COORDINATION AND CONTROL: (1) THE NERVOUS SYSTEM 115 



The static sensation receptors fall into two functional groups. Those 

 of the first group are stimulated by rotational movements of the head, 

 and are located in enlargements at one end of each of the semicircular 

 canals. The receptor cells have hairlike projections that extend into the 

 cavity of the canal. When the head is turned in any direction, the semi- 

 circular canals move with it, but inertia causes the endolymph within the 

 canals to tend to remain at rest. Any rotation having a component 

 parallel to the plane of one or more of the canals will therefore cause a 

 movement of the endolymph relative to the canal and thus stimulate the 

 projecting ends of the receptor hair cells. 



Information concerning fixed positions of the head comes from the 

 other group of receptors. Patches of hair cells are located in the walls 

 of the sacculus and utriculus. They are stimulated by the position of a 

 gelatinous mass which rests upon them — the otolithic membrane. This 

 shifts its position as a result of gravity when the head is moved, and 

 remains displaced as long as the head keeps its new position. 



These two sets of receptors (as well as the sound receptors located in 

 the cochlea) stimulate afferent 1 neurons whose axons go to the medulla 

 by way of the eighth pair of nerves. Unlike the auditory stimuli, those 

 from the static receptors are routed primarily to the cerebellum. Here 

 they are correlated with stimuli from kinesthetic receptors located in 

 muscles, tendons, and joints, thus providing information about the posi- 

 tion and movements of the rest of the body relative to the head, and 

 making possible such actions as balancing the body. These two sets of 

 afferent neurons make synapses in the cerebellum with neuron chains 

 ending in muscles, so that this part of the brain is largely responsible for 

 motor coordination, much of which is accomplished without the necessity 

 of conscious control. Only occasionally do static and kinesthetic sensa- 

 tions rise to the level of consciousness. 



THE NERVOUS MECHANISM AND THE BRAIN 



Our knowledge and inferences as to the functioning of the human 

 nervous system come largely from three sources: (1) its structure, as de- 

 termined by both gross and microscopic dissection; (2) experiment on 

 living animals, including many types of experiments with human subjects 

 as well as those that must be confined to laboratory animals; and (3) 

 subjective observation of one's own mental and nervous functioning. Each 

 of these has inherent advantages and limitations, and much of our con- 

 siderable, although still highly incomplete, knowledge of how the nervous 

 system works is due to pooling the information derived from each type of 

 observation. 



Any detailed summary of what is known of the functioning of the nerv- 

 1 Afferent — carrying impulses to the central nervous system; see p. 116. 



