Man may normally be expected to respond in a particular way to a particular stimulus because men throughout the history of the race have so responded.  Certain connections have been established in his nervous system and he acts accordingly—­he does what he does because he is man.  We cannot here go into a detailed discussion of the physiological processes involved in thinking and other forms of behavior, but perhaps we may well set down a statement or two relative to man’s tendencies to act, and their explanations: 

“The nervous system is composed of neurones of three types:  Those that receive, the afferent; those that effect action, the efferent; and those that connect, the associative.  The meeting places of these neurones are the synapses.  All neurones have the three characteristics of sensitivity, conductivity, and modifiability.  In order for conduct or feeling or intellect to be present, at least two neurones must be active, and in all but a few of the human activities many more are involved.  The possibility of conduct or intelligence depends upon the connections at the synapses,—­upon the possibility of the current affecting neurones in a certain definite way.  The possession of an ‘original nature,’ then, means the possession, as a matter of inheritance, of certain connections between neurones, the possession of certain synapses which are in functional contact and across which a current may pass merely as a matter of structure.  Just why certain synapses should be thus connected is the whole question of heredity.  Two factors seem to affect the functional contact of a synapses,—­first, proximity of the neurone ends, and second, some sort of permeability which makes a current travel on one rather than another of two neurones equally near together in space.  This proximity and permeability are both provided for by the structure and constitution of the nervous system.  It should be noted that the connection of neurones is not a one-to-one affair, but the multiplicity of fibrils provided by original nature makes it possible for one afferent to discharge into many neurones, and for one efferent neurone to receive the current from many neurones.  Thus the individual when born is equipped with potentialities of character, intellect and conduct, because of the pre-formed connections or tendencies to connections present in his nervous system.

“Types of Original Responses.—­These unlearned tendencies which make up the original nature of the human race are usually classified into automatic or physiological actions, reflexes, instincts, and capacities.  Automatic actions are such as those controlling the heart-beats, digestive and intestinal movements; the contraction of the pupil of the eye from light, sneezing, swallowing, etc., are reflexes; imitation, fighting, and fear, are instincts, which capacities refer to those more subtle traits by means of which an individual becomes a good linguist, or is tactful, or gains skill