302 INTRODUCTION TO NEUROLOGY 



that is, the hereditary component of the commonly recognized 

 instincts (p. 61). There is no clear boundary between reflexes 

 and instinct-actions as just denned. These actions may be 

 exceedingly complex and their neuro-muscular mechanisms may 

 be complicated apparently without limit. The available evi- 

 dence suggests that they are always unconsciously performed. 



Most of our common activities include all three of these types 

 of behavior in varying proportions, and accordingly they fre- 

 quently have not been distinguished. The first and third types 

 are especially liable to confusion, for both are manifested as 

 stereotyped, non-intelligent behavior. They can sometimes be 

 separated only by a study of their origins; nevertheless this dis- 

 tinction is of great importance, especially to educators. 



The nervous organs of the invariable reactions are fairly well 

 known and are characterized in their more highly elaborated 

 forms by a closely knit system of nerve-centers and distinct con- 

 necting fiber tracts so organized that particular stimuli may call 

 forth a response or a combination of several responses selected 

 from a fixed number of possible actions. The range of possible 

 reactions of any given functional system of this type is limited 

 by the structural complexity of the nerve-centers involved. 

 This complexity may be very great, with a correspondingly great 

 number of movements necessary to complete the reaction, and it 

 may include the capacity for discriminating between two or more 

 structurally possible modes of response by means of variable 

 internal functional states of the nerve-centers. But in all of 

 these cases the response is finally determined within rather nar- 

 row limits by the nature of the stimuli and the innate structural 

 organization not only of the nervous organs, but of the body as a 

 whole. 



In some cases an elaborate nervous reflex or instinctive act 

 may involve a more extensive nervous apparatus than is required 

 by an intelligent act. It is not a mere question of the size of the 

 nervous mechanisms involved. For instance, a comparison of 

 the brains of the two species of fishes shown in Fig. 136 shows 

 that in the medulla oblongata of these rather closely related 

 species there is an astonishing difference between the size of 

 certain reflex centers. The greater size of the medulla oblon- 

 gata of Carpiodes over that of Hyodon is due almost entirely to 



