246 NUTRITIONAL PHYSIOLOGY 



existence of materials, tools, and a model, made the result 

 inevitable. 



In the previous discussion of reflex action (Chapter V) 

 the conception was developed that the nervous system con- 

 sists of pathways capable of transmitting energy in the 

 form of "nerve-impulses" to and from its central portion. 

 That central part is represented in the higher forms by 

 the brain and the spinal cord. The afferent or incoming 

 paths begin in localities where external influences or stim- 

 uli can be brought to bear. The nerve-endings which lie 

 thus exposed are called "receptors." They may be simple 

 terminal twigs or bulbs subject to stimulation by pressure 

 or temperature. They may be connected with elaborate 

 organs like the eye and the ear. The eye is a device for 

 converting the energy of certain ether-waves into the im- 

 pulses that traverse the optic nerve. The ear is, at least 

 in part, a device for transforming the energy of certain 

 air-waves into impulses that run along the auditory 

 nerve. 



The impulses which arrive within the brain or the cord 

 may cause the immediate or delayed return flow of im- 

 pulses along the efferent or centrifugal paths of the system. 

 These lead in most cases to the contractile tissues which 

 give objective expression to animal life. It must not be 

 forgotten that the efferent branches of the nervous system 

 extend also to various glands and may modify secretion as 

 well as contraction. It will be recalled that efferent im- 

 pulses may initiate the flow of tears, of the saliva, the 

 gastric juice, and the adrenal principle. Other instances 

 are almost equally clear. 



In animals of all grades responses of the reflex type are 

 the constant duty of the nervous equipment. Those forms 

 which we regard as highest in the scale are distinguished by 

 a second property, that of the modification of the re- 

 sponses as a result of individual experience. By this is 

 meant the capacity for training, forming habits, or learn- 

 ing to profit by the past. It is what makes different mem- 

 bers of the same species vary in their behavior. It is what 



