ACTION OF THE NERVOUS SYSTEM. 67 



est as among the highest animals, we must condude 

 that the controlling nervous activity is everywhere 

 of the same fundamental character. A distinct, 

 separate, and visible nervous structure is there- 

 fore not essential to the display of nervous activity. 

 In fact an elementary form of nervous activity is 

 recognisable in that sensitiveness or irritability, 

 which is common to all living matter. The move- 

 ments which result from this so-called irritability, 

 we must believe to be caused by internal molecular 

 changes of the mass ; and these molecular changes 

 must in turn be caused by the action of some 

 external force. 



As there are certain animals, of a low degree of 

 organisation, which possess no nervous structures, so 

 also there is a period in the life of each of the higher 

 animals when there is no nervous structure present. 

 At the beginning of their individual existence, all of 

 the higher animals resemble closely the full-grown 

 forms of the very lowest of animals. They consist 

 of a single cell, and even after many cells have de- 

 veloped, there is still no trace of nervous structures. 

 Yet during this time the functions of nutrition, as- 

 similation, growth, and development are performed 

 harmoniously, all the activities of the organism unit- 

 ing to produce a common result. Long before the 

 cells of the nerve tissues have attained their com- 



