GENERAL ORGANOLOGY 



117 



thousands, may remain separate. In a definite region of the epithelial 

 wall the sensory cells are developed into the crista aciistica or the auditory 

 ridge ; they are in connection with the auditory nerve and bear the auditory 

 hairs projecting into the endolymph. The otoliths are usually free in the 

 centre of the vesicle, or are often held in place by bundles of cilia which 

 project from the non-sensitive epithelial cells. 



Every auditory vesicle develops from a pitlike invagination of the 

 skin, and consequently is for a time an auditory pit. Therefore it is not 

 surprising that in many animals the organ has stopped at the lower stage 

 of development; for example, the crayfish has an open auditory pit 

 (fig. 378). On the other hand, the auditory vesicle may develop a com- 

 plicated system of cavities as in mammals (fig. 83), where it is divided by 

 a constriction into the sacadus and the utriculiis. The sacculus is pro- 



O 



'—U 



Fig. 83. — Diagram of the human labyrinth. U, utriculus with the semicircular 

 canals; 5, sacculus connected with the cochlea (C) by the canalis reuniens; R, recessus 

 labyrinthi; V , blind sac of the cochlea; K, apex of the cochlea. 



vided with a spirally-wound blind sac, the cochlea, the utriculus with the 

 three semicircular canals, the whole being called the labyrinth. In addi- 

 tion there is formed in most vertebrates, a sound-conducting apparatus, 

 so that the auditory organ acquires a very complicated structure. 



Other Forms of Auditory Organs. — Since there are animals without 

 auditory vesicles which hear well, like the spiders and insects, we must 

 assume that there are auditory organs of another type. Still we have 

 no certain knowledge of these except in the case of the tympanal auditory 

 organs of the grasshoppers (see p. 4ro). 



Function of the Semicircular Canals. — Experiments upon repre- 

 sentatives of the different classes of vertebrates have led to the conclusion 

 that the three semicircular canals, standing at right angles to each other, 

 are organs of equilibrium, while the cochlea of the mammals and the 

 homologous lagena of the other vertebrates is the seat of hearing. Corre- 

 sponding to the poor development of the lagena, hearing is so poorly 

 developed in the fishes that it was long believed to be absent and the laby- 

 rinth was regarded as a balancing organ, since when it was destroyed the 



