THE STORY OF THE BRAIN AND NERVE 107 



diameter from the 1 -500th or the 1 -600th part of an 

 inch to the 1 -5000th part of an inch, many of the 

 cells in the brain exhibiting even more minute 

 dimensions. The characteristic feature of a typical 

 nerve cell is that it gives off branching threads or 

 processes of its substance called dendrons. Certain 

 of these connect the cell with neighbour cells, for as 

 a rule we find in the brain and spinal cord cells are 

 gathered together into groups to which the general 

 name of nerve centres has been applied. Amongst 

 the processes or dendrons which a nerve cell 

 possesses, one stands out more prominently than 

 the rest. This process is termed the axis cylinder, 

 and differs from other branches or dendrons in that 

 it can be traced passing from the cell outwards to 

 the body. When so traced, this axis cylinder is 

 found to become one of the fibres of a nerve. In 

 other words, just as the wires of the telegraph 

 system are directly connected with the batteries, so 

 the nerve cells give off their axis cylinders, which 

 pass forth and become veritable wires of the body. 

 It is unnecessary to detail the different forms and 

 shapes which nerve cells assume, save to record that 

 in the spinal cord and elsewhere, the cells give off 

 many dendrons or branches, and are called multi-polar 

 cells, whereas those which are characteristically 

 typical of the brain are triangular in shape, giving off a 

 branch at each angle, and one from the centre and 

 base, this last representing the process which 

 ultimately will pass to become a fibre in one of the 

 nerves of the brain. The nature of the second 

 element in the nervous system, the nerve fibre, has 

 already been sufficiently indicated. Even the finest 

 nerves of the body are made up of bundles of fibres 

 which originate from nerve cells. If these fibres be 



