14 THE RIDDLE OF MIGRATION 



viously received. Any given nerve is made up of a 

 very large number of fibres and every fibre, be it 

 short or long, possesses a nucleus and cell-body, of 

 which the fibre is merely a process. Fibre and cell- 

 body together constitute the complete nerve-cell 

 which is termed a neurone. An individual fibre can 

 transmit an impulse in but one direction, never in 

 two. It thus comes about that nerves containing 

 only fibres of one kind are either pure sensory or 

 pure motor. If they contain both, they are mixed 

 nerves and, as such, can transmit impulses either 

 in or out. All the spinal nerves, but only some of 

 the cranials, are mixed. 



In its simplest form a nervous response is as fol- 

 lows. A sense organ somewhere on the body receives 

 a stimulus. The sensory fibres supplying it convey 

 the message inward to the central nervous system. 

 Here minute processes on the tip of the sensory fibre 

 arborize and come into contact with similar fibrils 

 of a motor fibre or fibres. (Fig. 1, A.) The impulse 

 is thus transferred to the motor fibres and so re- 

 transmitted outward to a muscle (or gland) which 

 makes a suitable response. We can illustrate this 

 by a simple experiment. If a frog is killed by having 

 its brain destroyed, and a drop of irritating fluid is 

 then placed somewhere on its body, one of the hind 

 legs will go through the appropriate action of kick- 



