THE SENSE-ORGANS 493 



phatic and endolymphatic cavities respectively. The mem- 

 branous labyrinth is held in place by scattered strings of the 

 original connective tissue, which connects it with the Sony wall, 

 aside from which the two come into close contact at the places 

 of entrance of the various branches of the auditory nerve. In 

 the lower vertebrates the outer labyrinth remains as a mold 

 imbedded in the petrosal bone, but in higher forms, especially 

 in mammals, the bony labyrinth appears over certain regions, 

 especially the semicircular canals and the lagena, as a thin but 

 very hard wall, with a space between its outer surface and the 

 main mass, thus reproducing from without also, the main de- 

 tails of the membranous labyrinth. 



The transition from water to air, undoubtedly the greatest 

 change which vertebrates have ever experienced, and one which 

 demanded modifications affecting every part, affected the organ 

 of hearing directly, for a change was made from a denser 

 medium, which readily transmitted sound vibration, to a 

 lighter one in which transmission was more difficult. This 

 disadvantage was undoubtedly felt by the urodeles, which ex- 

 hibit a new organ, evidently destined to assist in the reception 

 of less powerful vibrations. In the cartilaginous otic capsule 

 surrounding the labyrinth, that which partly corresponds to the 

 " bony labyrinth " of higher forms, there exists an oval opening 

 with a reinforced rim, closed by x an ossicle in the form of a lid, 

 usually with a process projecting from its center [cf. Fig. 39, 

 op~\. The opening, which persists in all higher vertebrates, is 

 the fenestra ovalis, and the osseous lid, which is, in origin, a 

 portion cut off from the wall of the capsule, is the operculum. 

 This latter is fitted to the rim of the fenestra ovalis by a 

 membrane, and, as it is nearly subcutaneous, it is set in mo- 

 tion by the impact of sound waves, and thus serves to slightly 

 intensify the vibrations. 



This apparatus proves sufficient for urodeles, which are 

 much in the water, but in the tailless forms (Anura), far more 

 terrestrial than the salamanders, the sound-receiving apparatus 

 is much improved by an important addition, the tympanum, or 

 cavity of the middle ear. This is developed from the gill 



