lOO NERVOUS SYSTEM OE VERTEBRATES. 



tactile stimuli, but the responses given by fishes to such stimuli 

 are less constant and precise than the reactions to combined gusta- 

 tory and tactile stimulation. Usually the localization of objects 

 detected by the gustatory or olfactory organs requires concurrent 

 somatic stimuli. This is easily illustrated in our own experience. 

 We can not tell with certainty the location of an object which we 

 smell. If a wind brings the odor to us we can tell the direction 

 of the object, but not its distance. The direction is known only 

 by the pressure (somatic) stimuli due to the wind. If we are 

 bhndfolded in a room and wish to find the position of a boquet 

 of flowers which we detect by its odor we must go about and sniff 

 until we find the place where the odor is strongest. In general, 

 while somatic movements may be called forth by visceral stimuli 

 they are more typically called forth by somatic stimuh and are more 

 precise when they are directed in response to somatic stimuli. The 

 nerve centers and peripheral nen-es which direct somatic move- 

 ments constitute a distinct portion of the nervous system called 

 the somatic efferent (motor) division. 



The visceral activities consist of contractions of visceral muscles, 

 secretory processes, vaso-motor regulation, etc. These all con- 

 tribute directly or indirectly to the processes of nutrition in the 

 vddest sense, or of reproduction. These activities are aroused 

 chiefly by visceral afferent impulses, including gustatory and 

 olfactory impulses. How far they may be called forth by somatic 

 stimuh is not fully understood. It is a fact of common experience 

 that the concept of food aroused by sight or tactile perception 

 may cause salivary secretion. Vaso-motor regulation is in large 

 part the result of heat and cold stimuli in the skin. Perspiration 

 is called forth by rise in temperature within the body. The nerve 

 centers and fibers which control visceral activities constitute 

 the visceral efferent division of the nervous system. 



In the life of any vertebrate animal four kinds of nervous activi- 

 ties are called for: (i) the reception of somatic stimuh; (2) the 

 direction of somatic movements; (3) the reception of visceral 

 stimuh; (4) the direction of visceral activities. Corresponding 

 to these four sorts of activities there are four anatomically distinct 

 divisions of the nervous system: somatic afferent and efferent. 



