568 COMPARATIVE PHYSIOLOGY. 



Auditory Judgments. — Such are much more frequently erro- 

 neous than are our visual judgments, whether the direction or 

 the distance of the sound he considered. As in the case of the 

 eye, the muscular sense, from accommodation of the vihratory 

 mechanism, may assist our judgments, heing aided hy our 

 stored past experiences (memory) according to the law of asso- 

 ciation. Sounds are, however, always referred to the world 

 without us. The animals with movable ears greatly excel man 

 in estimating the direction, if not the distance, of sounds. There 

 are few physiological experiments more amusing than those 

 performed on a person blindfolded, when attempting to deter- 

 mine either the distance or the direction of a sounding tuning- 

 fork, so gross are the errors made. 



One who makes such observations on others may notice that 

 most persons move the ears slightly when attempting to make 

 the necessary discriminations, which of itself tends to show how 

 valuable mobility of these organs must be to those animals that 

 have it highly developed. 



SPECIAL CONSISIiRATIONS. 



Comparative. — Among invertebrates steps of progressive de- 

 velopment can be traced. Thus, in certain of the jelly-fishes 



we jSnd an auditory vesicle (Fig. 

 414) inclosing fluid provided with 

 one or more otoliths or calcareous 

 nodules and auditory cells with at- 

 tached cilia, the whole making up 

 an end-organ connected with the 

 auditory nerve. A not very dis- 

 similar arrangement of parts exists 

 in. certain moUusks (Fig. 415). The 

 vesicle may lie on a ganglion of 

 the central nervous system. On 

 the other hand, the vesicle may be 

 open to the exterior, as in decapod 

 crustaceans ; and the otoliths be re- 

 FiG. 4i4.-Auaitory vesicle of Oery- placed by grains of sand from with- 

 a"i?Ser*a^-TK He?t' «^t- ^^ ^^ difficult to decide what 

 wigi. N and JT', the auditory the function of otoliths may be in 



nerves; 0«, otolitlij J2s, aiidito- , , ,, . , 



ry cells; flZi, auditory cilia (type mammals; but there seems to be 



°Tra(ihymedmm). *"^^™ " * little reason to doubt that they com- 



