chap, xii.j EAR OF MAMMALS. 471 



continued into a tube which enters into connection 

 with the subarachnoid cerebral cavity. Eight different 

 sensory regions are now to be distinguished ; the 

 cochlear region has a commencing outgrowth or lagena, 

 and within is a space which is covered over by a very 

 thin membrane, the membrana basilaris. 



In the Reptilia we observe several stages in 

 the outgrowth of the cochlea, and these are most 

 marked in crocodiles, which, in this character, as in so 

 many others, stand nearest to the birds. In these last 

 the lagena is quite prominent (Fig. 198; B, b), and 

 begins to take on a spiral course. 



With the exception of the Prototheria, all Mammals 

 have their cochlea coiled into a heliciform spiral, the 

 canal of which is wound round the bony axis or 

 modiolus ; the coil may be flat, as in the Cetacea, or 

 very steep, as in some Rodents (guinea- pig) (Fig. 198 ; 

 c). The internal structure of the cochlea has been 

 fully described in the " Elements of Histology " (chap, 

 xii.) ; here it need only be said that the seal a vestibuli, 

 the membrane of Reissner, and the rods of Corti are 

 peculiar to the mammalian ear; as to 

 the last, we have so far evidence that 

 it has been developed within the 

 limits of the mammalian series that 

 we find them to be much more simply 

 arranged in the Prototheria than in 

 the higher mammals. The absence of 

 this organ from the ears of birds, 

 many of which are, as we know, 

 capable of being attracted by musical 

 sounds, makes it impossible for us at present to accept 

 the doctrine that these rods are physiologically impor- 

 tant as the means of distinguishing different notes of 

 music. 



The otoliths found in the lymph of the membranous 

 labyrinth are ordinarily larger in fishes than in higher 



