310 PHYSIOLOGY 



doctrine of continuity through the central nervous system. Even if it be 

 true, it would not in any way upset the importance of the neuron theory. 

 Every plant or animal individual must be regarded as a protoplasmic con- 

 tinuum. With growth of the living matter, its metabolic functions demand 

 the dispersion of nuclear material through the protoplasm, and this is 

 effected by division of the nucleus. Considerations of strength and rigidity 

 demand the division of the protoplasm into compartments or cells, which, 

 at first at any rate, remain in protoplasmic continuity. This division has 

 probably a further advantage in that lesions of parts of the individual entail 

 merely the death of the cells immediately affected and do not necessarily 



m. 



FIG. 151. Schema of the neurofibrillar continuum, involved in an ordinary reflex 

 act, in a vertebrate nervous system. (BETHE.) 



spread to the whole organism. Thus in the central nervous system injury 

 to one axon causes degeneration of the axon below the point of section, but 

 the degeneration stops short at the end arborisation and does not spread 

 into the next neuron. If we assume that, in consequence of the straitness 

 of the path, the propagation through the fibrillse is especially difficult in the 

 synapse, most of the phenomena described above as characteristic of the 

 reactions which take place in the central nervous system can be easily ex- 

 plained on the theory of continuity of the fibrillse. The serious difficulty 

 in the acceptance of this theory is, however, the ' Law of Forward Direction,' 

 i.e. the fact that an impulse will pass from an axon to the next neuron, but 

 will not pass backwards across the synapse from the cell body to the con- 

 tiguous axon. Bethe suggests that this rule of Forward Direction, which 

 is possibly present only in the more highly developed nervous systems, may 

 be due to a species of " polarity " of the nerve-fibril, of such a nature that 

 an impulse is strengthened and so assisted on its passage in the normal 

 direction, but is diminished and finally abolished when it passes in the 

 opposite direction. Such an explanation is unsatisfactory, since there is 

 absolutely no experimental evidence of the existence of such polarity in a 



