Synonyms
Latin: Aruis externa English: external ear
Definition
The outer ear is the first level of the sound conduction apparatus, next to the middle ear. The outer ear includes the pinna (auricle), the external auditory canal (external acoustic meatus) and the eardrum (tympanic membrane), which forms the boundary with the middle ear. The first important component of the outer ear is the pinna.
It encloses an elastic cartilage plate (Cartilago auriculae). The skin lies closely against it. From the outside, each person’s auricle has an individual shape.
This is formed by the cartilaginous structures helix, anthelix, tragus and antitragus. The earlobe (Lobus auriculae) is the only part free of cartilage and can be fused together or hang freely as a bulge. The muscles of the ear belong to the mimic facial musculature and are innervated by the 7th cranial nerve (facial nerve).
In most cases, however, they are severely degenerated and functionless. That is why very few people can consciously wiggle their ears. The auricle is very well supplied with blood, which is used for temperature regulation.
If the body temperature is too high, more blood is directed into the auricle and cooled by the external air flow. Everyone is certainly familiar with the phenomenon of “red ears” in embarrassing or fearful situations. However, as there is no insulating layer of fat around the auricle, frostbite can quickly occur, especially in the upper area.
The temperature-regulating effect via the ears is certainly of secondary importance in humans, as sweat glands and other mechanisms can regulate body temperature more effectively. In the animal kingdom, e.g. with elephants, the success is more obvious. There are also various lymph nodes on the outer ear, which can be swollen during inflammatory processes.
The auricle collects the incoming sound as a kind of funnel, which then continues its path via the external auditory canal. This funnel function is particularly important for directional hearing. The distinction is made between “up/down” and “front/back”, which is ensured by the folds of the pinna, as these reflect or amplify the different sound frequencies differently.
Central neurons evaluate this information. The external auditory canal (part of the outer ear) is about 3cm long and has an average diameter of 0.6cm. In the initial part it consists mainly of elastic cartilage.
Towards the eardrum the walls are more and more formed by a bony wall. It has an S-shaped course, which is particularly important when examining the eardrum with an otoscope. Here, the auricle must be pulled backwards and upwards so that the cartilaginous part is stretched and straightened out, the funnel of the otoscope can be inserted and the view of the eardrum is revealed.
Especially in the anterior section there are more sebaceous and ceruminal glands. The latter produce a thin fluid secretion which, together with the sebum and dead cells, forms earwax (cerumen). Normally, this lard serves as a protection against the penetration of foreign bodies and against drying out of the skin in the auditory canal.
However, if it is produced in excess, it can reduce hearing performance. Also, swelling of the secretion on contact with water and also subsequent hearing loss is possible. The healthy eardrum (part of the outer ear) is pearl grey in colour, is round-oval and has an area of approx.
75 mm2. It can be divided into four quadrants in a clockwise direction: This division is made along a light stripe (Stria mellearis), which belongs to the translucent hammer handle, and a perpendicular to this line, which runs through the navel (umbo). The navel forms the lower end of the eardrum fused to the hammer handle.
This division is clinically important, as it allows a better description of the localisation of pathological changes. In a normal eardrum, a light reflex is produced in the II quadrant, which provides information about the tension of the eardrum. In principle, however, the eardrum can be divided into a small flaccid part (pars flaccida, shrapnel membrane) and a larger, tensioned part (pars tensa).
The centre of the eardrum is funnel-shaped and is drawn in towards the navel. The function of the eardrum is to transmit sound to the ossicular chain and thus into the tympanic cavity (middle ear). The incoming sound causes the eardrum to vibrate mechanically, which is then transmitted via the hammer, anvil and stapes to the oval window, causing the inner ear fluid to vibrate. The actual conversion of the sound waves into electrical impulses then takes place in the inner ear. – I: front top
- II: front bottom
- III: bottom rear
- IV: top rear.