CHAP. XI.] PERIPHERAL NERVOUS END-ORGANS. 449 



epithelial structures which are directly continuous with fibres of the 

 auditory nerve. The sac contains a liquid termed endolymph and 

 is usually separated from the bony or cartilaginous structures wherein 

 it lies, by a liquid termed perilymph, through which sound-waves 

 have to be transmitted before they can affect the structures contained 

 in the membranous labyrinth. On the inner surface of the mem- 

 branous labyrinth, in certain situations, are crystalline bodies usually 

 termed otoliths or otocoma. 



Perilymph and Endolymph. 



Peril b According to Dahnhardt 1 the perilymph of the had- 



dock is a somewhat tough gelatinous liquid, rich in 

 mucin, and containing a proteid matter precipitable by acids but not 

 coagulated by heat. It contains from 2'1 to 2'2 per cent, of solid 

 constituents. Its chief saline constituent is common salt. 



Endo- According to the same observer the endolymph of 



lymph. ^ ne haddock is a clear liquid, containing 1*5 per cent, of 



solid matter ; the quantity of mucin is small, and albumin is absent. 



Otoliths, Lapilli, or Otoconia. 



In the vestibular sacs of the membranous labyrinth of most 

 (though not of all) fishes, lying free upon the surface of the epi- 

 thelium and bathed by endolymph, lie small concretions, termed 

 otoliths, lapilli, or otoconia, which are either pulverulent, as in the 

 plagiostomatous fishes, or hard and stony, as in the osseous fishes. In 

 these cases the otoliths rest freely on the surface of the long pro- 

 jections of the hair-cells which line the otolith sacs. 



Although by no means universally distributed, similar concretions 

 are met with in the vestibular sacs and in the ampullar commencements 

 of the semicircular canals throughout the various groups of vertebrate 

 animals, though as a general rule they do not present themselves as 

 individual lapilli, lying free, but rather as pulverulent crystalline 

 concretions lying imbedded in the epithelial lining of the sacs. In 

 the latter case the individual crystals are surrounded and held 

 together by a slimy organic matter. Otoliths also occur in many in- 

 vertebrate groups. 



According to Johannes Muller, the otcliths of the osseous fishes 

 have a structure similar to that of the enamel of the teeth, though 

 the statement is one which cannot, on morphological grounds, be 

 comprehended and invites further examination 2 . 



1 Dahnhardt, "Endolymphe u. Perilymphe." Arbeit, d. Kieler physiol. Instituts, 

 p. 103. 



3 For much interesting information on otoliths consult Miiller's Elements of Phy- 

 siology, translated by Baly. Vol. n. p. 1129 et seq. 



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