THE AUDITORY APPARATUS 199 



sulcus spiralis (Fig. 94). Prentiss claims that it is in part formed from the 

 embryonic cells which develop into the spiral organ, and that its connection 

 with the spiral organ is retained in the adult (Fig. 94) ; but Hardesty (1915) 

 says that the cells of the embryonic spiral organ contribute little or nothing 

 to the formation of the tectorial membrane and that this membrane is free 

 from the spiral organ in the adult. Prentiss describes the membrane as 

 growing in thickness by the secretion of a cuticulum formed between the 

 ends of the epithelial cells, thus giving to the mature membrane a cham- 

 bered or honey-comb structure. Hardesty, however, regards it as produced 

 by fibrils growing out from the free ends of the epithelial cells lying be- 

 tween the embryonic spiral organ and the axis of the cochlea, these fibrils 

 being embedded in a gelatinous matrix. 



Shambaugh concludes that the tectorial membrane takes the part of a 

 physical resonator by responding in its various parts to tones of different 

 pitch, depending on the size of the membrane, tones of higher pitch being 

 taken up by the hair cells located near the beginning of the basal coil, those 

 of lower pitch by the cells near the apex of the cochlea, where the tectorial 

 membrane attains its maximum size. The stimulation of the hair cells is 

 effected only through the medium of their projecting hairs, these being 

 excited by vibrations of the tectorial membrane to which they are attached. 



In fishes the organs of the internal ear are intimately associated with an 

 extensive series of subcutaneous canals containing numerous sense organs 

 and with naked cutaneous sense organs of the same type, the entire complex 

 forming the system of lateral line sense organs (see p. 110 and Fig. 95). 

 The nerves which in fishes supply the lateral line sense organs (lateralis 

 roots of the VII and X cranial nerves) and the organs of the internal 

 ear (VIII nerve) are intimately associated and terminate together in the 

 acoustico-lateral area of the medulla oblongata (Figs. 43 and 44, pp. Ill, 

 112), and all of these end-organs have the same type of structure as those 

 of the human internal ear (Fig. 32, p. 88). 



The internal ears of fishes are essentially similar to those of man save 

 that they lack the cochlea and the organ of Corti. They possess a small 

 sense organ in the saccule, the lagena, supplied by a special branch of the 

 VIII nerve (Fig. 95, RL), from which the cochlea of higher vertebrates has 

 been developed. The researches of Parker have shown that fishes hear, 

 though there is no evidence that they possess the power of tone analysis, 

 and the sense organs of the saccule are the essential receptors for sound 

 waves. The sense organs of the lateral line system are said by Parker to be 

 sensitive to water vibrations of slower frequency than the sound waves to 

 which the ear responds, while Hofer is of the opinion that these organs are 

 stimulated only by streaming movements of the water in which the animals 

 live. Probably the lateral line organs also participate in the equilibratory 

 reactions of the fish. 



Though our knowledge of the functions of the various parts of the acous- 

 tico-lateral system of fishes is still very imperfect, it is evident that all of 

 these organs are both structurally and physiologically of common type, and 

 it is probable that they have had a common evolutionary origin from a more 

 generalized form of cutaneous tactile organ. This is the explanation of the 

 intimate association in the human ear of sense organs of so diverse functions 

 as the cochlea for hearing and the semicircular canals for equilibration, the 

 former being an exteroceptor whose reactions may be vividly conscious, and 

 the latter being a proprioceptor whose reactions are almost entirely uncon- 

 sciously performed. For further consideration of the semicircular canals 

 and their central connections see p. 183. 



