The External Ear
The ear can be split into three parts; external, middle and inner. This article is about the external ear. We shall look at its structural anatomy, vasculature, innervation, and the clinical conditions that can occur.
The external ear can be functionally and structurally split into two sections; the auricle (or pinna), and the external acoustic meatus.
The auricle is an external, lateral paired structure. Its function is to capture and transmit sound to the external acoustic meatus.
Most of the auricle has a cartilaganious framework, with the lobule the only part not supported by cartilage. The outer curvature of the ear is called the helix. Moving inwards, there is another curved elevation, which is parallel to the helix – this is known as the antihelix. The antihelix divides into two cura – the inferoanterior crus, and the superoposterior crus.
In the middle of the auricle is a hollow depression, called the concha of auricle. It continues into the skull as the external acoustic meatus. The concha acts to direct sound into the external acoustic meatus. Immediately anterior to the start of the external acoustic meatus is an elevation of tissue – the tragus. Opposite the tragus is the antitragus.
Cutaneous innervation to the skin of the auricle comes from the greater auricular, lesser occipital and branches of the facial and vagus nerves. Patients can complain of an involuntary cough when cleaning their ears – this is due to stimulation of the auricular branch of the vagus nerve, the nerve responsible for the cough reflex.
With respect to the vasculature, the main vessels involved are the posterior auricular, superficial temporal and occipital arteries and veins.
Clinical Relevance: Auricular Haematoma
A auricular haematoma is when blood collects between the cartilage and the overlying pericondrium. It is usually a result of trauma, commonly seen in contact sports.
The accumulation of blood disrupts the vascular supply to the cartilage of the pinna. If it is not drained quickly, a gross deformity results, called ‘cauliflower ear‘.
External Acoustic Meatus
The external acoustic meatus is a sigmoid shaped tube that extends from the deep part of the concha to the tympanic membrane. The walls are given their structure by cartilage from the auricle, and bony support from the temporal bone. This part of the external ear gets its sensory innervation from branches of the mandibular and vagus nerves.
The external acoustic meatus does not have a straight path, and travels in an S-shaped curve:
- Initially travels in a superoanterior direction.
- Turns slightly to move superoposterior.
- Ends in an inferoanterior direction.
The external acoustic meatus ends at the tympanic membrane. The tympanic membrane has a double layered structure, covered with skin on the outside and a mucus membrane on the inside. At the core of the membrane is connective tissue. It is connected to the surrounding temporal bone by a fibrocartilaginous ring.
The tympanic membrane is translucent, therefore structures within the middle ear can be seen. On the inner surface of the membrane, the handle of malleus attaches to the tympanic membrane, at a point called the umbo of tympanic membrane.
The handle of malleus continues superiorly, and at its highest point, a small projection can be seen, called the lateral process of malleus. The parts of the tympanic membrane moving away from the lateral process are called the anterior and posterior malleolar folds.
Clinical Relevance: Perforation of the Tympanic Membrane
The two most common causes of tympanic perforation are trauma and infection. An infection of the middle ear (otitis media) causes pus and fluid to build up. This causes an increase in pressure, and eventually the eardrum ruptures.
In many cases the tympanic membrane heals itself, but in large perforations surgical intervention might be necessary.