THE NERVOUS SYSTEM 203 



in the vestibulum. Vibrations thus set up in the perilymph 

 travel up the scala vestibuli to the apex of the cochlea, and then 

 down the scala tympani to the fenestra rotunda, whose mem- 

 brane moves in and out with the vibrations. In passing the 

 scala media the vibrations in the perilymph are transmitted 

 through the walls of the scala media, eventually stimulating the 

 hair cells of the organ of Corti. Whether the vibrations reach 

 the hair cells through the vestibular membrane and endolymph 

 or directly through the basilar membrane remains an open 

 question. 



Equilibration. — The semicircular canals serve as organs of 

 equilibration. Every movement of the head causes the endo- 

 lymph and the contained otoconia to stimulate the cristae 

 acusticae in the semicircular canals from which impulses are car- 

 ried to the brain by the vestibular branch of the eighth cranial 

 nerve. Since the three canals lie in planes at right angles to each 

 other, movement in any direction is certain to affect one of them. 

 The action of the fluid on the hairs can be illustrated by a simple 

 experiment. If a glass of water is given a sudden rotary twist 

 the glass moves, but the water tends to lag behind, so that if the 

 glass had contained fine hairs extending from its sides into the 

 water, these hairs would have been bent or otherwise disturbed 

 by the water. In a similar way, when the head moves, the 

 endolymph in the canal lying in the plane of motion tends to 

 remain stationary while the sensitive hairs are dragged through 

 it and stimulated. Thus means are afforded for recognizing the 

 direction and components of any motion. At the same time 

 sensory cells in other parts of the utriculus and sacculus are 

 stimulated by the endolymph. 



Integrative Action of the Nervous System. — The first appear- 

 ance of the nervous system in the lower invertebrates, as has 

 been already pointed out, does not involve the creation of a new 

 functional activity so much as a specialization of the functions of 

 excitation and transmission, which are common properties of all 

 forms of life. Protozoa are sensitive to the same stimuli that 

 affect higher forms, yet they lack a nervous system. The 

 importance of the nervous system becomes more and more pro- 

 nounced with increase in size and complexity of the animal body, 

 because of the greater dependence on it as a means for providing 

 rapid excitation and transmission. The significance of the ever- 



