THE NERVOUS FUNCTIONS 



25 



nervous system is the apparatus of these more perfect adjust- 

 ments and its protoplasm is highly modified in different direc- 

 tions. Some parts may be especially sensitive to particular 

 forms of energy (such as light waves, sound waves, etc., this 

 being termed the adequate stimulus in each case) ; other parts, 

 the nerves, are highly modified so as to conduct nervous impulses 

 from part to part with a minimum expenditure of energy and loss 

 of efficiency; still other parts of the nervous system serve as 

 centers for receiving and redistributing nervous impulses some- 

 what after the fashion of the central exchange of an automatic 



jKin 



Fig. 1. Diagram illustrating the simplest spinal reflex arc consisting 

 of two nervous elements or neurons (see Chapter III), a sensory neuron 

 connected with the skin and a motor neuron connected with a muscle. 

 Physiological connection between the two neurons is effected within the 

 spinal cord. (Modified from Van Gehuchten.) 



telephone system. These are the correlation centers, and they 

 are larger and more complex in proportion to the range of diver- 

 sity in the possible reactions of the animal. 



The simpler reactions to stimulation of the sort here under con- 

 sideration are called reflexes (Fig. 1 ; see also p. 56), and the essen- 

 tial mechanism is a reflex arc consisting of (1) a sensitive receiv- 

 ing organ (receptor or sense organ); (2) a conductor (afferent 

 or sensory nerve) transmitting the nervous impulse inward from 

 the receptor; (3) a correlation center or adjuster, generally 

 located within the central nervous system; (4) a second con- 

 ductor (efferent or motor nerve) transmitting the nervous im- 



