THE NERVOUS SYSTEM 



389 



The simplest 

 fluid, in which 



waders have the largest olfactory nerves. It is most 

 acute in the carnivorous quadrupeds, and in some wild 

 herbivores, as the deer. In man it is less delicate, but 

 has a wider range than in any brute. 



Hearing is the perception of sound, 

 form of the organ is a sac filled with 

 float the soft and delicate ends of 

 the auditory nerve. Usually the vibra- 

 tions of the fluid are strengthened by 

 the presence of minute hard granules, 

 called otoliths. Most invertebrates _ 



FIG. 347. Ear of a Mol- 



have no more complicated apparatus lusk (Cycias\ greatly 



- . , ., . , , , ., enlarged, showing the 



than this ; and it is probable that they 

 can distinguish one noise from an- 

 other, but neither pitch nor intensity. 

 The organ is generally double, but not always located in 

 the head. In the clam, it is found at the base of the 

 foot ; some grasshoppers have it in the fore legs ; and 

 in many insects it is on the wing. Lobsters and crabs 

 have the auditory sacs at the base of the antennas. 145 



otolith in the center of a 

 cavity which is filled with 

 fluid, and whose walls are 

 lined by ciliated cells. 



FIG. 348. Brain and Auditory Apparatus of the Cuttlefish: a, b, brain; c, auditory 

 apparatus; d, the cavity in which it is lodged ; e,f,g, eyes ; i, 2, 3, otoliths. 



A complex organ of hearing, located in the head, 

 exists in all vertebrates, save the very lowest fishes. 



