VAN 121: Veterinary Gross Anatomy-II (2+2)

EXTERNAL EAR

• The external ear consis of two parts, the auricula or pinna - a funnel shaped organ with its muscles, which collects the sound waves and the external acoustic meatus, which conveys these waves to the drum of the ear which separates the external ear from the middle ear.
• The auricula or pinna is the visible ear, has two surfaces, two borders, base and an apex.
• The anterior surface is irregularly concave and presents an opening of the external ear.
• There are three or four ridges and two or three furrows, which become gradually effaced towards the extremities.
• The posterior surface is convex and presents grooves and elevations corresponding to those on the anterior surface.
• This surface is almost circular at the base and flat and narrow at the extremity.
• The middle portion forms the widest part. The superior border in convex.
• The inferior border is thicker and more strongly convex.
• The base is convex and is attached to the external auditory process of the petrous temporal bone in such a way as to be freely movable.
• It is surrounded by a flat in its anterior, internal and posterior aspects.
• Its inferior and external parts are overlapped by the parotid gland.
• The auricula is made up of elastic cartilages, muscles and skin.
• The cartilages of the external ear are conchal, annular and scutiform.
  • The conchal cartilage determines the shape of the ear. The base of the cartilage is coiled to from a tube that encloses the cavum conchae that is funnel shaped. Its medial surface is convex and bears a prominence the eminentia conchae. The lower part of the medial margin bears a pointed prolongation -the styloid process. Behind the base is a foramen for the auricular branch of the vagus.
  • The annular cartilage is a quadrilateral plate curved to form three fourth of a ring and it embraces the external acoustic process. It forms with the lower part of the conchal cartilage and the cartilaginous part of the external acoustic meatus.
  • The scutiform cartilage is a quadrilateral piece situated in front of the base of the concha. Its superficial face is convex from side to side and deep face is concave. Its anterior end is thin and rounded and posterior end is thicker and wider and it gives attachment to the muscles. The cartilage moves freely over the underlying parts.
• The auricular muscles have already been described under myology.
• The external acoustic meatus leads from the cavum concha to the tympanic membrane. Its direction is downwards, forwards and inwards.
• It consists of a cartilaginous part and osseous part.
• The annular cartilage forms the cartilaginous part while the external acoustic process of the temporal bone forms the osseous part.
• These are united by elastic membranes to form a complete tube.
• Its lumen at the inner end is one half of that at the outer end.