COORDINATION AND BEHAVIOR 



549 



labyrinth filled with endolymph. The 

 cochlea is a spiral canal of VA turns. The 

 auditory nerve is divided into two, one part 

 becoming the cochlear nerve and the other 

 the vestibular nerve. Air waves entering the 



external ear bring about vibrations of the 

 eardrum which are communicated through 

 the 3 bones, the malleus (hammer), incus 

 (anvil), and stapes (stirrup), to the fluid in 

 the internal ear; this stimulates the nerve 



■Malleus 

 -Incus 

 Stapes • 



Semicircular canal 

 Auditory nerve 



Eustachian tube 



Pharynx 



Figure 396. A dissection of the human ear to show the parts that have to do with receiving 

 sound and the part which is concerned with balance. 



endings in the organ of Corti, and impulses 

 are transmitted by the auditory nerve to the 

 center of hearing in the brain. The organ of 

 Corti lies within the cochlea and contains 5 

 rows of sensory "hair cells" which are the 

 sound receptors. 



Equilibrium in man is a complex thing 

 that depends on vision, proprioceptors, 

 sensitiveness to pressure on the soles of the 

 feet, and organs of equilibrium within the 

 internal ear. The latter consist of two small 

 sacs, the saccule and utricle, and semicircular 

 canals. On the bottom of the saccule and 

 the utricle there is a membrane of sensory 

 hairs on which rest many tiny crystals of 

 lime (otoliths). This balancing mechanism 

 gives knowledge of linear accelerations, and 

 tilt and pull of gravity, due to the inertia of 



the crystals which causes the sensory hairs 

 to bend. The semicircular canals also con- 

 tain "hair cells" like those of the saccule and 

 utricle. These hair cells are sensitive to the 

 movement of a fluid which fills the canals. 

 Any acceleration or deceleration of head 

 movement causes the fluid to flow in differ- 

 ent directions in the canals, thus stimulat- 

 ing the hair cells to send impulses to the 

 brain. The receptors in the canals are 

 strongly affected by vertical movements like 

 those of an elevator and by the rolling and 

 pitching motions of ships and planes. 



The eyes are the most complex of the 

 sense organs of vertebrates. The principal 

 elements of structure and the method of 

 action may be pointed out by means of a 

 diagram of the human eye (Fig. 397). The 



