The auricle starts to develop when 6 hillocks appear around the first pharyngeal groove, which lies between the first and the second branchial arches. Three hillocks develop on each side of the groove; as their growth progresses they contribute to the development of the auricle. The bulk of the auricle is developed from the mesenchyme of the second branchial arch (Hyoid arch). The cartilage of the auricle extends inwards partially to surround the future external meatus. A rudimentary pinna is formed by the 60 day of embryonic life, and by 4months the convolutions are formed resembling the adult pinna.
The pinna projects from the side of the skull to varying degrees. It has two surfaces; the lateral which is the exposed surface, and the medial which is hidden. The lateral surface have several prominences and depressions giving it a unique shape. The curved rim of the pinna is known as the helix. At the posterior superior aspect of the helix is a small tubercle known as the auricular tubercle (Darwin's tubercle). Anterior and parallel to the helix another prominence is present known as the antihelix. Superiorly the antihelix divides into two crura encompassing a fossa known as the triangular fossa. Above the superior of the two crura lies another fossa known as the scaphoid fossa. In front of the antihelix, infact partly encircling it is the concha. Theantero superior portion of the concha is covered by the descending limb of the antero superior portion of the helix. This region of the cochais known as the cymba concha. This cymba concha has an important surface relationship with the suprameatal triangle. Infact the suprameatal triangle or (Mceven's triangle) lie just under the cymbaconcha. This triangle acts as a surface marking for the mastoid antrum during mastoidectomy procedures. Opposite to the concha, and lying across the external auditory meatus (partially covering it) is a small blunt triangular prominence known as the Tragus. This prominence points posteriorly. Opposite to the tragus lie another prominence known as the antitragus.
The body of the auricle is covered with fibroelastic cartilage, covered with skin. It is connected to the surrounding parts by ligaments and muscles. It is also continuous with the cartilage lining the external auditory canal. The skin of the auricle is thin and closely bound to the perichondrium on the lateral side. On the medial side the skin is not adherent to the cartilage of the auricle, there is a layer of subdermal adipose tissue separating the skin from the cartilage. The skin is covered with fine hairs which have sebaceous glands opening into their root canals. The glands are most numerous in the concha and the scaphoid fossa. The fibroelastic cartilage is absent in the lobule area of the pinna. The auricular cartilage depends on the perichondrium for its nourishment. The cartilage is connected to the temporal bone by two extrinsic ligaments, the anterior and the posterior ligaments.Intrinsic ligaments connect various parts of the cartilagenous auricle. The muscles of the auricle belong to two groups; the extrinsic and the intrinsic groups. The extrinsic muscles are supplied by temporal and post auricular branches of the facial nerve. The extrinsic muscles are functionally not so important,but they give rise to the post auricular myogenic response following auditory stimulation. The extrinsic muscles are auriculares anterior,auriculares superior and auricularis posterior. The intrinsic muscles are 6 in number, all small and inconsistent and without useful function.
The post auricular branch of the external carotid artery supplies the medial surface of the pinna, and extends around the helix to supply the extremities of the lateral surface. The anterior auricular branch of the superficial temporal artery supply the bulk of the lateral surface.Auricular branch from the occipital artery assists the post auricular artery in supplying the medial surface.
Lymphatic drainage from the posterior surface of the pinna is to the lymph nodes at the mastoid tip, from the tragus and from the upper part of the anterior surface is to the preauricular nodes, and from the rest of the auricle to the upper deep cervical nodes.
Sensory innervation of auricle:
|Nerve||Derived from||Region supplied|
|Greater auricular||Cervical Plexus C2 C3||Medial surface and posterior part of lateral surface of pinna|
|Lesser occipital||Cervical plexus C2||Superior portion of medial surface|
|Auricular||Vagus||Concha and Antihelix|
|Auriculo temporal||Mandibular division of trigeminal nerve||Tragus, Crus of the helix and adjacent helix|
|Facial||Supplies root of concha|
Physiology of pinna:
1. Pinna increases the pressure at the tympanic membrane in a frequency sensitive way, thus emphasizing certain frequencies in the input.
2. It helps to localise the direction of the source of sound.
3. Protects the middle ear and inner ear from extraneous insults