SENSE ORGANS AND NERVOUS COORDJNATJON 



581 



for the lens is moved back and forth in camera fashion in fishes and 

 amphibians and does not change shape. 



244. The Lateral Line and Ear 



Equilibrium. AH vertebrates have the ability to perceive differences 

 in the orientation of their bodies with respect to their surroundings and 

 to maintain their equilibrium. Although vision and proprioceptive im- 

 pulses from the muscles play a part, this ability is primarily a function 

 of the inner ear. The inner ear is embedded within the otic capsule of 

 the skull and consists of a complex of membranous walled sacs and 

 canals, the membranous labyrinth, which are filled with a liquid endo- 

 lymph and surrounded by a protective liquid cushion, the perilymph 

 (Fig. 29.5). The dorsal part of the membranous labyrinth consists of 

 three semicircular canals, each of which is perpendicular to the other 

 two. Two lie in the vertical plane, but at right angles to each other, 

 and one is in the horizontal plane at right angles to the other two. Each 

 has a round swelling, an ampulla, at one of its ends in which there is a 

 patch of hair cells— receptor cells bearing hairlike processes. The three 

 semicircular canals connect with a chamber known as a utriculus, and 

 this in turn connects with a more ventral chamber known as a sacculus. 

 Both of these chambers contain patches of hair cells. Calcareous otoliths 

 are in contact with these cells. Different parts of the membranous labyrinth 

 are concerned with different aspects of equilibrium— static equilibrium, 

 linear acceleration and angular acceleration. Differences in the position of 

 the head and body (static equilibrium) affect the way in which gravity pulls 

 the otoliths uik)ii the underlying hair cells. Rapid forward movement 

 (linear acceleration) cause the otoliths, which have more inertia than 

 the surrounding endolymph and hence lag, to push back upon certain 



Utriculus 



Ante-rior vert iced 

 Semicircula-r csLnal 



Ampiilla.' 



Horizontal 

 Scrnicirculair canal 



EndolyiTLphatic sac 



Perilymph 



^-Posterior ve.rtical 

 .'se-micircular canal 



Patches of hair cells 

 Sa.cculus 



ena 



Figure 29 5 The left ear of a fish seen in a lateral view. Only an inner ear is 

 present, embedded within spaces in the otic capsule of the skull. (Modified after 

 Kingsley.) 



