The external ear
'Γhis consists of the following structures:
The pinna - a funnel-shaped plate of cartilage whose functions are:
- To guide sound waves picked up from the external environment down the external auditory meatus to the tympanic membrane tear drum)
- To act as a means of facial expression and communication between animals of the same species.
Pinmic are used in conjunction with other parts of the body. e.g. the eyes and tail, to Communicaledominance. fear, acceptance, etc.The pinna varies in size and shape between breeds and has been altered by general ions of selective breeding, leading to the development of characteristics, many of which predispose* to ear disease. 'Γhe original wolf-like dog. from which all breeds arc thought to be derived. has upright V- shaped ears - this is still seen in (kτman Shepherds. Chows and I Iuskics. All cats have upright pointed cars which, until the recent introduction of the Scottish Eold with its twisted Ilattcncd ear pinnae, have been left untouched by selection. Eiich pinna is formed from a sheet of elastic Cartikige known as the Uiiricidarcartilaye. This continues as an incomplete tube known as the external auditory meatus linked al the base to the Unmdarcartilaye. This, in turn, articulates with the CXtermiI acoustic process of the tympanic bulla, forming a series of interlinking cartilages which enables the pinna to move freely to pick up sound waves.
Both sides of the auricular cartilage are covered in hairy skin - in most breeds the outer side is more hairy than that of the inner side. The covering of skin continues into (he external auditory meatus where there arc fewer hair follicles: the PocxJIc is particularly noted for its hairy ear canals. The skin is well supplied w ith ceruminous ylands - mcxlilied sebaceous glands that secrete wax and protect the ear canal from damage and infection.
The external auditory meatus - this may also be called the external car canal. The opening of the canal faces dorsolaterally: it runs vertically dowτι the side of the skull - the vertical canal - and then turns inwards to run horizontally - the horizontal canal. The canal ends al a delicate sheet of tissue known as the tympanic membrane or ear drum.