i8o 



EVOLUTION AND DISEASE. 



In order to appreciate fully the bearing of these facts 

 and some others which will be mentioned, it will be 

 necessary to take into consideration the chief points 

 connected with the development of the pinna as an 

 illustration of the care requisite in interpreting variations 

 and abnormalities from the evolutionary point of view. 

 The details of the development of the human pinna have 

 been carefully described by Professor His. At the end of 

 the first month of embryonic life, the first branchial cleft is 

 surrounded by six rounded, slightly prominent tubercles, 



FIG. 95. Human Ears with so-called supernumerary auricles. 



numbered I to VI in fig. 96, and it is by the coalescence of 

 these six tubercles that the pinna is formed. The details 

 of their fusion are briefly these : tubercles I and V 

 unite across the fissure and form tragus and antitragus, 

 the gap, or notch, between them is represented in the 

 adult as the fissura intertragicum ; tubercle VI unites 

 with these to form the lobule; tubercle II forms the 

 helix, whilst III lengthens out and forms the conchal 

 rim and the tail, or cauda helicis, whilst the tubercle 

 marked IV becomes the antihelix. 



These facts explain readily enough the mode of origin 



