above, below, in front of, behind, and to either side of the human head. This is due to the fact that the pinna, along with the brain and inner ear, allow us to create a three-dimensional mental map pinpointing the source of a noise. When a sound is heard by both ears, the differences in timing and reception angle for each ear allow the listener to figure out where the sound is at relative to the body and how close the source is. Many species, humans in particular, use this biological mechanism to pick up the slack and supplement the limited range of perception that they receive from their eyes. Since the eyes only allow most species to see a small part of the world around them, the ears serve a crucial function, allowing the listeners to determine if something requires attention.
Where Is the Pinna Located in the Ear?
The pinna is an outer ear structure. As mentioned previously, it's made up of cartilage tissue covered in skin. The pinna surrounds the external auditory canal and is the only visible portion of the human ear. Pinnas can vary in size and shape, with some people inheriting the ability to wiggle them. Scientists think that it's an evolutionary holdover, as many animal species swivel their ears to better pick up sounds in different directions.
The Evolution of Human Ears
As the latest species in the great ape family, humans inherit many qualities from their hominid and primate ancestors. They had ears that looked and functioned similarly to our own. However, current evidence suggests that human ear evolution began billions of years ago after fish developed gills.
The gills-to-ears evolution concept may sound a little strange. The key to this concept lies in Eusthenopteron, an ancient fish that lived around 370 million years ago during the Devonian Period. This fish had gills like its contemporaries, except with one notable difference — it possessed a small bone with a slight kink that obstructed the opening of each gill.
Another species that lived during this period, Panderichthys, had the same type of bone. However, it was wider and allowed the fish to breathe air while swimming underwater. Scientists call this bone a spiracle and believe it was an important first step in the evolution of human ears. Of course, other structures had to develop in the meantime: the hyomandibular bone plus middle ear adaptations that allowed mammals to detect higher-pitched sounds.
What Does the Pinna Do?
The pinna has two main jobs: collecting sound and shielding internal ear structures. Our pinna's shape is critical to their ability to channel and direct sounds in our environment. They behave like funnels, routing sound so it enters into our middle ears. They also help mitigate pressure differences between the outside air and the internal ear assembly. By offering a transition between these pressure differences, sound can more efficiently pass into the auditory canal.
More Evolutionary Ear Facts
You may have noticed that reptiles, birds, and amphibians do not have pinnas. In contrast, mammalian ears are among the most sophisticated in the animal kingdom. They're the only ones with auricles and outer ear structures to route sound into the middle and inner ear.
Mammalian pinnas develop to suit the unique environments in which each animal lives. They vary in size, shape, structure and mobility. Consider the ears of the domestic (or not-so domestic) feline: They point upward, but they can also swivel up to 180 degrees. This feature enables cats to pick up faraway sounds in different directions. Since cats are both predators and prey animals, their hearing abilities help them ambush their prey and detect threats.
Animals' ears can also display their emotional and mental states, a feature not present in modern humans. If you've ever seen a cat with "airplane ears," you may already understand how this works. Airplane ears signal that a cat is frightened and uncomfortable. Conversely, a cat's ears that are up and forward in the neutral position show that the animal is relaxed.
