Integrative Systems 



209 



Fig. 180. The left membranous labyrinth of vertebrates as seen in lateral view. 

 (A) Myxine. (B) Petromyzon. (C) Teleost. (D) Frog. (E) Crocodile. (F) Bird. 

 (G) Mammal. (After Hesse. Courtesy, Neal and Band: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



outer is a thicker layer of connective tissue. All cavities within the 

 labyrinth are filled by a watery endolymph. The space everywhere 

 present between the labyrinth and the internal surface of the carti- 

 laginous or bony otic capsule is filled by a similar lymphlike fluid, 

 the perilymph. It is only in sharklike fishes that the primary em- 

 bryonic communication between the cavity of the otic vesicle and the 

 exterior is retained in the adult, appearing as the slender endolymph 

 duct which opens externally on top of the head. The pair of endo- 

 lymph pores are not far apart on either side of the middorsal line. 

 In at least some of these fishes, the fluid filling the interior of the 

 labyrinth is probably sea water, not endolymph. Calcareous particles 

 (otoconia), or even large solid calcareous masses (otoliths), are 

 commonly present in the endolymph of the utriculus, sacculus, and 

 ampullae. 



The sensory structures of the otic labyrinth are restricted to a 

 few small, definitely localized areas which are fairly constant in loca- 

 tion in all vertebrates. Each sensory area consists of a group of rela- 

 tively long columnar cells (i.e., "long" in contrast to the flatfish cells 

 of the non-nervous epithelium), each sense-cell bearing a brush of 

 "sensory hairs" projecting from its free surface into the endolymph 

 (Fig. 181A). A sensory area may be a roundish spot or macula, or it 

 may be an elongated ridge or crista. On the main wall of the utriculus 



