302 PHYSIOLOGY 



and along a branch of the entering nerve fibre which runs right across the 

 cord to terminate in the neighbourhood of the anterior horn-cells. Here 

 the impulse must be transmitted in some way to the dendrites or body of 

 one of the large motor nerve-cells in the anterior horn, whence it is carried 

 along the axon of the cell, leaving the cord by the anterior root and passing 

 down a peripheral nerve to the end-plate on a muscle fibre. Here again 

 by some means the arrival of the impulse excites the muscle to contract. 

 This reaction never takes place in the contrary sense, i.e. no impulse started 

 in the motor nerve can travel back through the spinal cord and along the 

 sensory nerve. Although an impulse excited in the nerve passes easily 

 to the muscle, an excitatory process started in the muscle itself is confined 

 to this tissue and never extends to the nerve fibre. Apparently the same 

 rule holds good within the grey matter of the central nervous system, where 

 two neurons come into relation with one another. An impulse passes easily 

 from the axon of one into the dendrites and cell of the other neuron, but, 

 so far as we are aware, it is impossible by exciting an axon to cause a 

 retrograde wave of excitation to pass through its corresponding cell and 

 into t he terminations of the axons in immediate contact with the cell. 

 This statement has been called by Sherrington the ' Law of Forward 

 Direction.' It might be also spoken of as the irreciprocal conduction of the 

 nerve arc. The character of a reaction to any stimulus, applied to the 

 surface of the body, is determined by the course which the impulse, excited 

 in the afferent nerves, takes on entrance into the central nervous system. 

 This course is laid down by the connections of the neurons through which 

 the nerve impulse passes. In the central nervous system therefore, more 

 than in any other part- of the body, function is directly dependent on struc- 

 ture. Theoretically if we had a perfect knowledge of the connections of 

 the neurons in the central nervous system and knew the nerve fibres affected 

 by any given stimulus, we should be able to prophesy exactly the result 

 of such stimulus. In the case of the simpler reactions this is already possible, 

 but in the higher parts of the nervous system the enormous complexity of 

 the systems of neurons excludes any possibility of our forming more than 

 a general idea as to the nerve paths traversed in any given reaction ; and 

 the variations which exist from individual to individual must always prevent 

 in the intact animal an absolute prediction of the results of any stimulus. 



