106 HUMAN PHYSIOLOGY 



from body to brain, it requires the co-operation and, 

 if it may be so termed, the supervision of nerve 

 cells. This definition must be kept fairly in mind, 

 because in common language the term " nerve " is 

 used as if the bodily telegraph wires represented the 

 most essential feature of the nervous apparatus. 



NERVE FORCE. The particular form of energy 

 which nerve cells generate, and which represents 

 the electricity of our bodily telegraph system, is 

 termed nerve force. It is this force which flows 

 along nerve fibres, and represents the messages 

 which stimulate the body to action on the one hand, 

 and arouse the brain and nerve centres on the other. 

 The rate at which nerve force passes along nerve 

 fibres has been estimated in warm-blooded animals 

 at about 200 feet per second. This is a slow rate as 

 compared with that of electricity or with the speed 

 of light waves, which pass through space at a rate 

 of 1 86,000 miles per second. The phrase " as quick as 

 thought" is therefore to be accepted in a relative 

 sense only, though the rate at which our nerve force 

 speeds along nerve fibres is quite sufficient for the 

 perfect carrying out of the work of the nervous 

 system. 



NERVE CELLS AND FIBRES. Nerve cells form, in 

 one sense, the central figure of the nervous system. 

 Like all other cells a nerve cell is a mass of living 

 protoplasm, and, having regard to the duties it dis- 

 charges, we may safely conclude that the protoplasm 

 of the nervous system is of higher order than that 

 represented in the other cells of the body. This 

 much, indeed, we are legitimately entitled to assume 

 from the character of the work it performs. Like 

 all other cells, nerve cells are microscopic in size, 

 some of them extremely minute. They vary in 



