CHAPTER VII 

 The Functions of Nerve-Fibres and Nerve-Cells 



ON the view which we take as to the nature and amount 

 of control which the nervous system exerts over the organs 

 depends to a certain extent our conception of the causes 

 which lead the organs to do their work. There is no other 

 problem in physiology of so general a character as this. 

 The simplest animals which exist at the present time are 

 destitute of any tissue specially set apart for the control of 

 the other tissues, and it may be assumed that the animals 

 which earliest appeared upon the earth were in this respect 

 like the simplest animals now extant. The unicellular 

 amoeba which crawls about the stalks of duckweed in our 

 ponds exhibits in its movements what in higher animals we 

 should regard as evidences of purpose. It moves in the 

 direction of its food. Yet the appreciation of the direction 

 in which food lies and the guidance of its movements are 

 due to the properties of its general body-protoplasm, and 

 not to any specialized internal structure. The simplest 

 multicellular animals, if certain composite animals which 

 may be regarded as colonies of cells rather than compound 

 individuals, are excluded, set aside certain cells for the 

 reception of information from the outer world, and certain 

 other cells, prolonged into fibres, for the transmission of 

 messages to its contractile tissue. A nerve-cell and its fibre 

 come into existence in order that they may establish a com- 

 munication between the outer world and the muscle-cells by 

 which an animal moves. There is no d priori reason why 



