120 AN INTRODUCTION TO SCIENCE 



the spinal cord, grow down into the peripheral portion until 

 they find the muscle-fibres from which they have been 

 severed. If a sensory nerve be divided, its fibres also grow 

 centrifugally, because they are the processes of cells which 

 lie in the spinal ganglia, just outside the spinal cord. But 

 the attempt to cause the central stump of a cut motor nerve, 

 A, to grow downward into the peripheral portion of a cut 

 sensory nerve, B, is an experiment foredoomed to failure. 

 Nothing would be gained by crossing two motor nerves, 

 since they both carry impulses of the same kind, and 

 unfortunately the nerves of special sense do not allow of the 

 experiment. It is not possible, for example, to cross the 

 nerve of taste with the nerve of hearing. 



Miiller was mistaken in attributing the specific effects 

 of stimulating the several sensory nerves to the nerves 

 themselves. It was recognised by Vulpian (1866) that "all 

 nerves sensory, motor, vaso-motor, and others have the 

 same properties, and are only distinct in their functions. 

 This question is of the highest importance for general 

 physiology. It dominates the whole physiology of nerve- 

 fibres." Many observations made since Vulpian wrote have 

 shown that a nerve has no functions more specific than 

 those of a telegraph wire. It conducts impulses. If a 

 motor nerve is stimulated in the middle of its course by 

 an electric current, a nerve-current is started braimvards 

 (where it will produce no effect) as well as towards the 

 muscles which it supplies ; and the same is true of a sensory 

 nerve. The specific effects depend upon the sense-organ from 

 which its message starts and upon the receiving apparatus to 

 which it delivers it. If a telegraph wire were cut and the 

 sending apparatus, or transmitting key, were moved from the 

 town from which the wire started to the cut end of the wire, 

 the clerk in charge of the indicator would suppose that any 

 message he received came from the town with which the 

 wire ought to place him in communication, and he would 

 read his message in this belief. That the brain is in the 



