59 o SCIENCE PROGRESS 



These peculiarities are probably extremely important parts 

 of the mechanism by which complex tones are resolved into 

 their components and are essential to the due performance of 

 the higher functions of hearing. But before entering further 

 into this question we must return and study for a moment the 

 evolution of the perilymph spaces connected with the cochlea — 

 the scala vestibuli and tympani. 



As mentioned above, the history of the perilymph spaces is 

 essentially one of simplification, as pointed out by Dr. Gray. 

 When we left these spaces in the amphibia (fig. 3, amphibian), 

 there was only a little rudiment of the scala tympani pressed 

 against the mesial surface of the basilar membrane but as yet no 

 signs of a scala vestibuli on the outer surface of the pars 

 basilaris. The scala tympani was nothing but a slight pro- 

 trusion from the side of a tube (fig. 3, D. PLPH.) that connects 

 the great perilymph chamber lying between the saccule and the 

 oval window (fig. 3, Sp. sacc.) with the cranial cavity and the 

 exterior. In reptiles the arrangement is essentially the same 

 (fig- 3> reptilian) except for the advent of a scala vestibuli (Sc. 

 vest), which is represented by a downward prolongation of the 

 saccular perilymph chamber upon the outer surface of the pars 

 basilaris or cochlear canal. The two scalae, although present, 

 are not connected directly through their apices but indirectly 

 through the perilymph duct and the saccular perilymph chamber. 



In crocodiles, so far as our information goes, in birds, 

 certainly, there is a direct connexion, though an imperfect one, 

 by means of a loose spongework of tissue that surrounds the 

 apex of the cochlear canal (fig. 3, Bird) and is in open connexion 

 with the cavities of both scalae. As soon as this direct con- 

 nexion appears the indirect connexion through the perilymph 

 duct is lost. Finally in monotremes a free passage (fig. 3, 

 monotreme, HLCTR) is opened up between the apex of the scala 

 vestibuli and the apex of the scala tympani and the two scalae 

 become a continuous tube running down the outer surface of 

 the cochlear canal from the vestibular perilymph space (foramen 

 ovale) and up the mesial surface to the foramen rotundum. 



The loss of the perilymph duct in crocodiles and birds, 

 however, is not complete. A considerable part, somewhat 

 swollen, remains between the scala tympani and the cranial 

 cavity and the exterior (foramen rotundum), forming a definite 

 perilymph sac (fig. 3, SAC. PLPH). Very pronounced traces of 



