SENSE ORGANS. 73 



above and below, by similar outgrowths of the perilymphatic 

 space. From the resemblance which these structures present to 

 a spiral stairway these divisions are called scalae, that part con- 

 nected with the membranous labyrinth being the scala media, 

 the upper of the perilymphatic spaces being the scala vestibuli, 

 the lower the scala tympani. This whole structure, from its 

 resemblance to a snail-shell, is called the cochlea. In the scala 

 media the macula lagenae of the lower vertebrates becomes de- 

 veloped into a highly specialized sensory structure, the organ 

 of Corti. Besides ' hair cells ' (sensory cells) and other cells, 

 the organ consists of series of hard rods (pillar cells) arranged 

 like a A at right angles to the axis of the scala. As the spiral 



qa POH 



FIG. 78. Organ of Corti in section, after Stohr. AT, auditory tooth; CC 9 

 cells of Claudius; DC, Deiter's cells; HC, Hensen's cells; HP, head plate; Iff, 

 inner hair cells ; MB, basal membrane ; N, nerve ; OH, outer hair cells ; IP, OP, in- 

 ner and outer pillar cells ; /*, phalanges ; SS, sulcus spiralis ; T, tunnel. The ' mem- 

 brana tectoria,' being decidedly problematic in character and relations, omitted. 



diminishes in size, from apex to base these A's also diminish in 

 size, a fact which led to the view formerly held that these were 

 in some way connected with the recognition of pitch. 



The middle ear or tympanum first appears in the anura. 1 

 It is formed by the expanded end of the first visceral cleft 

 (spiracle of elasmobranchs), which does not break clear through 

 to the exterior, but is closed externally by a thin tympanic 

 membrane, with an external wall of ectoderm, an inner of ento- 

 derm, and a middle layer of mesenchyme. Internally the tym- 

 panic cavity remains in connection with the pharynx by means 

 of the proximal portion of the cleft, here known as the Eusta- 

 chian tube. Sound waves are conducted across the tympanic 



1 In the urodeles and caecilians the tympanic cavity is lacking, and there is but a 

 single auditory ossicle, the stapes, which usually articulates with the quadrate. 



