THE SENSE-ORGANS 491 



papilla lagenae appears, sometimes as a separate area, some- 

 times as a lobe of the macula sacculi, thus giving four acoustic 

 maculae for most fish. In amphibians the papilla basilaris ap- 

 pears while the others are retained, although there is a slight 

 transposition of the macula neglecta. This condition, with 

 five acoustic areas, is retained by the Sauropsida with some 

 variation, such as the division of the papilla basilaris in certain 

 lizards and a great reduction of the macula sacculi in turtles, 

 points referable to special adaptation and of no general signifi- 

 cance. In mammals there are important changes. The 

 macula neglecta has entirely disappeared and the papilla lagena 

 is found only in Ornithorhynchus (monotreme), leaving in 

 this Class but three acoustic areas aside from the three cristae 

 acusticse of the semicircular canals. 



The most important difference in the mammalian labyrinth 

 is the great development of the lagena. The tendency, already 

 shown in crocodiles and birds, to prolong this part and to curve 

 its axis, results here in an excessive elongation which becomes 

 wound into a close spiral, the nerve forming the central axis. 

 The number of complete coils in man is about 3, but varies 

 among mammals between the limits of ij and 5.* This 

 coiled lagena becomes complicated by the addition of parts of 

 the outer bony labyrinth, to be explained later, which form two 

 additional coiled passages, scala vestibuli and scali tympani, 

 that receive between them the lagena under the anatomical 

 name of ductus cochlearis [scala media'}. This entire organ, 

 including both this part of the labyrinth and its accessory or- 

 gans, is called the cochlea. 



The papilla basilaris lies along the floor of the coiled lagena 

 (scala media) and becomes highly differentiated into a number 



* Examples are as follows : 



Erinaceus (Hedgehog) i^ 



Whales and porpoises 1^2 



Rabbit 2^ 



Cat , 3 



Ox 354 



Swine 3 l / 2 



Ccelogenys (South American rodent) 5 



