210 THE HEAD AND NECK 



first visceral cleft, and the auricle is formed by the development 

 and fusion of several tubercles, which arise from the mandi- 

 bular and hyoid (first and second) arches. This double origin 

 is indicated in the adult by the double nerve supply from the 

 auriculo-temporal and great auricular nerves (pp. 108, 174). 

 Owing to faulty union of the various tubercles with one another, 

 pre-auricular fistulse may occur. The interval between the 

 helix and the tragus is the commonest site of this congenital 

 deficiency, and it is said to indicate the line of union between 

 the first and second arches. 



The Tympanum or Tympanic Cavity and the Auditory (Eustachian) Tube 

 are the remains of the first visceral pouch (Fig. 46). They are enclosed in the 

 temporal bone, which consists of three pieces at birth. The Petromastoid 

 Portion encloses the internal ear, and forms the medial part of the roof and 

 the medial wall of the tympanum, the osseous part of the auditory tube, and 

 the tympanic (mastoid) antrum. The Squamous Portion forms part of the 

 lateral wall of the skull and of the tympanum, and the upper part of the 

 osseous external acoustic meatus. It also forms the lateral wall of the 

 tympanic antrum and the lateral part of the roof of the tympanum. The 

 Tympanic Ring forms the remainder of the osseous external meatus. 

 Although the tympanic antrum is present at birth, the mastoid process does 

 not begin to develop until the end of the second year, and the mastoid air- 

 cells begin to appear as outgrowths from the antrum at the same period. 



The External Acoustic Meatus leads from the concha to the 

 membrana tympani. Its lateral cartilaginous portion is first 

 directed medially, forwards and upwards, and then, at a distance 

 of about a quarter of an inch from the surface, medially and 

 backwards to meet the osseous portion of the meatus, which is 

 directed medially, downwards and forwards. The membrana 

 tympani is obliquely placed so that the lower and anterior walls 

 of the canal are definitely longer than the upper and posterior. 

 The total length of the meatus is about one inch, and its 

 narrowest part is found at three-quarters of an inch from the 

 surface. In the examination of the tympanic membrane the 

 canal is straightened by drawing the auricle upwards and back- 

 wards. This procedure brings the cartilaginous portion into line 

 with the osseous portion, but in the infant the auricle must be 

 drawn downwards and backwards, as the canal is almost entirely 

 cartilaginous, and the outer surface of the membrana tympani 

 is directed mainly downwards. 



The external acoustic meatus is lined by a layer of skin, 

 which is reflected over the outer surface of the tympanic mem- 

 brane. It is firmly bound down to the periosteum and peri- 

 chondrium, and therefore septic infections of the canal, although 



