THE NERVOUS SYSTEM OF VERTEBRATES 



339 



(c/. Fig. 133). The nerve impulse so started is carried by the nerve 

 fibre past the T-shaped junction in the posterior root ganglion into 

 the cord, 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 



FIGS. 146 and 147. Nerve-cells from spinal cord. (BETHE.) 



Fig. 146, showing Golgi network, and neurofibrils : d, e, f, junctions of 

 axons with Golgi network. Fig. 147, showing neurofibrils and Nissl bodies. 



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 the terminations 

 of the axons in immediate contact with the cell, This statement 



