128 NERVOUS SYSTEM OF VERTEBRATES. 



that many factors enter into this, including all forms of somatic 

 afferent impulses and the brain centers concerned with them. 

 In simple animals without special sense organs the maintenance of 

 equilibrium depends upon tactile stimuH from without and upon 

 impulses aroused by changes of pressure due to movements of 

 parts of the body upon one another. These factors are apparently 

 sufficient for the purpose in these animals and they undoubtedly 

 play an important role in equilibration in all animals. As special 

 sense organs are developed all those whose impulses may affect bodily 

 movements take part in the maintenance of equilibrium. When 

 the eyes are well developed and much used they are important 

 for equilibrium, as our own experience of dizziness and a stumbHng 

 gait after being long blindfolded, and the feeling of equilibrium- 

 weariness after walking or riding a bicycle in extreme darkness, 

 give evidence. In fishes the neuromasts must be of great impor- 

 tance directly or indirectly, because of their wide distribution 

 over the body which enables them to receive stimuli from many 

 directions, and because they are sensitive to vibratory stimuli 

 to which the general cutaneous endings do not respond. Finally, 

 the structure of the ear with its semicircular canals and contained 

 sense organs is such as to make it the especial organ of equili- 

 bration. In case of the ear it is just the change of position of the 

 animal's own body which arouses the vibrations that serve as the 

 appropriate stimuli to the sense organs. Since the control of 

 equilibrium in response to stimuli aroused from within is the 

 most direct and effective, the ears have become the dominant factors 

 in equilibration. It will appear below that the portion of the 

 ear concerned with equilibration is measurably separate from 

 that concerned with sound, and this separation extends to the 

 brain centers. 



The nerves which innervate the neuromasts are connected 

 with the dorso-lateral surface of the medulla oblongata by three 

 chief roots, one of which may be subdivided into two. As already 

 stated (p. 20) the caudal root is distributed to the organs of the 

 lateral Hne, the middle root supplies the organs of the ear, and the 

 cephalic root, which may be double, innervates the three canals 

 of the head. On their way to their areas of distribution the fibers 



