SENSITIVITY IN PLANTS AND ANIMALS 77 



from the central element by an efferent nerve to the 

 contractile or secretory cell (Fig. 34A). 



In the higher types of animal life a fourth 

 element is added, so that there are two central 

 elements, one to receive the impulse from the sensory Nerves, 

 organ, and another connected with the former to 

 transmit, by means of the HERVE 



efferent nerve, the impulse 

 to the muscle, gland, or 

 other body affected. When 

 the response takes place 

 without any consciousness 

 being aroused, it is termed 

 a reflex action. Usually, 

 however, the two nerve - 

 centres are connected with 

 a nerve cell complex form- 

 ing a central nervous 

 system, by whose means FIG. 34. Scheme of nervous 



a definite and determinate 

 co-ordination of the various 

 parts of the organism is 

 insured.. It is the develop- 

 ment and elaboration of this central nervous system 

 that furnishes the key-note to the history of the 

 evolution of the animal line of life. 



system. A, simple reflex 

 action; B, 1, 2, 3, 4, reflex 

 action; 1, 2, 5, 6, 3, 4, con- 

 scious nervous response. 



